Heat Index Calculator
With air temperature 90 °F, relative humidity 70 percent, heat index comes to 105.9 °F — feels like. It is reached in 6 steps, the last of which is temperature < 80 ? simple : rothfusz, and each one is printed on the page with its numbers filled in. The formula is the one published by NOAA National Weather Service, not an approximation fitted to it.
What the temperature feels like at a given humidity, by the National Weather Service regression, with the risk category it falls in.
Formula and sources checked · How we check
Air temperature 90 °F, Relative humidity 70
105.9 °F
Feels like for the example below. Editing a field recomputes the calculator below; this figure holds the answer the page was loaded with.
It is written into the HTML rather than drawn by a script, so a search engine reading this page without running JavaScript still finds an answer.
- Rothfusz regression
105.922 °F- Simple form, used below 80 °F
0.5 * (90 + 61 + (90 - 68) * 1.2 + 70 * 0.094)91.99 °F- Heat index
105.922 °F- Heat index in Celsius
(105.92202 - 32) * 5 / 941.068 °C- How much it adds to the air temperature
105.92202 - 9015.922 °F- Risk band, 0 to 4
3
Worked example
90 °F at 70% humidity feels like 106 °F — sixteen degrees above the air temperature, and inside the National Weather Service’s danger band where heat exhaustion is likely with prolonged exposure.
How to work it out yourself
- 1.Enter the air temperature in the shade and the relative humidity. Direct sun adds up to 15 °F that the index does not include.
- 2.Treat the figure below 80 °F as approximate: the regression is fitted to hot conditions and the simpler form takes over underneath.
- 3.Read the risk band rather than the number alone. The difference between 100 and 106 is the difference between caution and danger in the published categories.
What humidity does at 90 °F
| Relative humidity (percent) | Feels like | How much it adds to the air temperature |
|---|---|---|
| 10 | 85.9 °F | -4.09 °F |
| 20 | 86.3 °F | -3.66 °F |
| 30 | 87.9 °F | -2.07 °F |
| 40 | 90.7 °F | 0.68 °F |
| 50 | 94.6 °F | 4.6 °F |
| 60 | 99.7 °F | 9.68 °F |
| 70 | 105.9 °F | 15.92 °F |
| 80 | 113.3 °F | 23.33 °F |
| 90 | 121.9 °F | 31.9 °F |
| 100 | 131.6 °F | 41.64 °F |
Air temperature held at 90 °F.
The formula
- Rothfusz regression
- Simple form, used below 80 °F
0.5 * (90 + 61 + (90 - 68) * 1.2 + 70 * 0.094) - Heat index
- Heat index in Celsius
(105.92202 - 32) * 5 / 9 - How much it adds to the air temperature
105.92202 - 90 - Risk band, 0 to 4
Source: NOAA National Weather Service — heat index and the Rothfusz regression
Questions people actually ask
- Why does humidity make heat worse?
- Because sweating cools by evaporation, and evaporation slows as the air fills with water vapour. At 100% humidity sweat cannot evaporate at all and the body loses its main cooling mechanism. That is the whole content of the heat index: not more heat, but less ability to shed it.
- Is the heat index measured or calculated?
- Calculated, from a regression fitted to a heat-balance model of a specific hypothetical person — 5 ft 7 in, 147 lb, walking at about 3 mph in the shade, wearing long trousers and a short-sleeved shirt. Change any of those assumptions and the real apparent temperature changes with it, which is why the index is a guide rather than a measurement.
- Does the index account for sunshine?
- No. The published figure is a shade value, and full sun can add as much as 15 °F to what a body actually experiences. Wind cuts it the other way, up to a point — above about 95 °F, moving air hotter than skin stops helping and starts adding heat.
- What do the risk categories mean?
- The National Weather Service uses four: caution from 80 °F, extreme caution from 90, danger from 103 and extreme danger from 125. They describe the risk from prolonged exposure or activity rather than from stepping outside, and heat stroke becomes likely rather than possible in the top band.
Related
- Temperature ConverterConverts between Celsius, Fahrenheit and Kelvin in any direction, with the reference points to sanity-check it.
- Water Intake CalculatorDaily fluid target from body weight and exercise, against the NASEM total-water reference intakes.
- Calories Burned CalculatorEnergy burned by an activity from its MET value, body weight and duration. Net of what you would have burned sitting still, which trackers leave in.
- Wind Chill CalculatorWhat the cold feels like once wind is taken into account, by the National Weather Service formula, with the frostbite time it implies.
- Dew Point CalculatorDew point from temperature and humidity by the Magnus formula, with what that dew point actually feels like.
- Oven Temperature ConverterAn oven setting in Fahrenheit, Celsius, gas mark and the fan equivalent, with the dial rounding real ovens actually offer.
Put this calculator on your site
Free, no attribution required beyond the link.
<iframe src="https://rulecalculators.com/embed/heat-index" width="100%" height="420" style="border:1px solid #e7e4de;border-radius:12px" title="Heat Index Calculator"></iframe>