Heat pace calculator: how much slower you will run a marathon
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A warm morning at the start of a marathon can take ten or fifteen minutes off a result you worked half a year for. This calculator does not guess how much. From temperature and humidity it estimates the WBGT index, then turns to a study that measured the slowdown at seven marathons over several decades and gives the loss relative to a cool day. It gives it as a lower bound, because in that study amateurs lost more than the leaders.
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<iframe src="https://easytrackly.app/en/embed/heat-pace" title="Running pace in the heat" width="100%" height="760" style="border:0;max-width:48rem" loading="lazy"></iframe>
<p><a href="https://easytrackly.app/en/running-calculator/heat-pace">EasyTrackly running calculator</a></p>What WBGT is
Air temperature alone describes poorly how hard running in the heat is. Humidity decides whether sweat evaporates, the sun adds heat by radiation, and wind takes it away. The WBGT index combines these factors into one number, and it is what studies of exercise in the heat and the rules of many races use. It is measured with a dedicated instrument. The calculator uses an approximation published by the Australian weather service that estimates WBGT from temperature and humidity for full sun and light wind. On an overcast day the real WBGT will be lower, in full sun without wind slightly higher.
What the study showed
Ely, Cheuvront, Roberts and Montain collected in 2007 the results of seven North American marathons, including 36 years of Boston and 29 of New York, together with the weather on race day. They divided the days into four WBGT bands: 5 to 10 degrees, 10 to 15, 15 to 20 and 20 to 25. The men's winners were slower than the course record by 1.7 percent on average in the coolest band and by 4.5 percent in the warmest. Women followed a similar trend. The most important sentence in the paper concerns the rest of the field, though: the runners in 25th, 50th, 100th and 300th place slowed down more with rising WBGT than the leaders.
Why we give a lower bound
The calculator subtracts the slowdown of the coolest band from the slowdown in your conditions and gets the loss relative to a cool day: from zero to 2.8 percent for the leaders. For a runner finishing a marathon in four hours the loss is larger, but the authors gave no figures for such groups that could be used in a formula, so the calculator does not invent them. That is why the result says "at least". For a 4:00 in the cool that means 4:07 or more in the heat, and coaching experience says that for amateurs it is more often 4:15.
How to run in the heat
Start slower than the pace from your cool-weather plan, from the first kilometre, not from the moment it gets hard. Drink at every station in small amounts, and if the course has shade, stay in it even at the cost of a few metres. Before a race in a warmer climate, acclimatisation counts: after 7 to 14 days of training in the heat the body sweats earlier and more, and heart rate at the same pace drops. Symptoms such as chills in the heat, confusion or sweating that stops are a reason to stop and seek help.
Frequently asked questions
Does this work for 10K and the half marathon?
The study concerned the marathon. Over shorter distances the percentage loss is usually smaller, because the body does not heat up as much, but we have no data of this quality for them, so the calculator does not cover them.
What about the cold?
Below 5 degrees WBGT the study had no data, and other work indicates that the fastest times fall around 10 to 13 degrees of air temperature. In the cold the calculator assumes a loss of zero.
How do I know the humidity at the start?
From the hourly forecast for race day. If you do not know it, leave 50 percent: for most spring mornings in Europe that is a middle value.
Why does my watch show a different heat correction?
Watches correct pace with their own algorithms and give no sources. The calculator shows one number from a published study and says plainly what it does not know.
Sources
- Ely M. R., Cheuvront S. N., Roberts W. O., Montain S. J., Impact of weather on marathon-running performance, Medicine and Science in Sports and Exercise, 2007. Slowdown in four WBGT bands at seven marathons.
- Australian Bureau of Meteorology, Thermal comfort observations. The WBGT approximation from temperature and humidity for full sun and light wind.