How much does heat slow you down in a marathon?
Expect to be slower when it is warm. In elite marathoners, performance fell 0.3–0.4% for every °C of heat stress outside a 7.5–15 °C sweet spot. Whether slower runners lose more is still debated. If your race will be hotter than your training, 1–2 weeks of heat acclimation is the standard preparation.
What to do
- Check the race-day WBGT. Above 15 °C, slow your goal pace by at least 0.3–0.4% per °C, and more if you are a slower runner.
- If the WBGT forecast is around 21–25 °C or higher, switch your goal to finishing safely.
- If the race will be hotter than your training, do 5–14 easy sessions of 45–60 minutes in the heat over 7–14 days in the final 2–3 weeks.
- Drink to thirst, even in heat. Base any plan on a sweat rate measured in warm weather, and never drink more than you sweat.
- Keep your carbohydrate target, and favour glucose–fructose products. Cold drinks and cooling before the start may help.
- If you feel unwell in the heat, stop, cool down and get medical help.
Measuring heat: WBGT
WBGT (wet-bulb globe temperature) is a heat-stress measure that combines air temperature with humidity, sun and wind into one number.
How much heat costs
The clearest numbers come from elite marathoners. Their performance was best when WBGT was between 7.5 and 15 °C. Outside that range, performance fell 0.3–0.4% for each °C (Mantzios and colleagues, 2022).
These are elite runners. Whether slower or faster runners lose more in the heat is contested (NYC analysis, 2020).
Adjusting your pace
The research report behind this site turns the elite data into a pace rule, graded C because it extrapolates from elites:
- WBGT above 15 °C: slow down 0.3–0.4% per °C if you are a faster runner.
- Slower runners: multiply that by 1.5–2. This part is the report’s judgement, not a measured result.
- WBGT of about 21–25 °C or more: change the goal from a time to finishing safely.
An example for a runner aiming at 4:00 on a day with a WBGT of 20 °C, which is 5 °C above the sweet spot:
| Assumption | Slow-down per °C | Total for 5 °C | Time added to 4:00 |
|---|---|---|---|
| Elite rate | 0.3–0.4% | 1.5–2% | about 4–5 minutes |
| Slower-runner rate (× 1.5–2) | 0.45–0.8% | 2.25–4% | about 5–10 minutes |
That gives a revised goal of roughly 4:04–4:10. Our suggestion: start at that pace rather than trying to bank time early. Going out too fast is the main cause of hitting the wall, and a race-time prediction made in cool weather will be too fast on a hot day.
Heat acclimation
Heat acclimation means training in the heat so your body adapts before race day. Acclimation over 1–2 weeks is the consensus preparation for a hot race (Racinais and colleagues, 2015).
The report’s practical version, graded C and based on that consensus: if the race will be hotter than your training, do 5–14 sessions of 45–60 minutes of easy exercise in the heat over 7–14 days, in the final 2–3 weeks before the race. Our suggestion: keep these sessions genuinely easy, since they overlap with your taper.
Heat training is not altitude training
Some runners use heat suits as a “poor man’s altitude camp”. In elite skiers, five weeks of heat-suit training raised haemoglobin mass, the oxygen-carrying protein in the blood, by 2.8%. But VO2max, running economy and performance did not change (Rønnestad and colleagues, 2022).
Use heat training to prepare for a hot race, not as a general performance booster.
Fuelling and drinking in the heat
The report’s heat rules for fuelling and drinking are graded C, based on consensus guidance (McCubbin, 2020; Yu, 2024; Young, 2024):
- Fluids: base any drinking plan on a sweat rate measured in warm weather, and still never plan to drink more than you sweat.
- Sodium: 500–800 mg per hour.
- Cooling: cold drinks and cooling before the start.
- Carbohydrate: keep your usual target (see carbs per hour), favour glucose–fructose products, and drop by 10–15 g per hour if you have a history of stomach problems.
Don’t overdrink. Heat is one of the risk factors for low blood sodium (hyponatraemia), which comes from drinking too much. At the Boston Marathon, finishing in more than 4 hours and gaining weight during the race were the key risk factors (Almond and colleagues, 2005). The consensus advice is to drink to thirst (Hew-Butler and colleagues, 2015). Sodium supplements do not prevent hyponatraemia if you overdrink (Hoffman and colleagues, 2015). Read how much to drink in a marathon before a hot race.
Sweat losses vary a lot. Endurance athletes lose about 1.28 L of sweat an hour on average, with wide differences between people (Barnes and colleagues, 2019). Measure your own sweat rate in both cool and warm conditions if you can.
Don’t trust a skin sensor to keep you safe
Skin-based core-temperature sensors lack validated decision value (CORE validation). Go by how you feel. See how accurate your running watch is.
Red flag
If you feel unwell while running in the heat, stop, cool down and get medical help. No finish time is worth pushing on.
What we don’t know
- The best data are from elite runners. How much heat slows recreational runners, and whether slower runners lose more, is unresolved.
- The slower-runner multiplier and the “finish safely” threshold are judgement calls, not tested cut-offs.
- The acclimation dose is a consensus range, not a precise recipe, and no study here tested it against marathon times in recreational runners.
- The fuelling and drinking numbers for heat come from guidance, not marathon trials.
Sources
- Mantzios et al. (2022). Heat stress and elite marathon performance.
- NYC analysis (2020). Heat and finish times across runner levels in the New York City Marathon.
- Racinais et al. (2015). Consensus on heat acclimation for training and competing in the heat.
- Rønnestad et al. (2022). Heat-suit training, haemoglobin mass and performance in elite skiers.
- McCubbin (2020). Guidance on fuelling and drinking in the heat.
- Yu (2024). Guidance on fuelling and drinking in the heat.
- Young (2024). Guidance on fuelling and drinking in the heat.
- Almond et al. (2005). Hyponatraemia among Boston Marathon finishers.
- Hew-Butler et al. (2015). International consensus on exercise-associated hyponatraemia.
- Hoffman et al. (2015). Sodium supplementation and hyponatraemia in endurance athletes.
- Barnes et al. (2019). Sweat and sodium losses in athletes.
- CORE validation. Validation of a skin-based core-temperature sensor.
Last reviewed 13 November 2026.