Racing · Free tool
Marathon Heat Adjustment Calculator
Enter the goal time you trained for in cool conditions and the forecast race-day temperature. We estimate how much the heat will cost you, and the realistic adjusted finish time and pace.
Why heat slows you down
Running generates a lot of heat, and in warm conditions your body diverts blood to the skin to shed it — blood that would otherwise carry oxygen to your legs. Core temperature rises, perceived effort climbs, and the only way to avoid overheating is to slow down.
Endurance performance peaks around 10–12 °C (50–54 °F). Above that, every few degrees adds a measurable penalty — modest at first, then increasingly punishing past 25 °C (77 °F).
Dew point matters too
This calculator uses air temperature, the single biggest lever, but humidity matters almost as much. When the dew point is high, sweat evaporates poorly and your cooling system stalls, so a humid 22 °C can feel worse than a dry 27 °C. Treat the estimate as a floor on race-day difficulty when the air is also sticky.
On a hot day, start slower than your adjusted target and let the clock come to you. Negative-splitting a hot marathon is rare; the runners who fade least are the ones who back off early.
Frequently asked questions
How much does heat slow marathon time?
Roughly 1% slower around 15 °C, 3% near 20 °C, 6% near 25 °C, and 10% or more by 30 °C, relative to cool conditions. This tool interpolates between those points to estimate your specific slowdown.
Should I change my pace or my goal in the heat?
Adjust your goal. Trying to hold a cool-weather pace in the heat is the fastest route to blowing up. Set out at the heat-adjusted pace from the gun and reassess at halfway.