Heat Index Calculator
Enter temperature and humidity to get the heat index and its health risk band — the same formula behind US heat warnings.
Open calculatorLast 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.
Worked example
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).
Heat index (°F) by air temperature and relative humidity
| Air temperature | 40 % RH | 50 % RH | 60 % RH | 70 % RH | 80 % RH |
|---|---|---|---|---|---|
| 80 °F | 80 | 81 | 82 | 83 | 84 |
| 85 °F | 84 | 87 | 89 | 93 | 97 |
| 90 °F | 91 | 95 | 100 | 106 | 113 |
| 95 °F | 99 | 105 | 113 | 123 | 134 |
| 100 °F | 109 | 118 | 130 | 143 | 158 |
| 105 °F | 122 | 134 | 149 | 166 | 187 |
The heat index is defined for warm conditions, from about 80 °F and 40 % humidity upwards.
Worked scenario: 90 °F and 60 % humidity
The calculator returns 99.7 °F (37.6 °C). The air is 90 °F, but at that humidity sweat evaporates more slowly, so the body cools less and the strain on it is closer to that of a hotter, drier day.
Limits of the index
- It describes conditions in the shade. The US National Weather Service notes that direct sunshine can raise the effective value by up to 15 °F.
- It is a guide for people at rest or in light activity. Heavy work, age, health and medication change how heat affects a person.
- It does not apply to cold weather. For that, use wind chill.
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.