A walkthrough, end to end.
- 1
Enter the air temperature and relative humidity.
- 2
Pick units: Fahrenheit or Celsius.
- 3
See the heat index (apparent temperature) and the heat-stress category.
NWS Rothfusz heat index
Standard formula valid for T ≥ 80°F and RH ≥ 40%. Adjustments apply at RH extremes. Below 80°F, HI = T.
What you can do with this.
Outdoor work safety
OSHA uses heat index to set safety levels.
Sports & athletics
Coaches monitor practice in hot climates.
Travel comfort
Estimate how hot it'll feel in humid destinations.
Construction sites
Plan break frequency by heat index.
Outdoor events
Evaluate risk of heat illness.
Pets and kids
Avoid extended exposure when HI is dangerous.
Crop and livestock
Heat stress thresholds for agriculture.
Heat index 2026 — what's current
NWS Rothfusz remains the standard public-facing model.
Frequently asked.
Standard formula targets T ≥ 80°F and RH ≥ 40%. Below that, heat stress is minor.
≥ 90°F: caution. ≥ 103°F: extreme caution. ≥ 125°F: extreme danger.
No. All math runs in your browser.
Dew point measures absolute humidity; heat index combines temp and humidity into a feels-like temperature.