Heat Index Calculator

Heat index is the "feels like" temperature when humidity is factored in. High humidity slows the evaporation of sweat, so your body sheds heat less efficiently and the air feels hotter than the thermometer reads. A 95 F day at 60% humidity feels like around 113 F, while the same temperature at 15% humidity feels closer to 90 F.

It uses the NWS Rothfusz regression (1990) with two corrective adjustments: a downward correction when humidity is below 13% and temperature is 80-112 F (dry air feels cooler than the formula predicts), and an upward correction when humidity is above 85% and temperature is 80-87 F (muggy, low-heat conditions). The calculator first computes the simpler NWS weighted average, and only applies the full regression when that average reaches 80 F or higher.

Caution starts at 80 F, where fatigue is possible with prolonged exposure or activity. Extreme caution begins at 90 F, where heat cramps, heat exhaustion, and even heat stroke become possible. Danger starts at 103 F, where heat cramps or exhaustion are likely and heat stroke is possible. Extreme danger is 125 F or above, where heat stroke is imminent.

The Rothfusz regression was built for hot conditions, so whenever the input temperature is below 80 F the calculator flags the result as an approximation. At cooler temperatures humidity has little effect on how hot it feels, so the number is still a useful rough estimate but should not be read as precise.

No. Heat index assumes shade and light wind. The NWS notes that full sunshine can add up to 15 F to how hot it actually feels on top of the value shown here. Treat the result as a baseline, since real "feels like" temperatures in direct sun run higher.