Tool
Heat-Adjusted Pace Calculator
Marathon physiology has an optimum around 12 °C dry-bulb. Above that, evaporative cooling fails faster than your fitness can keep up. Plug in the forecast and a target pace to see how much to give back.
Effective slowdown vs. an ideal-temperature day.
Adjusted pace: 5:17/km
| Distance | Ideal day | In these conditions |
|---|---|---|
| 10 km | 50:00 | 52:45 |
| Half marathon | 1:45:29 | 1:51:17 |
| Marathon | 3:30:59 | 3:42:35 |
What model is this?
The penalty curve combines the Maughan/Shirreffs work on thermoregulation, the El Helou marathon-temperature analysis (PLoS ONE, 2012), and ACSM heat-acclimatization findings. Above 50 % relative humidity, each 10 % RH adds an effective-temperature bonus that grows with absolute temperature — capturing the fact that humidity matters more when it's already hot.
How accurate is this for a real race?
The slowdown is a population average — individual runners vary by ±2 percentage points. Lean runners, low body-fat athletes, and well-acclimatized racers do better than the model suggests; high-body-mass runners and non-acclimatized runners do worse. For a full race-specific projection that combines heat with elevation and wind, use TrainingFlow's full pacing engine.