How much does heat slow your pace?
Heat slows marathon pace progressively. A study of marathon results found that performance slowed steadily as the wet bulb globe temperature rose from 5 to 25°C, in men and women. Top male finishers ran 1.7% to 4.5% slower than course records across four groups of race conditions. Studies disagree on whether faster or slower runners lose more.
What the wet bulb globe temperature is
The wet bulb globe temperature (WBGT) is a heat-stress measure that combines air temperature, humidity, wind and sun into one number. It is the measure used in the main marathon studies below, because air temperature alone leaves out humidity and sun. An estimate for your area is available from the US National Weather Service.[5]
What the main study found
Ely and colleagues analyzed marathon results from several major races. They reported a progressive slowing of marathon performance as the WBGT rose from 5 to 25°C, in men and women of a wide range of abilities.[1] A companion review by Montain and colleagues, using 140 race-years of data, described the same pattern: times slow progressively as the weather warms above 5 to 10°C WBGT, and men and women are affected similarly.[2]
For top male runners, the paper reports the slowdown against the course record for four groups of race conditions, labeled Q1 to Q4. The slowdown rises from group to group, in line with its overall finding that heat slows performance.[1]
| Group of race conditions | Top men slower than course record |
|---|---|
| Q1 | 1.7% (± 1.5) |
| Q2 | 2.5% (± 2.1) |
| Q3 | 3.3% (± 2.0) |
| Q4 | 4.5% (± 2.3) |
These figures describe elite men against a course record. Do not apply them directly to your own time.
Who is affected most: the studies disagree
Ely and colleagues found that the 25th-, 50th-, 100th- and 300th-place finishers slowed more than faster runners as the WBGT rose, and concluded that slower runners are more negatively affected.[1] The review by Montain and colleagues also reported a greater penalty for slower runners than for elite runners.[2]
A later study of the New York City Marathon found the opposite. Under identical thermal exposure, the fastest runners experienced a larger decline in performance than slower ones. The authors suggested that faster runners may suffer more because they perform the race more intensely, even though they run for a shorter time.[3] The two sets of studies used different races and methods, so the evidence does not give one answer for every runner.
What RunRepeat found about temperature
An analysis of 107.9 million race results by RunRepeat found that the best running temperature was between 40 and 50°F.[4] It is a general temperature figure, not a WBGT reading, so it is a rough guide for race-day forecasts. It agrees with the pattern above: performance is best in cool conditions and falls as it warms.
How to plan your pace in the heat
These are practical points, not research findings.
- Check the forecast and the heat index. Look at the WBGT estimate for race morning, not only the air temperature.[5]
- Run by effort. Heat raises heart rate and the feeling of effort at a given pace, so hold back early rather than defending the pace on your watch.
- Choose a more conservative goal. Pick a slower goal time, then turn it into pace and splits with the marathon, half marathon or 10K calculator.
- Run your own pace. Do not match the pace of a partner who may handle heat better than you.[5]
- Stop if you feel unwell. Dizziness, confusion or nausea in the heat are reasons to stop and get help. This is general information, not medical advice. See the disclaimer.
Example of the mechanics, not a heat rule: a 4:00:00 marathon goal needs 9:09 per mile. If you choose a 4:10:00 goal instead, the pace becomes 9:32 per mile, which is 0:23 per mile slower. The calculators give exact splits for whichever goal you pick.
What we cannot tell you
MilePaceLab does not offer a heat adjustment calculator. The studies show the direction and the scale for elite runners, but I could not verify published values for other paces, and the studies disagree on who loses more. A single number for everyone would sound more certain than the science is. If you find a published table or regression you trust, send it to us, and we will test it before using it.
Frequently asked questions
Does heat slow all runners the same?
No. One set of studies found slower marathon runners lost more performance in heat. A New York City Marathon study found the fastest runners lost more. The evidence does not give one answer.
At what temperature does pace start to slow?
In marathon results, times slowed progressively as the WBGT warmed above about 5 to 10°C. RunRepeat’s analysis found the best running temperature was between 40 and 50°F, which is about 4 to 10°C.
Should I adjust my goal pace for heat?
Heat slows most runners, so a more conservative goal is sensible on a hot day. The research does not give a single adjustment, so decide with the forecast, your experience and how you feel, then plan splits with a race pace calculator.
Sources
- Ely, M.R., Cheuvront, S.N., Roberts, W.O. and Montain, S.J. (2007). Impact of weather on marathon-running performance. Medicine and Science in Sports and Exercise, 39(3), 487–493.
- Montain, S.J., Ely, M.R. and Cheuvront, S.N. (2007). Marathon performance in thermally stressing conditions. Sports Medicine, 37(4–5).
- Gasparetto, T. and Nesseler, C. (2020). Diverse effects of thermal conditions on performance of marathon runners. Frontiers in Psychology, 11, 1438.
- Andersen, J.J. The State of Running 2019. RunRepeat, in collaboration with the IAAF.
- Outside Online. How much should you slow when running in the heat? Reports on the Ely et al. study and on estimating WBGT from the National Weather Service.
Related tools and guides
About the author
Shaheer Bin Ramzan is a software engineer and SEO specialist with five years of professional experience. He builds and checks the calculators and writes the guides on MilePaceLab. He is not a running coach or doctor. See the author page, the methodology and the disclaimer. Report errors through the contact page.