What is the best running temperature?

In big-city marathon data, finish times were fastest in cool air, about 3.8–9.9 °C depending on the runner's level, and slower as it warmed. Getting used to the heat is the step a heat consensus statement calls most important.

By RunFast · Updated 16 October 2026 · 7 min read

The research closest to a best running temperature comes from marathon races, and it points to cool air rather than warm.¹ In 1,791,972 finishes across six big-city marathons from 2001 to 2010, finish times were fastest at air temperatures from 3.8 °C to 9.9 °C, depending on the runners’ level of performance.¹ Above those temperatures running speed fell and more runners withdrew, and large drops in temperature below them were also linked with slower performances.¹

So the ideal running temperature isn’t one number, and it isn’t a rule. These figures come from observational data on marathon race days, and they describe an association between weather and finish times.¹ They say nothing about when to train or what is safe. What the stronger evidence adds is practical: heat makes endurance exercise harder, and getting used to it over 1–2 weeks is the step a heat consensus statement calls most important.²

What the marathon data show

The largest analysis, of 1,791,972 finishes in six big-city marathons over 60 races, found that air temperature and performance were related by a curve: speed was highest around an optimum and fell on both sides of it.¹ The optimal running temperature for maximal speed, for men and women, varied from 3.8 °C to 9.9 °C according to each level of performance.¹ Above the optimum, running speed decreased and withdrawal rates increased.¹

A second study looked at finishers of seven US and Canadian marathons, each over 6 to 36 years.³ It measured heat as wet-bulb globe temperature (WBGT), and found that marathon performance slowed progressively as WBGT rose from 5 °C to 25 °C.³ For the top three men in each race, the slowdown compared with the course record grew with each band of WBGT.³

These are elite men, and for the top women the differences were not statistically significant.³ The same study also looked at runners placed 25th to 300th, and found that slower runners’ performance was more negatively affected as it got warmer.³

A 2026 systematic review of 76 publications on the New York City Marathon points the same way: performance declined with increasing temperature, especially among slower runners, men aged 30–64 and women aged 40–64.⁴ It summarises the studies in words rather than pooling them into one figure.⁴

Why heat slows you down

The 2015 consensus recommendations on training and competing in the heat, from 17 authors, say that exercising in the heat causes thermoregulatory and other physiological strain that can impair endurance exercise capacity.²

What helps: getting used to the heat

Heat acclimatisation

The same consensus calls heat acclimatisation the most important thing you can do to reduce that strain and run as well as you can in the heat.² It says acclimatisation should consist of repeated exercise-heat exposures over 1–2 weeks.²

A 2019 meta-analysis of 35 studies found that heat acclimation improved time-trial performance, with an effect size of 0.49.⁵ Time to exhaustion improved most, with an effect size of 0.86.⁵ Most of the studies tested men only, though some reported data on women.⁵ The results are effect sizes, not minutes saved in a race.⁵

Summer does some of it

A 2022 systematic review of 29 studies with 561 participants found that seasonal heat acclimatisation occurs in healthy adults across various climates.⁶ The studies were mostly in healthy men.⁶

The first hot days

The 2015 consensus asks that events be scheduled to minimise athletes’ health risks, especially in mass-participation events and during the first hot days of the year.² It is written for organisers, but the point carries to your own running: the first warm spell of the year is when you have had the least time to get used to it.

A 2023 International Olympic Committee (IOC) consensus statement, written for event organisers and medical teams, says exertional heat stroke is one of the leading life-threatening conditions for athletes, and that it is preventable with proper risk mitigation.⁷ This guide doesn’t cover how to recognise or treat heat illness. For medical advice, ask a doctor, or on race day the event’s medical team.

How to apply it

• Expect to be slower when it is warm. The marathon studies above link warmer race days with slower times.^{1, 3, 4} Plan for that rather than chase your usual pace.
• Run by effort, not pace, on warm days. Easy runs should still feel easy; see how easy runs fit an 80/20 plan. If you’d rather run indoors, the treadmill pace chart converts speed to pace.
• Rethink a goal time set for cool conditions. If race day is forecast warm, a goal and splits set for cool weather may need to change; the marathon pace chart and the pace calculator help you work out new splits.
• Give the heat 1–2 weeks. If your race is somewhere hotter than where you train, plan repeated runs in the heat over the weeks before it.
• Treat the first hot days of the year with care. Ease off rather than chase your usual paces.

Where the evidence is thin or mixed

The 3.8–9.9 °C range comes from observational data on marathon race days.¹ It shows how weather and finish times went together in those races, not the best temperature for running in training, in a shorter race or for your health.

On the cold side, the only finding in these sources is that large drops in temperature below the optimum were also linked with slower marathon performances.¹ This guide doesn’t give a temperature that is too cold or too hot to run in.

Humidity is unclear. One analysis of age-group runners in the New York City Marathon found higher humidity associated with faster running speeds.⁸ That is one race and one observational analysis, so it isn’t enough to build advice on.

The acclimatisation evidence is stronger, but its populations are narrower than “all runners”: most studies in the meta-analysis tested men only, though some reported data on women, and the review of seasonal acclimatisation was mostly in healthy men.^{5, 6}

Common questions

How much does heat slow a marathon?

It depends on the runner. For the top three men in seven North American marathons, times were 1.7 % slower than the course record in the coolest WBGT band (5.1–10 °C) and 4.5 % slower in the warmest (20.1–25 °C).³ Slower runners were more negatively affected than the fastest ones in that study, and in a systematic review of New York City Marathon research.^{3, 4} No source here gives a reliable slowdown per degree for recreational runners, so this guide doesn’t offer one.

Can you get used to running in the heat?

Yes, according to the 2015 heat consensus, which recommends repeated exercise-heat exposures over 1–2 weeks.² A meta-analysis of 35 studies found heat acclimation improved time-trial performance and time to exhaustion, though most of its studies tested men only.⁵ A systematic review found that summer itself brings some acclimatisation in healthy adults, mostly men in the studies it covered.⁶

Does humidity matter?

The evidence is thin. One study measured heat as WBGT, and found marathon times slowed as it rose.³ Another, of 830,255 New York City Marathon finishes from 1999 to 2019, found higher humidity associated with faster running speeds for both sexes, while running speed fell with rising temperature in age groups 20–59.⁸ Both are observational, so neither tells you how humidity on its own affects your run.

This guide is general information, not medical advice. If you have an injury, a health condition or are pregnant, ask a doctor or physiotherapist before changing how you train or eat.

Related guides

• Marathon pace chart: pace and splits for your goal timeFind your goal time for the even pace and splits to run it, per kilometre and per mile, then see what the research says about holding that pace over 42.195 km.
• 80/20 running: what the research says80/20 running means keeping most training easy. A 2024 meta-analysis found a small VO2peak advantage for polarised training, but small studies in recreational runners found no significant difference in performance.
• Treadmill pace chart: speed to pace per mile and per kmFind the pace for any treadmill speed in mph or km/h, and the speed to dial in for a target pace, then see what research says about the 1% incline.

Sources

1. El Helou N, Tafflet M, Berthelot G, Tolaini J, Marc A, Guillaume M, Hausswirth C, Toussaint JF. Impact of environmental parameters on marathon running performance. PLoS ONE. 2012. doi:10.1371/journal.pone.0037407
2. Racinais S, Alonso JM, Coutts AJ, Flouris AD, Girard O, González-Alonso J, Hausswirth C, Jay O, Lee JK, Mitchell N, Nassis GP, Nybo L, Pluim BM, Roelands B, Sawka MN, Wingo J, Périard JD. Consensus recommendations on training and competing in the heat. British Journal of Sports Medicine. 2015. doi:10.1136/bjsports-2015-094915
3. Ely MR, Cheuvront SN, Roberts WO, Montain SJ. Impact of weather on marathon-running performance. Medicine and Science in Sports and Exercise. 2007. doi:10.1249/mss.0b013e31802d3aba
4. Knechtle B, Thuany M, Nikolaidis PT, Forte P, Leite LB, Lepers R, Chlíbková D, Weiss K, Rosemann T, Vancini RL, Duric S. The New York City Marathon: a systematic review of performance, participation, pacing, and health-related outcomes. EXCLI Journal. 2026. doi:10.17179/excli2026-9354
5. Benjamin CL, Sekiguchi Y, Fry LA, Casa DJ. Performance changes following heat acclimation and the factors that influence these changes: meta-analysis and meta-regression. Frontiers in Physiology. 2019. doi:10.3389/fphys.2019.01448
6. Brown HA, Topham TH, Clark B, Smallcombe JW, Flouris AD, Ioannou LG, Telford RD, Jay O, Périard JD. Seasonal heat acclimatisation in healthy adults: a systematic review. Sports Medicine. 2022. doi:10.1007/s40279-022-01677-0
7. Racinais S, Hosokawa Y, Akama T, Bermon S, Bigard X, Casa DJ, Grundstein A, Jay O, Massey A, Migliorini S, Mountjoy M, Nikolic N, Pitsiladis YP, Schobersberger W, Steinacker JM, Yamasawa F, Zideman DA, Engebretsen L, Budgett R. IOC consensus statement on recommendations and regulations for sport events in the heat. British Journal of Sports Medicine. 2023. doi:10.1136/bjsports-2022-105942
8. Weiss K, Valero D, Villiger E, Scheer V, Thuany M, Cuk I, Rosemann T, Knechtle B. The influence of environmental conditions on pacing in age group marathoners competing in the New York City Marathon. Frontiers in Physiology. 2022. doi:10.3389/fphys.2022.842935