Case 1
- Air temperature in the shade
- 90°F
- Relative humidity
- 70%
105.9°F
Open with these valuesHeat Index Calculator
105.9°F
Result: 105.9 °FThe heat index is what a hot day feels like once humidity is added: 90 °F at 70 % feels like 106 °F. The regression behind it is written in Fahrenheit and fitted for 80 °F and up with at least 40 % humidity — below that, the heat index is simply the air temperature.
Worked examples
Case 2
- Air temperature in the shade
- 95°F
- Relative humidity
- 55%
108.9°F
Open with these valuesCase 3
- Air temperature in the shade
- 85°F
- Relative humidity
- 90%
101.6°F
Open with these valuesHow it's calculated
HI = c₁ + c₂T + c₃RH + c₄T·RH + c₅T² + c₆RH² + c₇T²RH + c₈T·RH² + c₉T²RH²
- StepEnter the shade air temperature in degrees Fahrenheit.
- StepEnter the relative humidity as a percentage.
- StepRead the feels-like temperature in °F; the table gives the °C equivalent.
- ResultFor the °C value yourself: subtract 32 and multiply by 5/9.
Reference table
| Temperature, humidity | In Celsius and NWS band | Heat index |
|---|---|---|
| 80, 40 | 26.6 °C · no added heat | 79.9 |
| 85, 90 | 38.7 °C · extreme caution | 101.6 |
| 88, 75 | 39.5 °C · danger | 103.1 |
| 90, 70 | 41.1 °C · danger | 105.9 |
| 95, 55 | 42.7 °C · danger | 108.9 |
| 100, 40 | 42.9 °C · danger | 109.3 |
Questions
What is the heat index?
The heat index is the feels-like temperature — how hot the air actually feels to the human body once relative humidity is taken into account. Humid air slows the evaporation of sweat, so your body cools less and a given temperature feels hotter. At 90 °F and 70 % humidity the heat index is about 105.9 °F, or 41.1 °C.
Why does this calculator work in Fahrenheit?
Because the Rothfusz regression is written in Fahrenheit, and swapping in Celsius values without changing the coefficients gives plausible-looking but wrong answers. A separate metric version exists with entirely different coefficients. The table above carries the Celsius equivalent of every row.
How is the heat index calculated?
This calculator uses the full Rothfusz regression from the US National Weather Service: a nine-term polynomial in the temperature T (°F) and the relative humidity RH (%). It is the same equation the NWS uses to issue heat advisories.
When is the heat index formula valid?
The regression is fitted for roughly 80 °F and up with at least 40 % relative humidity, which is why the inputs here start there. Below that — cooler or drier air — the heat index is essentially the air temperature itself. Above about 130 °F the raw regression overshoots, because the NWS applies extra adjustments this calculator does not carry.
Why does humidity make it feel hotter?
Your body cools mainly by evaporating sweat. When the air is already full of moisture, sweat evaporates slowly, so less heat leaves your skin and the same temperature feels hotter. That is why a humid 90 °F day can feel like 106 °F while a dry one feels close to the thermometer reading.
Is the heat index for shade or full sun?
It assumes the shade air temperature and a light wind. Direct sunshine can add up to about 15 °F to how hot it actually feels, so on a sunny day the real apparent temperature is higher than shown. Treat the result as a baseline and add caution in full sun.
Sources and last check
Information, not professional advice.