Heat Illness Risk Calculator
A free calculator that turns air temperature and relative humidity into a heat index, a risk level, and a list of water, rest and shade controls for that level.
The heat index is calculated with the National Weather Service Rothfusz regression, including its adjustments for very low and very high humidity. The result is sorted into five bands: below 80°F is Low Risk, 80 to 90°F is Caution, 91 to 102°F is Extreme Caution, 103 to 124°F is Danger, and 125°F or higher is Extreme Danger. The heat index assumes shade. Full sun can add up to about 15°F, and heavy exertion, PPE and poor airflow raise the risk further. Use it alongside OSHA and NIOSH guidance and your site's heat illness prevention plan.
How to use it
- Enter the air temperature in °F (40 to 130).
- Enter the relative humidity as a percentage (0 to 100).
- Read the heat index, the risk level and the recommended controls.