Heat cost calculator
Is it too hot to race?
A humid 70°F morning costs most runners 5 to 6%; 80°F about 8 to 9%.
Racing a specific event?
Load it to fill in its course and weather.
Distance
Your time on a cool, flat day
A recent race in good weather. Aiming for a goal instead? The last line of the answer covers it.
Temperature
Use the forecast for your start time, not the day's high.
Race weeks away? Load it above to use its typical morning.
Humidity
Dew point 58°F: a bit humid.
Sky
Very warm: start slower
Start around 9:27/mi rather than 8:58/mi and hold it, take fluids from the start, and speed up after halfway only if you feel good.Heat level: High
Expect on that day
4:07:56
Average 9:27/mi instead of 8:58/mi
The weather: 5.5% slower, +30 sec/mi, about 12:55 lost over the marathon.
Aiming for 3:55 in these conditions, or already ran it? That takes about the same effort as 3:42:45 on a cool, flat day.
How we got this
WBGT 66°F · dew point 58°F · 0.34% per °F at your pace
The wet bulb globe temperature (WBGT) rolls temperature, humidity and sun into one number, and heat starts to cost time once it passes about 50°F. Above that, pace slows 0.26 to 0.35% per °F depending on pace, calibrated on 6.3 million finishes across 1,686 race-years. How the model works
Questions
- Is it too hot to race?
- It depends on the wet bulb globe temperature (WBGT), which combines temperature, humidity and sun, more than on the air temperature. At a WBGT below about 50°F (10°C), heat costs nothing. From 73°F (23°C), race organizers warn every runner (the red flag): forget the PR and race by effort. 82°F (28°C), the black flag, is the threshold most commonly cited for cancelling or postponing a race; consider not racing, or run it easy. The calculator works out the WBGT for your morning and says which side of those lines you're on, with the pace to start at.
- How much does heat slow a marathon?
- Nothing below a wet bulb globe temperature of about 50°F (10°C), then roughly 0.26 to 0.35% of pace per °F above it, depending on pace: calibrated on 6.3 million finishes across 1,686 race-years. A 72°F (22°C), 65% humidity morning in some sun costs a 3:30 runner around 5 to 6%, ten minutes or more.
- Why humidity and sun, not just temperature?
- Runners cool by sweating, and humidity decides how well that works; sun adds radiant heat. The calculator combines them into a WBGT, the standard sports-medicine heat index, and prices that.
- Does my pace matter?
- Yes. Faster runners produce more heat but are out for less time; the heat slope per degree is lower at fast paces and higher for the back of the pack, which is what the race results show.