Heat index calculator, how hot it really feels.

A heat index calculator turns the air temperature and the humidity into one number: how hot the air feels to a person in the shade. When the air is humid, sweat dries slowly, the body cools badly, and 96 F can feel like 121 F. This heat index calculator uses the formula the US National Weather Service (NWS) publishes, so its numbers match the NWS heat index chart. It takes Fahrenheit or Celsius, accepts either relative humidity or dew point, and shows the NWS risk band that goes with the result.

How to use the heat index calculator

1. Type the temperature in Air temperature and pick Fahrenheit or Celsius under Unit. Changing the unit converts the number you typed.
2. Under Humidity given as, choose Relative humidity or Dew point. The box below it changes its label to match.
3. Type the humidity. The result cards update as you type: the heat index, the risk band, how much hotter it feels, the effect of direct sun, and which part of the formula was used.
4. Look at the table under the cards. It shows the heat index for every temperature and humidity in a grid, with your cell outlined.
5. Use Copy result to copy one line of text, or Copy link for a link that opens with your numbers.

The NWS heat index formula

The NWS describes two steps (page checked 26 Oct 2026 at wpc.ncep.noaa.gov). T is the air temperature in degrees Fahrenheit and RH is the relative humidity in percent.

1. Work out the simple formula, HI = 0.5 × (T + 61.0 + ((T − 68.0) × 1.2) + (RH × 0.094)), then average that with T. If the average is under 80 F, that is the heat index.
2. If it is 80 F or higher, use the Rothfusz regression: HI = −42.379 + 2.04901523 T + 10.14333127 RH − 0.22475541 T RH − 0.00683783 T² − 0.05481717 RH² + 0.00122874 T² RH + 0.00085282 T RH² − 0.00000199 T² RH².

Two adjustments follow the regression. If RH is under 13 percent and T is between 80 and 112 F, subtract ((13 − RH) / 4) × √((17 − |T − 95|) / 17). If RH is over 85 percent and T is between 80 and 87 F, add ((RH − 85) / 10) × ((87 − T) / 5). The NWS notes the regression is not appropriate when the heat index would be below about 80 F, and says it can be off by about 1.3 F.

Worked examples

96 F and 65 percent. The simple formula gives 0.5 × (96 + 61 + 33.6 + 6.11) = 98.36, and the average with 96 is 97.18, so the regression applies. Its nine terms add up to 121.03, and neither adjustment applies. The heat index is 121 F, which is the example the NWS itself gives for these two numbers. That is in the Danger band.

100 F and 10 percent. The regression gives 94.75. Humidity is under 13 percent, so subtract (3 / 4) × √(12 / 17) = 0.63. The heat index is 94.12, or 94 F: dry heat feels cooler than the thermometer says. That is Extreme caution.

85 F and 90 percent. The regression gives 101.58. Humidity is over 85 percent and the temperature is between 80 and 87 F, so add (5 / 10) × (2 / 5) = 0.2. The heat index is 101.78, or 102 F, which is Extreme caution, one degree short of Danger.

75 F and 50 percent. The simple formula gives 74.55, and the average with 75 is 74.78. That is under 80, so it is the answer: about 75 F, and no risk band.

35 C and 60 percent. 35 C is 95 F. The regression gives 113.09 F, which is 45.05 C. That is Danger.

The NWS risk bands

The bands and their wording come from the NWS page at weather.gov/ama/heatindex, checked 26 Oct 2026. The Celsius figures are converted from the Fahrenheit ones and rounded.

What the number does not include

• Sun. The values are for shade. The NWS says direct sunlight can raise the heat index by up to 15 F, and the Direct sun card adds that.
• Wind. Light wind is assumed. The NWS warns that strong wind with very hot, dry air can be dangerous too.
• You. Age, health, medicines, how hard you work and how used you are to heat all change the risk. This page does the maths. It is not medical advice, so follow your local weather service and your doctor.

Using the dew point

Weather apps often show the dew point instead of relative humidity. If you pick Dew point, the calculator turns it into relative humidity with the Magnus approximation, a common textbook formula, and then uses the NWS steps. The Magnus step is ours, not an NWS formula, so a result from a dew point can differ from one using a measured humidity by a fraction of a degree. The dew point can never be above the air temperature, and the page says so if it is.

Common problems

• If the answer is close to the air temperature, you are below 80 F, where the heat index barely differs from the thermometer.
• Very high temperatures, above about 112 F, are past the range the NWS gives for its adjustments, so treat those numbers with care.
• Your phone's feels-like temperature may use a different model, such as one with wind and sun, and will not match this number.

For other weather and unit maths, try the Fahrenheit to Celsius page or the percentage calculator.