Running in the Heat — Hydration, Pacing and Safety
Heat changes running more than any other condition: the same pace that feels easy at 15°C feels hard at 30°C, the heart rate climbs, the sweat pours, and the risks — dehydration, heat exhaustion, heatstroke — are real. Yet heat is also the most manageable condition, because the adjustments are simple: slow down, drink to a plan, and respect the warning signs.
This guide covers running in the heat properly: how sweat and hydration actually work, how much to drink, how much to slow down, how to acclimatise, and the warning signs that mean stop now. It is written for anyone running through a hot summer, training for a warm-weather race, or simply facing an unexpectedly hot day.
Use the calculator
Start with your own numbers: try our water intake calculator, heart rate zone calculator to work out your figures before applying anything on this page. The worked examples below use round numbers so you can follow the maths; the calculators use your real weight, height, age and goal.
Related reading: Running Nutrition Guide — What to Eat Before, During and After, How to Breathe While Running — Techniques That Actually Help, Long Runs Explained — How Long, How Slow, How Often. You can also browse the full guides section for the rest of the series.
The key points
Heat raises heart rate and perceived effort at the same pace. At 30°C, the heart rate at a given pace is typically 10–20 beats per minute higher than at 15°C, because blood is diverted to the skin for cooling. The pace does not change; the cost of the pace does. The practical result is that a tempo run in the heat feels like a race, and a race in the heat needs a slower target — roughly 5–10 seconds per kilometre slower for every 5°C above 20°C, depending on humidity.
Sweat rates vary enormously — 0.5 to 2 litres per hour. Some runners sweat 0.5 litres an hour; others sweat 2 litres. The only way to know your rate is to weigh yourself before and after a run (1 kg lost is about 1 litre of sweat, minus any fluid drunk). A runner losing 1.5 litres an hour who drinks 0.5 litres an hour is dehydrating by a litre an hour — and a 2% loss of body weight (1.5 kg for a 75 kg runner) measurably impairs performance.
Drink to a plan, not just to thirst. Thirst lags behind loss, especially in the heat, so a plan is better: 400–800 ml per hour during running, adjusted to your sweat rate and the temperature, with electrolytes (sodium) for runs over 90 minutes or in heavy sweating. But do not overdrink either — drinking far more than you sweat risks hyponatraemia (low blood sodium), which is rare but dangerous. The target is to replace most of the sweat, not all of it, and to finish the run no more than 2% lighter than you started.
Acclimatisation takes 10–14 days. The body adapts to heat over 10–14 days of regular exposure: sweat starts earlier, sweat rate rises, heart rate falls, and the same pace feels easier. The adaptation requires actual heat exposure — 60–90 minutes a day in the heat, which can be easy running, walking, or even a sauna after training. A runner who arrives in a hot climate the day before a race has none of this adaptation, and should slow down accordingly.
Slow down before you are forced to. The correct response to heat is to reduce the pace target before the run starts, not after you are already struggling. A tempo run becomes an easy run, a long run gets 30–60 seconds per kilometre slower, and a race target is adjusted down. The fitness is not lost — it is banked, and it returns when the temperature falls. Runners who refuse to slow down in the heat are the ones who end up walking, or worse.
Know the warning signs of heat illness. Heat exhaustion — heavy sweating, nausea, dizziness, headache, weakness — means stop, cool down and rehydrate. Heatstroke — confusion, slurred speech, hot dry skin, a body temperature above 40°C, collapse — is a medical emergency: call for help immediately and cool the person by any means available. The difference between the two is the brain: confusion means heatstroke, and heatstroke kills. Never run through confusion.
At a glance
| Temperature | Pace adjustment | Fluid per hour | Extra advice |
|---|---|---|---|
| Below 15°C | None | 300–500 ml | Normal training |
| 15–20°C | None to 5 s/km slower | 400–600 ml | Ideal racing weather |
| 20–25°C | 5–10 s/km slower | 500–700 ml | Light clothing, sunscreen |
| 25–30°C | 10–20 s/km slower | 600–800 ml | Run early or late; electrolytes |
| Above 30°C | 20–30+ s/km slower, or shorten | 700–1,000 ml | Consider moving indoors |
| Above 35°C | Do not run hard; easy only | To thirst + plan | High risk — take it seriously |
Humidity matters as much as temperature: high humidity prevents sweat from evaporating, which is how the body cools, so a humid 28°C can be harder than a dry 32°C.
The formula behind it
The sweat-rate calculation is a weigh-in: sweat rate = (weight before − weight after + fluid drunk) ÷ hours. A 75 kg runner who weighs 73.8 kg after a 2-hour run, having drunk 1 litre, lost 75 − 73.8 + 1 = 2.2 litres over 2 hours — a sweat rate of 1.1 litres an hour. That runner should plan to drink about 0.8–1.1 litres an hour in similar conditions, with electrolytes for runs over 90 minutes.
The dehydration maths is the 2% rule: performance falls measurably once fluid loss exceeds 2% of body weight. For a 75 kg runner that is 1.5 litres; for a 60 kg runner it is 1.2 litres. The calorie maths is the Compendium (Ainsworth et al., 2011): running at 6 mph (9.8 MET) burns 9.8 × weight (kg) per hour, so an hour in the heat at 75 kg burns about 735 kcal — and the heat adds to the cost, because the heart works harder to cool the body. The water-intake formula used by the water calculator (35 ml per kg of body weight a day as a baseline, plus sweat losses) gives a 75 kg runner a baseline of about 2.6 litres a day before running is counted.
Your plan, step by step
- Check the temperature and humidity before the run, and adjust the pace target using the table — before you start.
- Weigh yourself before and after a hot run once, to learn your sweat rate, and plan fluids from it.
- Drink 400–800 ml per hour during the run, with electrolytes for runs over 90 minutes; do not overdrink.
- Run early in the morning or late in the evening, wear light clothing and sunscreen, and carry water on runs over an hour.
- Acclimatise over 10–14 days of regular heat exposure before a hot-weather race.
- Stop immediately for confusion, slurred speech, nausea with dizziness, or a failure to sweat — and get help.
How to use this guide
Use the heat as a training variable, not an enemy. Hot-weather running, done sensibly, builds heat adaptation that pays off in any warm race — and the pace adjustment is temporary, not a loss of fitness. The runners who handle heat best are the ones who respect it most: they slow down early, drink to a plan, and never let pride override the warning signs.
Plan hot runs like you plan long runs: route with water, shade and a way home; tell someone the route; carry a phone; and have a bail-out option. A hot run that ends with a walk home is a good outcome; a hot run that ends with an ambulance is not. The difference between the two is usually a decision made in the first kilometre, not the last.
What this guide does NOT tell you
This guide is general information, not medical advice, and heat illness is a medical matter: the warning signs described here are for recognition, not diagnosis, and anyone who has had heatstroke, or who takes medicines that affect sweating, blood pressure or temperature regulation, should talk to a clinician before running in the heat. Children, older adults and people with heart or kidney conditions are at higher risk, and the pace and fluid figures here are population averages.
Sweat rates, fluid needs and heat tolerance vary enormously between individuals, and the table is a starting point to adjust from your own measurements, not a prescription. This guide cannot judge whether it is safe for you to run on a given day — if the temperature, humidity or your own symptoms say no, the correct decision is not to run, or to run indoors.
Frequently asked questions
How much water should I drink when running in the heat?
About 400–800 ml per hour, adjusted to your sweat rate — which you can measure by weighing yourself before and after a run (1 kg lost is about 1 litre of sweat). For runs over 90 minutes, or when sweating heavily, add electrolytes (sodium) to replace what sweat removes. Do not overdrink: finishing a run more than 2% lighter than you started is the target, and drinking far more than you sweat risks hyponatraemia, which is rare but dangerous.
How much slower should I run in the heat?
Roughly 5–10 seconds per kilometre slower for every 5°C above 20°C, depending on humidity — so a 5:00 per km runner might run 5:10–5:20 per km at 25°C and 5:20–5:30 at 30°C. Humidity matters as much as temperature: a humid 28°C can feel harder than a dry 32°C, because sweat cannot evaporate. Adjust the target before the run starts, not after you are already struggling.
What are the signs of heatstroke while running?
Confusion, slurred speech, disorientation, hot dry skin (sweating may have stopped), a body temperature above 40°C, or collapse. Heatstroke is a medical emergency: call for help immediately and cool the person by any means available — shade, water, ice, fanning. The difference from heat exhaustion (heavy sweating, nausea, dizziness, headache) is the brain: confusion means heatstroke, and heatstroke can kill. Never run through confusion.
How long does it take to acclimatise to heat?
About 10–14 days of regular heat exposure — 60–90 minutes a day in the heat, which can be easy running, walking, or even a sauna after training. The adaptations are real: sweat starts earlier, sweat rate rises, heart rate falls, and the same pace feels easier. A runner who arrives in a hot climate the day before a race has none of this adaptation and should slow down substantially — or, better, arrive a week early.
Should I run in a heatwave?
Easy running early in the morning or late in the evening is usually fine, with a slower pace and a hydration plan — but hard sessions, long runs and racing in extreme heat (above 35°C, or high humidity) carry real risk, and the sensible choice is often to move indoors, shorten the run, or skip it. No single run is worth heatstroke. If the official guidance says to avoid exertion, follow it — the fitness will still be there when the heat breaks.
Estimates only, not medical advice. Minimum safe intakes: 1,200 kcal/day women, 1,500 kcal/day men — consult a healthcare professional before a very-low-calorie diet.