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Heat Index Calculator: How Hot It Feels
Enter the air temperature and the relative humidity. The calculator applies the National Weather Service heat index procedure and shows the apparent temperature in the shade, with the NWS caution category.
How the heat index is computed
The heat index combines air temperature and relative humidity into an apparent temperature that reflects how hard the body must work to cool itself. When humidity is high, sweat evaporates slowly, so the same air temperature feels hotter. The US National Weather Service uses a multiple regression equation fitted by Lans Rothfusz to a physiological model by Steadman. With T in °F and R as percent humidity:
HI = −42.379 + 2.04901523T + 10.14333127R − 0.22475541TR − 0.00683783T² − 0.05481717R² + 0.00122874T²R + 0.00085282TR² − 0.00000199T²R²
The NWS procedure first computes a simple estimate, 0.5 × (T + 61 + 1.2(T − 68) + 0.094R). If the average of that and the air temperature is below 80 °F, the simple value is used. Otherwise the regression applies, with small corrections for very dry and for very humid conditions.
NWS caution categories
| Heat index | Category |
|---|---|
| 80 to 90 °F (27 to 32 °C) | Caution: fatigue is possible with prolonged exposure and activity |
| 90 to 103 °F (32 to 39 °C) | Extreme caution: heat cramps and heat exhaustion are possible |
| 103 to 124 °F (39 to 51 °C) | Danger: heat cramps and exhaustion likely, heat stroke possible |
| 125 °F (52 °C) and above | Extreme danger: heat stroke is highly likely |
These bands are guides used in public weather messages. Individual risk depends on age, health, medication, fitness, hydration and acclimatisation.
Validity and sources of error
- Shade and light wind. The index assumes a person in shade with a light breeze. Direct sun adds to the effective heat, and strong wind in very hot, dry air can add heat rather than remove it.
- Regression range. The formula is fitted over a limited range of temperatures and humidities. At extremes it can behave poorly, which is why the NWS adds correction terms.
- Measurement. Humidity sensors in a hot car or direct sun give false readings. Use shaded air.
- Dew point. The dew point calculator is another way to judge how humid the air feels.
Limits and safety
This calculator is educational and is not a medical or safety service. The heat index is an estimate for an average healthy adult. Children, older adults, pregnant people, outdoor workers and anyone taking medicines that affect hydration or temperature control can be at risk at lower values. Drink water, rest in shade or cool rooms, and never leave people or pets in parked vehicles. Follow local heat warnings, and seek urgent medical help for confusion, fainting, a very high body temperature or hot dry skin.
Worked example
An afternoon air temperature of 90 °F with 70% relative humidity.
Inputs
| Temperature unit | Fahrenheit (°F) |
|---|---|
| Air temperature (°F or °C) | 90 |
| Relative humidity | 70 % |
Result
Feels like 105.9 °F in the shade with light wind. NWS category: Danger.
At this heat index, heat cramps and heat exhaustion are likely with prolonged activity, and heat stroke is possible. Take heat precautions.
| Heat index | 105.9 °F / 41.1 °C |
|---|---|
| Air temperature | 90.0 °F / 32.2 °C |
| Relative humidity | 70.0% |
| Method | Rothfusz regression |
| NWS category | Danger |
- Full sunshine can raise the heat index by up to about 15 °F. The regression has an error of around 1.3 °F.
Estimating note: Quantities and ratings here are estimates for planning and learning. Products, local codes and site conditions differ, so check manufacturer data and local regulations, and use a qualified professional for electrical, structural or safety-critical work.
Frequently asked questions
What is the heat index?
The heat index is an apparent temperature that combines air temperature and relative humidity to show how hot it feels in the shade. Higher humidity makes the same air temperature feel hotter.
At what temperature does the heat index apply?
The NWS regression is used when the heat index is about 80 degrees Fahrenheit or higher. Below that, the simple Steadman formula gives a value close to the air temperature.
Does sunshine change the heat index?
Yes. The published index assumes shade. Direct sunshine can add up to about 15 degrees Fahrenheit to the effective value, so exposed outdoor work is more demanding.
How accurate is the formula?
The Rothfusz regression reproduces the underlying model to within about 1.3 degrees Fahrenheit. The model itself is a general estimate for a typical adult, not a personal measure.
What heat index is dangerous?
The NWS labels 103 to 124 degrees Fahrenheit as danger and 125 and above as extreme danger. Risk can start earlier for vulnerable people or during hard work.
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References and sources
- US National Weather Service: Heat Index equation (Rothfusz regression, 1990) and the heat index chart. Link
- Steadman, R. G. (1979). The assessment of sultriness. Journal of Applied Meteorology.