How it works
index = temperature°F + dew point°F → pace slowdown band
Dew point is a direct measure of how much moisture is in the air, so it captures the humidity that blunts evaporative cooling. Coach Mark Hadley’s widely used method adds the temperature and the dew point (both in °F) and reads the sum against a chart: at 100 or below there is no adjustment; each higher band adds more. For example, 80°F air with a 65°F dew point sums to 145, which calls for easing your pace by roughly 3–4.5%, turning a 9:00/mile effort into about 9:16–9:24. Above a sum of 180 the method advises against hard running altogether. The result is a range, not a single number, because body size, fitness and how well you are acclimated all change how much heat affects you.
Sources
- Hadley temperature + dew-point method Mark Hadley, Maximum Performance Running — heat & humidity pace-adjustment chart: temperature (°F) + dew point (°F) → percentage slowdown band.
- Why dew point, not just temperature Dew point quantifies absolute humidity; high humidity limits evaporative sweat cooling, raising heat strain independent of air temperature.
- Heat and endurance performance Endurance running pace declines as heat stress rises, with larger effects over longer races and for less heat-acclimated runners.
FAQ
How much does heat slow running pace?
It depends on both temperature and humidity. Using the temperature-plus-dew-point method, mild heat might cost under 1%, while hot, humid conditions can slow you 5% or more. This calculator gives a realistic range for your specific conditions rather than a single guess.
Why does the calculator use dew point?
Dew point measures the actual moisture in the air, which determines how well sweat can evaporate and cool you. Two days at the same temperature can feel very different depending on dew point, so combining the two predicts heat stress far better than temperature alone.
What is a comfortable dew point for running?
Generally, dew points below about 55°F feel comfortable, 55–65°F is noticeable, 65–70°F is hard, and above 70°F is oppressive. The higher the dew point, the more you should ease your pace and prioritise hydration.
Should I adjust pace by feel or by the number?
Both. Use the estimated slowdown as a starting point, then run by effort and how you feel — heart rate, breathing and perceived exertion. On a hot day, holding your cool-weather pace usually means working far too hard, so let the conditions dictate.
Does acclimation change the adjustment?
Yes. Runners who train regularly in heat adapt over one to two weeks and slow down less than the chart suggests, while those used to cool weather may need to ease more. That is why the result is a range — treat the faster end as well-acclimated and the slower end as not.
When is it too hot to run hard?
When the temperature-plus-dew-point sum climbs above about 180°F, the method advises against hard running. In those conditions, shift to easy effort, run at cooler times of day, and watch for signs of heat illness.
Heat adjustments are estimates from a population method and the conditions you enter, not a substitute for listening to your body. Heat illness is dangerous — hydrate, ease off, and stop if you feel unwell. General information, not medical advice.