Free tool
Race Weather Calculator
Hot, humid or windy race? Enter your finish time and the race day weather to see what you would have run in ideal conditions. A 4 hour marathon at 27°C and 70% humidity is worth about 3:42 on a cool, still day.
- Heat, humidity and wind
- Marathon, half, 10K, 5K
- Ideal conditions equivalent
How does the calculator work?
It turns temperature and humidity into a heat stress score called WBGT, using the simplified formula from the Australian Bureau of Meteorology. In the cold it uses wind chill instead. Each band adds a set percentage to your time, and the calculator takes that percentage off to show your ideal conditions time. The bands follow the heat and cold adjustments used in the Hansons Marathon Method race calculators.
| Conditions | Measured by | Time cost |
|---|---|---|
| Ideal, about 13°C and up with low heat stress | WBGT under 18.9°C | 0% |
| Warm | WBGT 18.9 to 21.7°C | 2% |
| Warm and humid | WBGT 21.7 to 24.4°C | 3% |
| Hot | WBGT 24.4 to 27.2°C | 5% |
| Very hot | WBGT 27.2°C and above | 8% |
| Cool, 4 to 13°C | Air temperature | 1% |
| Cold, under 4°C | Wind chill 1.7 to 12.8°C | 1% |
| Cold and windy | Wind chill −3.9 to 1.7°C | 3% |
| Very cold and windy | Wind chill −9.4 to −3.9°C | 5% |
| Freezing wind | Wind chill under −9.4°C | 7% |
How much does heat slow you down in a marathon?
Once the heat stress score passes about 19°C, most runners lose around 2 percent. In very hot, humid conditions the cost reaches about 8 percent, which is close to 20 minutes for a 4 hour marathoner. Humidity matters as much as the temperature, because sweat cannot cool you when the air is already wet.
How should I pace a hot race?
Use the second line of the calculator. Enter the time you are fit for in good weather and the forecast, and it tells you the time and pace to plan for. Start at that slower pace, drink early and pour water on your head. Runners who hold their cool weather pace in the heat usually fade badly after halfway.
Was my race better than the time says?
Often, yes. If you ran 4:00 on a 27°C humid morning, your fitness is closer to a 3:42 runner. Use the ideal conditions time to set your training paces in the marathon pace calculator, not the slower time from a hot day.
Questions runners ask
How much does heat slow down your running?
In warm conditions most runners lose about 2 to 3 percent. In hot, humid weather it can reach 5 to 8 percent, close to 20 minutes for a 4 hour marathon.
How do I adjust my marathon pace for heat?
Enter your goal time and the forecast into the calculator. It shows the time and pace to plan for. Start at that pace rather than your cool weather pace.
What is the best temperature to run a marathon?
Cool, still and dry conditions are best. In this model, races in heat stress below about 19 degrees WBGT carry no time cost.
What is WBGT?
Wet bulb globe temperature is a heat stress score that combines temperature and humidity. It is a better guide to how hard heat will feel than the temperature alone.
Does humidity slow you down when running?
Yes. At the same temperature, higher humidity raises heat stress because sweat cannot evaporate to cool you, so the time cost goes up.
Does wind and cold slow you down?
Yes. Below about 4 degrees, wind chill drives the cost, from about 1 percent on a calm cold day to 7 percent in freezing wind.
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