Training guide
Everyone runs slower in the heat, and that's your body protecting itself, not a lack of fitness. Big-city marathons are fastest on cool days of about 4 to 10°C, and finish times slow step by step as it warms, slower runners most. So run by effort rather than pace, go early or late, wear light kit, and drink to thirst.
Your body adapts. Over 1 to 2 weeks of easy runs in the warmth, your heart rate and body temperature fall at the same pace and you sweat sooner, with much of the change in the first week. This guide covers how much to slow down, how to acclimatise, how much to drink without overdoing it, clothes and timing, who is at more risk (older adults, some medicines, heart conditions, pregnancy), and the NHS signs of heat exhaustion and heatstroke.
Marathon times slow steadily as it warms, slower runners most, and the best marathon speeds are at about 4 to 10°C. At the hot 2018 London Marathon, surveyed runners finished 14% slower than they'd planned. Keep easy runs at a pace where you can talk, and take walk breaks.
Easy runs of about an hour in the warmth on most days lower your heart rate and body temperature, much of it in the first week; five days already helps, and it fades over a few weeks without heat.
Runs under an hour rarely need drinks, drinking to thirst did about as well as a schedule, and drinking well beyond thirst causes dangerously low blood sodium, as our run longer guide explains.
The NHS advises avoiding the sun between 11am and 3pm and wearing light-coloured, loose clothing. Strong sun and humidity make it harder still.
At the Boston Marathon, heatstroke struck about 4 in 10,000 starters, all on days of moderate heat, and younger, faster runners were at higher risk. Know the NHS signs, and call 999 for heatstroke.
Older adults, people with heart, lung or kidney conditions or diabetes, people on some medicines, pregnant women and anyone recently ill need more care in the heat. Don't stop a medicine to run: ask your GP or pharmacist.
Running makes heat. In hot weather your body sends more blood to the skin and sweats more to lose it, so the same pace raises your heart rate and feels harder. Your brain also eases the pace early: in 10 cyclists in a 20 km time trial at 35°C, power fell "before there is any abnormal increase in rectal temperature, heart rate or perception of effort". These are small lab studies.
Sweat cools you only when it evaporates. At 30°C, 8 men cycled for 68 minutes at 24% humidity and 46 at 80%, and 46 minutes in the shade against 23 under strong sun.
There's no single cut-off. Performance falls steadily on both sides of a cool optimum, and race heat illness rises with the wet bulb globe temperature (WBGT), a heat index that also counts humidity and sun. In the UK, the UK Health Security Agency issues heat-health alerts with the Met Office when it's hot enough to affect health.
Most people sweat about 0.5 to 2 litres an hour when exercising, and a few over 3. Sweat carries salt, roughly 10 to 70 millimoles of sodium a litre, and sweat rates vary several-fold between people.
Fitness and acclimatisation help, and trained athletes adapt faster. Older adults feel less thirst and lose heat less well, and some long-term conditions and medicines raise the risk. Children control their body temperature much like adults. This guide is for people who already run or walk comfortably; if you're starting to run in a heatwave, start in the cool of the day or indoors (our suggestion).
In 7 marathons, the leading men ran 1.7% slower than the course record at a WBGT of 5 to 10°C and 4.5% slower at 20 to 25°C, and "performance is more negatively affected for slower populations of runners". In 1,791,972 finishers of six big-city marathons, the best speeds came at 3.8 to 9.9°C, and as it warmed, speeds fell and more runners dropped out. In 1,258 elite races, each degree of WBGT outside the best range cost 0.3 to 0.4%. Our running pace calculator gives the same figures.
At the 2018 London Marathon (24.1°C, against a usual race-day high of 15.2°C), the mean finish was 20 minutes slower than in other years, and 364 surveyed runners finished 14% slower than they'd planned. At the Great North Run, higher temperatures, heat index and ozone slowed amateur runners.
No formula tells one runner how much to slow, which is why our pace calculator assumes a cool day. Start slower than usual, keep easy runs at a talking pace, add walk breaks, and drop your target on a hot race day, as our marathon plan says.
Our editorial sequence, built on the NHS heat advice, the acclimatisation consensus statements (Racinais 2015, 2023) and the drinking studies (Hew-Butler 2017; Goulet 2019).
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The movement on the figure: Treadmill jogging
The evidence comes from consensus statements and meta-analyses of studies mostly in young, fit men, many of them in labs.
A consensus statement advises repeated exercise in the heat "over 1-2 weeks", in sessions of at least 60 minutes a day: most of the drop in heart rate and body temperature, and the rise in sweating, comes in the first week, and it says the period should ideally pass 2 weeks to get all the benefits. The IOC's 2023 consensus gives 60 to 90 minutes a day for at least 2 weeks as the best, adds that shorter sessions still help, and suggests at least 4 sessions a week to acclimatise and 2 a week to keep it.
Across 211 papers (typically 8 sessions of about 90 minutes), heat acclimation lowered heart rate at the end of exercise by 17 beats a minute and core temperature by 0.43°C, raised blood plasma volume 5.6% and sweat rate by 163 millilitres an hour, and improved time trials in the heat by 3.1%. Across 96 studies, regimes shorter than 14 days helped, and the biggest gains came beyond 14 days.
Five or more days of heat sessions were enough to lower heart rate and body temperature. Without heat, the gains faded by about 2.5% a day, and coming back was 8 to 12 times faster than the loss: a month off was made up in about 4 days.
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Most of the drop in heart rate and body temperature comes in about 5 to 7 days of daily sessions; fuller acclimatisation, with sweating still improving, takes 10 to 14 days or more. Fewer or shorter sessions still give partial gains. Almost all of these studies were in young, fit men.
Across 29 studies of 561 people, mostly men, summer brought partial acclimatisation: sweat rate rose by up to about half a litre an hour and heart rate in a heat test fell by 3 to 8 beats a minute. Regular exercise in cooler weather gives partial acclimatisation too.
A review found evidence that older adults can acclimatise, over about 8 to 10 days.
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For a hot race or holiday, plan 1 to 2 weeks of easy runs in the warmth if you can, at least 4 a week. Start with about half your usual easy run at a slow pace, and build towards 60 minutes or more, adding time before pace. Keep it going with about 2 runs a week in the heat. The first hot spell of the year is when people who aren't used to the heat are most at risk, so ease into it. The steps are our editorial starting point, built on the consensus statements.
Drinking too much is the main cause of dangerously low blood sodium (exercise-associated hyponatraemia) and of every known death from it. Drinking to thirst prevents too much dehydration and greatly reduces that risk. Guidance for athletes aims to avoid losing more than about 2% of body weight in sweat, though that figure is debated: in a meta-analysis of time trials, losses of up to 4% didn't impair performance. If you're heavier after a run, you drank more than you sweated out.
At the 2002 Boston Marathon, 13% of 488 finishers had low blood sodium and 0.6% critically low; gaining weight during the race was the strongest link, and finishing in over 4 hours raised the odds too.
In 119 runners, runs of under an hour mostly needed no drink to avoid significant dehydration (31 of 35). On longer runs, needs ranged from nothing to 1.4 litres an hour, which is why no fixed schedule suits everyone.
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In 7 studies at about 28°C over about an hour and a half, drinking to thirst (about 0.5 litres an hour) against a plan (about 1.1 litres an hour) made a difference to performance the authors call "likely trivial", though people lost more weight drinking to thirst (2.1% against 1.0%).
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The ACSM's position stand aims to prevent losing more than 2% of body weight in sweat and suggests weighing before and after exercise to learn your sweat rate. But across 15 studies, losses of up to 4% didn't slow time trials, only fixed-pace tests.
Sweat contains salt, but we found no trial showing that recreational runners need electrolyte drinks on runs of an hour or two. On long, hot runs a sports drink is one way to take in carbohydrate and some salt (our suggestion). For someone with heat exhaustion, the NHS suggests an isotonic sports drink or rehydration powder as well as water.
Confusion, a headache, being sick or unusual tiredness after drinking a lot on a long run can be low blood sodium: get medical help straight away.
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Light, loose, light-coloured kit, a cap, and sunscreen of at least SPF 30. The IOC consensus says clothing "should impair sweat evaporation as little as possible".
The NHS advises staying in the shade between 11am and 3pm and avoiding exercise that makes you hotter in a heatwave; UKHSA advises planning activity for the early morning or evening and limiting strenuous activity in the hottest part of the day. On alert days, run early, shorter and slower, or move indoors.
Event guidance gives cut-offs for cancelling races, but heatstroke at one marathon rose steadily, by 1.2 cases per 10,000 runners for each degree of WBGT, and marathon results fall steadily as it warms. Go by the forecast, how you feel and who you are, not one number.
Ozone builds up as it gets hotter, and it slowed half marathon runners. In most situations the benefits of being active outweigh the risks of air pollution, though perhaps less so at high pollution in vulnerable groups, and reviews suggest cutting the dose rather than stopping. UK advice for adults and children with heart or lung conditions, and older adults, is to adapt hard outdoor efforts when pollution is high, especially if symptoms get worse.
The ACSM says exertional heatstroke can affect seemingly healthy athletes even when it's relatively cool. At the Boston Marathon (136,161 starters over 5 years), heatstroke struck 3.7 per 10,000 starters, all at moderate temperatures, and younger, faster runners were at higher risk. At the Vermont City Marathon it was 12 per 10,000, and at an 11.3 km summer road race about 2 per 1,000 finishers.
At races, medical teams put runners with heatstroke into cold water straight away: immersion within 30 minutes of collapse went with no deaths in more than 2,000 cases, and every one of 180 cases at one road race survived. Across 63 studies, immersion in water of 1 to 17°C cooled people faster than resting to cool down. That's for trained teams on site; if you're not at a race, call 999 and follow the NHS steps below.
At one road race, people who'd had heatstroke were about 3 times as likely to have it again in the next 2 years.
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The NHS lists "older people, especially those aged 65 and over", people who are pregnant or have serious long-term conditions such as heart or breathing problems, diabetes or kidney disease, people on some medicines, and people who are physically active outdoors, "such as runners, cyclists and walkers". A review found people over 60 and people with heart or lung disease or long-standing diabetes lose heat less well, and older adults feel less thirsty.
In 6,700 Australian veterans, starting some medicines raised the risk of hospital admission for dehydration or heat illness, from 1.17 times for one group of antidepressants to 2.79 times for a blood pressure tablet combined with a water tablet (diuretic). Water tablets, some blood pressure tablets, antidepressants and some others are named in reviews. UKHSA says to keep taking your prescribed medicines unless a medical professional advises otherwise: ask your GP or pharmacist how to exercise in the heat.
Across 12 studies of 347 pregnant women, including up to 35 minutes of hard exercise at 25°C, no woman's core temperature passed 39°C, and 45 minutes of moderate exercise at 32°C kept it under 38°C. These are small studies. Our pregnancy guide's advice is in the warning signs below.
Recent illness, especially diarrhoea, is a risk factor for heat illness, and so is having had heatstroke before. Don't run hard in the heat when you've just been ill (our suggestion).
Across 22 studies, children controlled their body temperature much like adults, though they sweat less. UKHSA still advises that children shouldn't do strenuous activity on very hot days.
Easy runs with walk breaks, as our easy run and run-walk sessions set them, by effort in the heat. On the hottest days, train indoors in the cool at a pace you can talk at: our easy bike ride runs for 30 to 75 minutes, and the rowing and elliptical move pages each give a starting dose. There's no swimming demo, but swimming counts too.
3D demoTreadmill walkingBeginner · Cardio machine
3D demoTreadmill joggingIntermediate · Cardio machine
3D demoTreadmill runningIntermediate · Cardio machine
3D demoStationary cyclingBeginner · Cardio machine
3D demoIndoor rowing (rowing machine)Beginner · Cardio machine
3D demoElliptical trainerBeginner · Cardio machineThe easy run already says to run early, go slower and take water on a hot day, and the long run's tips cover drinking without overdoing it. Use the run-walk sessions to slow down, and the bike or cardio sessions indoors on the hottest days.
A 40-minute easy run at a pace where you can talk in full sentences, with a brisk-walk warm-up and an easy cool-down: the base of every running plan.
40 minBeginner6 movesCardio machine, A wallA 30-minute beginner run-walk session: a brisk walk, then 1-minute jogs with 1-minute walks, building to 3-minute runs over your first three weeks.
30 minBeginner5 movesCardio machine, A wallA 35-minute run-walk session for weeks 4 to 8 of learning to run: 5-minute jogs with 2-minute walks, building to a 20-minute run.
35 minBeginner5 movesCardio machine, A wallYour weekly long run: 50 minutes at an easy, conversational pace with an optional 1-minute walk every 9 minutes, and how to fuel and drink on longer runs.
60 minIntermediate6 movesCardio machine, A wallA steady bike ride for a first triathlon or fitness: 5 to 10 minutes easy, a ride you can talk through, optional short faster bursts and an easy cool-down.
45 minBeginner4 movesCardio machine, A wallA steady 30 to 45 minute cardio session at talk-but-not-sing pace: walk, cycle, row or cross-train, counting toward weekly activity guidelines.
40 minBeginner3 movesCardio machine, A wallCouch to 5K's check-first note says to run early, slow down or walk more in hot weather, and the marathon plan has a heat note for race day.
A 9-week beginner plan with the same walk-to-run shape as Couch to 5K: three run-walk sessions a week, from 1-minute jogs to 30 minutes of running.
3 training days35 minBeginner4 days a weekAn 8-week beginner 10K plan: three runs a week plus strength, a long run that grows about 10% at a time, optional hills and tempo runs, and a light race week.
4 training days45 minBeginner4 days a weekA 12-week beginner half marathon plan: three or four runs a week, a long run building to about 15 km, one harder session, strength, a taper and race-day tips.
4 training days50 minBeginner4 days a weekAn 18-week first marathon plan from half marathon fitness: three runs a week, a long run built about 10% at a time, strength, a 3-week taper and race-day fuel.
4 training days60 minBeginnerThe NHS lists the signs of heat exhaustion: tiredness, dizziness, headache, feeling or being sick, heavy sweating with pale, clammy skin, cramps in the arms, legs and stomach, a high temperature, being very thirsty and feeling irritable. Move to a cool place, take off unnecessary clothing, drink water or a sports drink and cool your skin; you should start to feel better within 30 minutes.
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Call 999 (911 in the US) for signs of heatstroke: still unwell after 30 minutes of cooling, a very high temperature, hot skin that isn't sweating, a fast heartbeat, fast breathing or shortness of breath, confusion and lack of coordination, a seizure or fit, or loss of consciousness. Don't drive yourself to A&E.
Get help from NHS 111 if you're struggling to treat heat exhaustion, or need advice.
In the heat, tiredness, dizziness, a headache, feeling sick, heavy sweating and intense thirst can be heat exhaustion: stop, get somewhere cool, drink and cool your skin, and you should feel better within 30 minutes. If you don't, or there's confusion, hot skin that has stopped sweating, a fit or a collapse, call 999 or 911: that's heatstroke.
The NHS says to wrap the person in a cool, wet sheet, or fan them or sponge them down with cold water while you wait for help, and to put them in the recovery position if they lose consciousness (the NHS's advice, in our words).
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On a hot day, run early, go slower and take water. Dizzy, sick or very hot? Stop, get somewhere cool and drink.
Hot weather: run early, slow down or walk more.
Confusion, a headache, being sick or unusual tiredness after drinking a lot on a long run can be low blood sodium: get medical help straight away.
Drink water, wear loose clothing, and avoid hard exercise in hot, humid weather or the midday sun, particularly in the first trimester. Skip hot yoga and hot Pilates, and don't exercise on an empty stomach.
Moderate exercise doesn't raise your core temperature into the range of concern. Still skip hot classes, very hot and humid weather, and long hard sessions in the heat.
About 6 in 10 people with MS find heat makes their symptoms worse for a few hours, and cooling before and during exercise helps. Don't work through fatigue: stop with energy to spare.
On water tablets (diuretics), blood pressure tablets, antidepressants or other regular medicines, ask your GP or pharmacist how to exercise in the heat, and never stop a medicine to run. UKHSA says to keep taking prescribed medicines unless a medical professional advises otherwise.
If you get chest pain or pressure, feel faint or dizzy, are unusually breathless, or your heart races or beats irregularly during exercise, as our plans advise.
For chest pain that comes on suddenly and doesn't go away, feels tight or like squeezing, spreads to your arms, neck, jaw, stomach or back, or comes with sweating, feeling sick, light-headedness or breathlessness; severe difficulty breathing; someone who faints while exercising; palpitations that don't go away, or come with chest pain, shortness of breath or feeling faint; or a headache that starts suddenly and is extremely painful, especially while lifting or straining, or comes with numbness or weakness, difficulty speaking or loss of vision. The NHS says a bleed on the brain may be more likely when lifting something heavy. Don't drive yourself to A&E.
Call NHS 111 (in the US, your doctor or an urgent care centre) if you're suddenly more short of breath than usual, or have chest pain with feeling sick or a heartbeat that races, skips or flutters.
Call NHS 111 or ask for an urgent GP appointment (in the US, your doctor or urgent care) if you're suddenly more short of breath than usual, or your muscles are very sore and weak with dark, red or cola-coloured pee after an unusually hard run.
Lie down with your legs raised, or sit with your head lowered between your knees, and drink some water. Crossing your legs while standing, rocking up and down on your toes or clenching your fists can help. Call 999 if someone faints while exercising.
Ask for an urgent GP appointment or get help from NHS 111 for a headache that's triggered or made worse by coughing, sneezing, bending down or exercising. In the US, contact your doctor or urgent care.
Stiff, achy legs for a day or two are normal at first and ease as you repeat the runs. Stop and rest if a pain is sharp, makes you limp or change how you run, or builds as you go, and skip your next run if it's worse the next morning. The NHS advises not running when you're in pain, and seeing a GP or physiotherapist if it doesn't go away; a very painful or swollen knee, or a sudden sharp pain at the back of the ankle, needs a GP straight away.
Call 999 or go to A&E: loss of feeling or numbness around the genitals or bottom, new trouble peeing or controlling your bladder or bowels, changes in genital sensation during sex or problems with erections or orgasm, or pain, tingling, weakness or numbness in both legs. These are signs of cauda equina syndrome, which needs treating in hospital urgently. Don't drive yourself.
Pain, tingling or numbness spreading below the knee that isn't settling after a few weeks, is getting worse or stops your normal activities: see a GP.
Numbness, tingling or weakness that travels down the leg from the back. That points to a nerve (sciatica) rather than the knee.
A lump in the groin, or groin pain with coughing or straining (a possible hernia).
Get help from NHS 111 if you have a hernia (a lump in the groin or tummy) and pain in or around it, a bloated tummy, you feel sick or are being sick, you're vomiting blood or your vomit looks like coffee grounds, you're constipated, you have a high temperature or feel hot, cold or shivery, or you've suddenly become confused.
Call 999 or go to A&E now if you have a femoral hernia (a small lump at the top of the thigh, in the groin) and very bad tummy pain, you feel sick or are being sick, you cannot fart or poo, you have a high temperature or feel hot, cold or shivery, or you've suddenly become confused. It may be strangulated, which is an emergency. Don't drive yourself.
Speak to a GP first if you haven't exercised for some time, or if you have a medical condition or concerns. Get advice first, too, if you have a health problem, an injury or symptoms, have recently had a heart attack or an operation, or are pregnant or have recently given birth.
Answer the 7 questions of the Physical Activity Readiness Questionnaire (PAR-Q+) before you start. A yes to any (a heart condition or high blood pressure, chest pain, dizziness or fainting, another long-term condition or its medicines, a bone or joint problem, or being told to exercise only under medical supervision) means checking with your doctor or a qualified exercise professional first. It also asks people over 45 who aren't used to hard, all-out effort to check before exercising that hard: think races and time trials, not easy runs. The NHS suggests speaking to a GP first if you're worried about your health.
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If you have a heart condition, follow your cardiac team's advice on how hard to work, and stop for the warning signs above. People with long-term conditions may wish to ask their specialist before starting. With uncontrolled high blood pressure, avoid straining against a held breath; the high blood pressure guide has more.
Not these plans, or not yet: myalgic encephalomyelitis or chronic fatigue syndrome (ME/CFS), where the National Institute for Health and Care Excellence (NICE) advises against programmes with fixed step-by-step increases and the NHS against vigorous unsupervised exercise such as running; and pregnancy, unless you were already running, because the NHS advises against starting a new vigorous activity such as running.
Repeated exercise in the heat lowered heart rate by about 17 beats a minute and core temperature by about 0.4°C, and improved time trials in the heat by about 3%. Much of it comes within a week, and older adults can adapt too.
Drinking beyond thirst is the main cause of dangerously low blood sodium, weight gain during a marathon was the strongest warning sign, and drinking to thirst performed about as well as a schedule.
The ACSM aims to prevent losses over 2%, but losses up to 4% didn't slow time trials across 15 studies.
It happens at moderate temperatures in people running hard, as race records show.
Moderate exercise in warm conditions kept core temperature well under 39°C in small studies. Our pregnancy guide still advises avoiding hard exercise in hot, humid weather or the midday sun.
Sweating increases as you acclimatise, and sweat rates vary several-fold between people.
Runs under an hour rarely need any drink, and we found no trial showing recreational runners need electrolytes on runs of an hour or two.
It varies, so run by effort rather than a number. Marathon times slow steadily as it warms, slower runners most, and at the hot 2018 London Marathon runners finished about 14% slower than they'd planned. Keep easy runs at a pace where you can talk, and take walk breaks.
About 1 to 2 weeks of runs of an hour or so in the warmth, on most days, with much of the change in the first week. Five days already helps, and the fullest gains take about 2 weeks or more. It fades over a few weeks without heat but comes back quickly.
There's no single cut-off: the risk rises steadily with heat, humidity and sun. On hot days run early or late, go shorter and slower, or move indoors, and treat a UK heat-health alert as a cue to do that.
Drink to thirst, not to a schedule. On runs under about an hour you usually need nothing; on longer or hotter runs, carry water. If you're heavier after a long run than before, you drank more than you sweated out.
Yes. Drinking well beyond thirst can dilute the sodium in your blood, which can be dangerous: at one Boston Marathon, 13% of finishers had low sodium, and gaining weight during the race was the strongest link. Confusion, a headache or being sick after drinking a lot needs medical help straight away.
For most healthy runners who are used to the heat, an easy run early in the day is fine; the NHS advises avoiding exercise that makes you hotter in the hottest part of the day and keeping out of the sun from 11am to 3pm. Older adults, people with heart or lung conditions, and anyone on medicines affected by heat should take more care.
Tiredness, dizziness, a headache, feeling sick, heavy sweating with pale, clammy skin, cramps and intense thirst. Stop, get somewhere cool, drink and cool your skin; you should feel better within 30 minutes. If you don't, or there's confusion, hot skin that isn't sweating or a collapse, call 999.
Light-coloured, loose, thin kit that lets sweat evaporate, a cap and sunscreen of at least SPF 30. Avoid heavy layers and dark colours in full sun.
Not for most runs of an hour or so; water and normal meals are enough. On long, hot runs a sports drink is one way to take in carbohydrate and some salt.
If you already run, moderate exercise in warm weather kept body temperature well within safe limits in small studies. Avoid hard exercise in hot, humid weather or the midday sun, drink water, and see our pregnancy guide.
It helps you cope with heat, and acclimatisation improved time trials in the heat by about 3%. Whether it helps in cool weather is less clear, so don't chase pace in the heat.
Ozone builds up on hot, sunny days and slowed half marathon runners. For most people the benefits of running outweigh the risk; with a heart or lung condition, adapt hard efforts outdoors when pollution is high.
Exercise education, not a personal prescription, and clinical review of these guides is pending. Check with a GP before starting hard training if you have a heart condition, chest pain, dizziness or breathlessness when active, and stop for sharp or increasing pain. Health disclaimer.
