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 from sweat that your body relies on to dump heat during exercise — the same air temperature can feel and perform very differently depending on how much moisture is in it. Running effort at a given pace rises sharply as heat index climbs, since your cardiovascular system has to divert more blood flow to the skin for cooling, leaving less capacity for working muscles.

Runners training or racing in hot, humid conditions use heat-index-based pace adjustment to avoid pushing a cool-weather goal pace in conditions where it isn't physiologically sustainable, translating temperature and humidity into how many seconds per kilometre or mile to back off, and coaches use similar guidance to reset athlete expectations for summer races.

This calculator takes the temperature and humidity, computes the heat index, and returns the pace adjustment you should make to run safely and sustainably in those conditions.