Quick summary
- Slow down: in heat, run by effort or heart rate, not pace. Easy runs may be 20–60 seconds per km slower than in cool weather.
- Acclimatise: 10–14 days of regular training in the heat makes it noticeably easier.
- Time it: run early in the morning or late in the evening, and check humidity, not just temperature.
- Fluids and sodium: drink to thirst, add electrolytes on runs over 60 minutes, and weigh yourself to learn your sweat rate.
- Know the danger signs: confusion, stumbling, chills or stopping sweating mean stop, cool down and get help.
Heat changes everything about running. The same pace feels harder, your heart rate climbs, and long runs that felt fine in winter can leave you drained. For runners in places like the Gulf, the southern US or Australia, hot-weather training isn't a summer phase: it's most of the year.
You can train well in the heat. You just have to train differently.
Why heat makes running harder
Running produces a lot of heat. To get rid of it, your body sends more blood to the skin and sweats. That leaves less blood for your working muscles, so your heart beats faster to keep up. Sweat losses also reduce blood volume over a long run.
The result: at the same pace, your effort and heart rate are higher in heat. Humidity makes it worse, because sweat can't evaporate as easily, so it doesn't cool you as well.
How much to slow down
There's no exact formula, because heat affects runners differently and humidity matters as much as temperature. A practical guide:
| Conditions | Effect on most runners | What to do |
|---|---|---|
| Under 15°C, low humidity | Ideal | Run your normal paces |
| 15–20°C | Slight slowdown | Expect easy pace to be a few seconds per km slower |
| 20–25°C, or humid | Noticeable | Run easy days by effort; slow workout paces by about 10–20 s per km |
| 25–30°C, or very humid | Large | Shorten or split workouts; easy runs may be 30–60 s per km slower |
| Above 30°C, or high heat index | High risk | Move runs indoors or to the coolest time of day; cut duration |
The key is to keep effort right, not pace. Use the talk test, perceived effort, or heart rate. If you're following a pace-based plan like our sub-4 hour marathon training plan, treat the paces as cool-weather targets.
Heat acclimatisation
Your body adapts to heat surprisingly fast. With regular training in warm conditions, most runners see clear improvement over about 10–14 days:
- You start sweating earlier and sweat more.
- Your sweat contains less salt.
- Your blood plasma volume increases.
- Your heart rate and core temperature at a given pace drop.
How to acclimatise safely:
- Start with short, easy runs in the heat: 20–40 minutes.
- Increase duration gradually over 10–14 days before adding intensity.
- Keep at least some runs in the heat each week to maintain it; adaptations fade over a few weeks without exposure.
- If you're travelling to a hot race from a cool climate, arrive early if possible, or use controlled extra layers or warm indoor sessions in the weeks before, only after checking with a coach or doctor.
Hydration and sodium in the heat
- Know your sweat rate. Weigh yourself before and after a 60-minute run without drinking. Each kilogram lost is about one litre of sweat. In heat, many runners lose 1–2 litres per hour.
- Drink to thirst, and plan ahead. On long runs, carry fluid or plan loops past water. Don't aim to replace every drop, but avoid losing more than about 2–3% of body weight.
- Don't overdrink. Drinking much more than you sweat can cause dangerously low blood sodium. If you're heavier after a run than before, you drank too much.
- Add sodium. On runs over about 60 minutes in heat, use electrolyte drinks, tablets or capsules. Roughly 300–600 mg of sodium per hour is a common starting range; heavy, salty sweaters may need more.
- Pre-hydrate: drink normally through the day and have 400–600 ml of fluid in the couple of hours before a long run.
Our marathon fueling plan covers carbohydrate and fluid for race day.
What to wear in the heat
- Light, loose or fitted moisture-wicking fabric. Singlets and split shorts let air move and sweat evaporate.
- Light colours reflect sunlight; dark colours absorb it.
- A cap or visor, ideally one you can soak with water.
- Sunglasses and sweat-resistant sunscreen.
- Anti-chafe balm: sweat and salt make chafing much worse in heat.
Our race-day kit checklist covers kit for every temperature.
Practical hot-weather training tips
- Run at the coolest time of day, usually before sunrise.
- Choose shaded routes or loops with water access.
- Move hard sessions indoors to a treadmill on extreme days.
- Cool down actively: pour water over your head and neck, use ice in your cap, and get into shade after the run.
- Split long runs into two loops with a stop for fluids in between.
- Be flexible: swap a workout for an easy run if conditions are dangerous. One missed session won't change your marathon.
Racing a marathon in the heat
If race day is hot, adjust your goal before the start, not at 30 km. Start 10–20 seconds per km slower than your planned pace, take water at every aid station, pour water over yourself, and take extra sodium. A slower, controlled race in the heat often finishes faster than an aggressive start.
Warning signs of heat illness
Heat illness can become a medical emergency. Stop running, get into shade, cool down and get help if you or another runner shows:
- Confusion, slurred speech, stumbling or unusual behaviour
- Dizziness, fainting or a severe headache
- Nausea or vomiting
- Chills or goosebumps in the heat
- Hot, dry skin, or stopping sweating
- A racing heart that doesn't settle with rest
Confusion or collapse in the heat can mean exertional heat stroke. Call emergency services and start rapid cooling, such as cold water immersion or continuous cold water and ice, as soon as possible.
FAQ
How much slower should I run in hot weather?
It depends on temperature, humidity and how acclimatised you are. In warm, humid conditions, easy runs may be 20–60 seconds per km slower than in cool weather. Run by effort or heart rate rather than pace.
How long does it take to adapt to running in the heat?
Most runners see significant improvement after about 10–14 days of regular training in the heat, with sessions gradually increasing in length.
What is too hot to run?
There's no single cut-off. Risk rises sharply with high temperature and humidity together. When the heat index is very high, many runners move sessions indoors, run at dawn or cut the session short. Follow local heat warnings.
Should I take salt tablets when running in the heat?
Many runners benefit from extra sodium on runs longer than about an hour in the heat, especially heavy or salty sweaters. Electrolyte drinks, tablets or capsules all work. Practise in training first.
Can I train for a marathon in summer?
Yes, with adjustments: run early, slow easy paces, acclimatise gradually, hydrate with sodium, and move hard sessions to cooler times or indoors. Many runners train through summer for autumn marathons.
This article is general information, not medical advice. If you have a heart condition, are pregnant, take medication that affects heat tolerance, or feel unwell in the heat, speak to a doctor before training in hot conditions.