Heat Index Calculator
Enter temperature and humidity to get the heat index and its health risk band — the same formula behind US heat warnings.
Last updated: 2026-09-15
How the calculation works
- The Rothfusz regression models how humidity reduces sweat evaporation, raising the effective temperature on the body.
- Adjustments handle the dry-low-RH and humid-high-RH corners of the chart.
- Risk bands follow NWS/OSHA thresholds: caution at 80–90 °F, extreme caution 90–103, danger 103–125.
Formula
HI = −42.379 + 2.049T + 10.143H − 0.225TH − 0.0068T² − 0.0548H² + 0.0012T²H + 0.00085TH² − 0.000002T²H²
| Symbol | Meaning | Unit |
|---|---|---|
HI | Heat index | °F |
T | Air temperature | °F |
H | Relative humidity | % |
Worked example
92 °F at 65% RH: heat index ≈ 105 °F — 'danger' band. Workers need water/rest/shade cycles and the body cannot cool effectively through sweat alone.
Interpreting the result
Heat index is a health metric, not a comfort score: above 103 °F, heat exhaustion is likely with prolonged exposure and heat stroke becomes possible. It assumes shade and light wind — full sun adds up to 15 °F. Wet-bulb temperature is the stricter industrial metric for work/rest cycles.
Assumptions
- Shade, light wind, typical human clothing and activity.
- Valid from 80–112 °F and 13–85% RH (regression fitted range).
Limitations
- Direct sun can add up to 15 °F to the apparent temperature.
- Does not include wind (use wet-bulb globe temperature for outdoor work settings).
Frequently asked questions
What heat index is dangerous?
103–125 °F is NWS 'danger': heat exhaustion likely. Above 125 °F is 'extreme danger' — heat stroke is imminent without cooling.
Why does humidity make heat worse?
Your body cools by evaporating sweat; humid air is already near saturation, so evaporation slows and the cooling effect collapses even though the air temperature is unchanged.