Heat and humidity arithmetic
Enter your cool-weather pace and the race-day forecast to see how much heat and humidity should slow your plan, with a printable chart for every part of the day.
Your goal pace in cool weather, then the race-day forecast.
Printable heat pace chart
25.0 °C · 60% humidity · dew point 16.7 °C
+6.7% for well-trained runners
Moderate heat
5:36/km; elites slowed least
3:52:04 in cool weather
| Air temperature | WBGT | Heat stress | Slowdown | Pace | Finish |
|---|---|---|---|---|---|
| 10.0 °C | 7.2 °C | Cool | +0.0% | 5:30/km | 3:52:04 |
| 12.5 °C | 9.5 °C | Cool | +0.0% | 5:30/km | 3:52:04 |
| 15.0 °C | 11.9 °C | Neutral | +1.1% | 5:34/km | 3:54:40 |
| 17.5 °C | 14.2 °C | Neutral | +2.5% | 5:38/km | 3:57:54 |
| 20.0 °C | 16.5 °C | Neutral | +3.9% | 5:43/km | 4:01:08 |
| 22.5 °C | 18.8 °C | Moderate heat | +5.3% | 5:47/km | 4:04:22 |
| 25.0 °C | 21.2 °C | Moderate heat | +6.7% | 5:52/km | 4:07:36 |
| 27.5 °C | 23.5 °C | High heat | +8.1% | 5:57/km | 4:10:50 |
| 30.0 °C | 25.8 °C | High heat | +9.5% | 6:01/km | 4:14:04 |
| 32.5 °C | 28.1 °C | Extreme heat | +10.9% | 6:06/km | 4:17:18 |
| 35.0 °C | 30.4 °C | Extreme heat | +12.3% | 6:10/km | 4:20:32 |
| 37.5 °C | 32.8 °C | Extreme heat | +13.7% | 6:15/km | 4:23:46 |
| 40.0 °C | 35.1 °C | Extreme heat | +15.1% | 6:20/km | 4:27:00 |
In a 24-hour or multiday race, read the row for each part of the day: the afternoon costs more than the night.
Well-trained estimate: 0.6% per °C WBGT above 10 °C (Ely 2007). Elite comparison: 0.3% per °C above 15 °C (Mantzios 2022). The shade estimate leaves out sun and wind; slower runners lost more than these averages.
Heat is scored as wet-bulb globe temperature (WBGT). Without a WBGT forecast, the calculator estimates a shade value as 0.7 × wet-bulb temperature + 0.3 × air temperature, with the wet-bulb temperature from Stull's formula. That reproduces the 2022 finding that 10–17.5 °C air corresponds to about 7.5–15 °C WBGT.
The slowdown is 0.6% for every °C of WBGT above 10 °C. That is the rate Ely and colleagues measured for well-trained runners across seven marathons, where times were fastest in the coolest range studied, 5–10 °C WBGT. Elite runners lost less: the 2022 analysis of 1,258 races found 0.3–0.4% per °C beyond 15 °C. Slower runners lost more than both, so treat the result as a minimum if you finish well back in the field.
The shade estimate leaves out sunshine and wind. Direct sun raises WBGT above it, and a breeze lowers it. If your race or weather service publishes a WBGT forecast, choose that option and enter it directly. Run by effort when the reading is above 28 °C, where World Athletics rates heat stress as extreme.
The best measurements come from marathons, so treat them as a floor for longer races. In a timed race, use the chart row for each part of the day: bank nothing in the cool morning, ease off in the afternoon and let the night give time back. The 24-hour pacing strategy shows how to build that into a distance goal, and the timed-race pace calculator turns it into lap targets. Heat also raises fluid and sodium losses; measure yours with the sweat rate calculator.
Heat illness can come on fast. Stop, cool down and get help if you become confused, unsteady or stop making sense. If you have a heart condition or take medication that affects heat tolerance, plan hot races with a clinician.
About 0.6% for every °C of WBGT above 10 °C, the rate measured for well-trained marathoners. At 25 °C (77 °F) and 60% humidity the shade WBGT is about 21 °C, so plan about 7% slower: a 4:00 marathon pace becomes roughly 4:16.
In a study of seven US and Canadian marathons, times were fastest in the coolest conditions studied, 5–10 °C WBGT. A 2022 analysis of 1,258 races put the best range at 7.5–15 °C WBGT, about 10–17.5 °C (50–64 °F) air temperature.
Wet-bulb globe temperature combines air temperature, humidity, sun and wind into one heat-stress number. World Athletics rates up to 10 °C as cool, 10–18 °C neutral, 18–23 °C moderate heat, 23–28 °C high heat and above 28 °C extreme heat.
It matters a lot, because humid air slows sweat evaporation. At 25 °C, raising humidity from 60% to 80% lifts the shade WBGT from about 21 to 23 °C and the estimated slowdown from about 6.7% to 7.9%.
Yes. In the marathon study behind this calculator, slower finishers lost more time as WBGT rose: roughly 0.1% per °C for elite men, 0.3% for elite women and 0.6% for well-trained runners.
Either works. Dew point stays steadier through the day while humidity swings with temperature, so a dew-point forecast is easier to carry from morning to afternoon. The calculator converts one to the other.