Free Heat Index Calculator Online
Use this free heat index calculator to find what the temperature really feels like when humidity is factored in. Enter the air temperature and relative humidity, and the heat index calculator instantly shows the apparent temperature along with the NWS danger level and safety recommendations.
Feels Like
Enter temperature and humidity
How the Heat Index Calculator Works
The heat index calculator uses the Rothfusz regression equation adopted by the National Weather Service. This polynomial equation takes air temperature and relative humidity as inputs and returns the apparent temperature that the human body perceives.
At 85°F with 50% humidity, the heat index calculator shows 86°F — barely above the actual temperature. But at 95°F with 70% humidity, it shows 124°F — in the "Danger" zone where heat stroke is likely with prolonged exposure. The heat index calculator makes this invisible risk visible.
The heat index calculator also accounts for the NWS adjustment factors for extreme conditions. Results are most accurate for shaded conditions; direct sun exposure can add 10-15°F to the perceived temperature.
NWS Heat Index Warning Levels
The heat index calculator displays the appropriate warning level for your conditions.
Heat Safety Tips
Hydrate Early
When the heat index calculator shows Danger levels, drink water before you feel thirsty. Aim for 8 oz every 15-20 minutes during outdoor activity. Dehydration sets in before thirst signals arrive.
Time Your Activity
The heat index peaks between 2-5 PM. Schedule outdoor exercise for early morning or evening when the heat index calculator typically shows lower values. Even a 2-hour shift in timing can make a significant difference.
Know the Signs
Heat exhaustion: heavy sweating, nausea, dizziness. Heat stroke: hot dry skin, confusion, loss of consciousness. When the heat index calculator shows Danger, watch for these symptoms in yourself and others.
Frequently Asked Questions
Disclaimer: This heat index calculator provides estimates for shaded conditions. Direct sun exposure can increase the heat index by up to 15°F. Follow NWS advisories for official guidance.
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