Heat Index Calculator
Humidity stops sweat evaporating, so the same temperature feels far hotter. The heat index combines the two into one "feels like" number, and this page also tells you what precaution that number calls for.
| Air temperature | °F |
| Relative humidity | % |
Result
Your result will appear here.
How it works
Sweat cools you only when it evaporates. In dry air it evaporates quickly and a 95 °F day feels like 95 °F; in humid air the same sweat stays on your skin, heat is trapped, and the body's core temperature climbs. The heat index converts temperature and relative humidity into the temperature that would feel equivalent in dry air.
The official version is a statistical fit to human response data (the Rothfusz regression), used by the US National Weather Service. It is only defined at or above 80 °F; below that, a simpler approximation is shown and the heat index is not really meaningful.
Formula
For 80 °F and above, the regression uses T for temperature in °F and R for relative humidity in percent.
- HI = −42.379 + 2.04901523T + 10.14333127R − 0.22475541TR − 0.00683783T² − 0.05481717R² + 0.00122874T²R + 0.00085282TR² − 0.00000199T²R²
- Dry-air adjustment (humidity under 13%): subtract ((13 − R) ÷ 4) × √((17 − |T − 95|) ÷ 17)
- High-humidity adjustment (humidity over 85%, temperature 80–87 °F): add ((R − 85) ÷ 10) × ((87 − T) ÷ 5)
Worked example
95 °F at 55% humidity gives a heat index of about 109 °F — fourteen degrees hotter than the thermometer, and squarely in the "danger" band where heat exhaustion is likely with prolonged activity.
Assumptions
The index assumes you are in the shade, in light wind, wearing light clothing, and are a healthy adult. Direct sunshine can add up to 15 °F. It is also not a substitute for the wet-bulb globe temperature used for workplace heat rules, which accounts for radiant heat and air movement.
Reading the result
The bands are practical, not decorative. Above 103 °F of heat index, outdoor work should move to early morning or evening. Above 125 °F, even brief exposure is dangerous. Humidity matters more than most people expect: 90 °F at 90% humidity is more dangerous than 100 °F at 20%, because the body cannot cool itself at all in saturated air.
For general information only. Follow local heat warnings and workplace heat-safety rules.
How to use the Heat Index Calculator
It takes a few seconds, and every figure is calculated in your browser.
- Enter the air temperature.
- Enter the relative humidity.
- Press Calculate — the result appears in the dark panel beside the form.
- Use Copy, CSV, Share or Print under the result if you want to keep it.
Heat Index Calculator — frequently asked questions
What formula does the Heat Index Calculator use?
For 80 °F and above, the regression uses T for temperature in °F and R for relative humidity in percent. HI = −42.379 + 2.04901523T + 10.14333127R − 0.22475541TR − 0.00683783T² − 0.05481717R² + 0.00122874T²R + 0.00085282TR² − 0.00000199T²R² Dry-air adjustment (humidity under 13%): subtract ((13 − R) ÷ 4) × √((17 − |T − 95|) ÷ 17) High-humidity adjustment (humidity over 85%, temperature 80–87 °F): add ((R − 85) ÷ 10) × ((87 − T) ÷ 5)
How does the Heat Index Calculator work?
Sweat cools you only when it evaporates. In dry air it evaporates quickly and a 95 °F day feels like 95 °F; in humid air the same sweat stays on your skin, heat is trapped, and the body's core temperature climbs. The heat index converts temperature and relative humidity into the temperature that would feel equivalent in dry air. The official version is a statistical fit to human response data (the Rothfusz regression), used by the US National Weather Service.
What assumptions and limitations should I know about?
The index assumes you are in the shade, in light wind, wearing light clothing, and are a healthy adult. Direct sunshine can add up to 15 °F. It is also not a substitute for the wet-bulb globe temperature used for workplace heat rules, which accounts for radiant heat and air movement.
Can you walk through a worked example?
95 °F at 55% humidity gives a heat index of about 109 °F — fourteen degrees hotter than the thermometer, and squarely in the "danger" band where heat exhaustion is likely with prolonged activity.
Last updated 10 November 2026 · Formula, worked example and assumptions are documented above, and every result is calculated in your browser.