Heat Index Pace Adjustment Calculator

About this calculator

The heat index combines air temperature and relative humidity into a single ‘feels-like’ number, because high humidity blunts the evaporative cooling your body relies on to dump heat while running. That extra physiological strain shows up directly as a slower pace at the same perceived effort, even when the thermometer reading alone doesn’t look extreme.

Runners and coaches use heat index-based pace adjustments to set realistic goal times for hot, humid race days, drawing on research that links heat index to measurable marathon slowdown, rather than chasing a pace that was only realistic in cool conditions.

This calculator takes the temperature, humidity and your goal pace or time, and returns the heat index along with your adjusted pace and estimated finish time.