Heat Index Calculator
Find the heat index, how hot it feels when humidity slows the cooling effect of sweat. Enter the air temperature in °F or °C and the relative humidity. The calculator uses the US National Weather Service method and shows the NWS risk level for the result.
How to use the Heat Index Calculator
- Enter your numbers in the Heat Index Calculator above.
- Press the Calculate button to see the result.
- To try other numbers, change any value and press the button again.
The NWS heat index formula
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
T is the air temperature in °F and RH the relative humidity in percent. This is the Rothfusz regression the US National Weather Service uses, as its Weather Prediction Center page The Heat Index Equation gives it. The NWS first works out a simpler formula, HI = 0.5 × (T + 61 + (T − 68) × 1.2 + RH × 0.094). If that comes to under 80°F, it is the answer, because the regression does not fit below about 80°F. Otherwise the regression applies, with two corrections. Below 13% humidity, at 80 to 112°F, it subtracts ((13 − RH) / 4) × √((17 − |T − 95|) / 17). Above 85% humidity, at 80 to 87°F, it adds ((RH − 85) / 10) × ((87 − T) / 5). In °C mode the calculator converts to °F, applies the formula and converts the answer back. The WPC notes that the regression is not valid beyond the range of data Steadman worked from. So the calculator takes air temperatures from 76°F to 126°F, the range of NOAA's full heat index chart, and prints figures up to 137°F, the top value on the NWS chart. 76°F is 24.4°C and 126°F is 52.2°C, rounded to a tenth. The calculator checks the °F value, so 24.4°C (75.9°F) falls just outside and 24.5°C is the lowest tenth it takes. A higher result shows as "Above 137°F" (or "Above 58.3°C") with the Extreme Danger level.
Worked example
At 90°F and 70% relative humidity the calculator shows 105.9°F, in the Danger level. At 85°F and 90% humidity it shows 101.8°F, which includes the +0.2°F high-humidity correction. Dry air works the other way: 100°F at 10% humidity feels like 94.1°F. In °C, 35°C at 60% humidity gives 45.1°C.
Frequently asked questions
Why does humidity make heat worse?
Sweating cools you by evaporation. Humid air is closer to saturation, so sweat evaporates slower. The body's primary cooling mechanism becomes less effective.
When should I worry about heat?
The National Weather Service heat index chart has four levels: Caution from 80 to 90°F, Extreme Caution from 90 to 103°F, Danger from 103 to 124°F and Extreme Danger at 125°F or higher, as listed on weather.gov/ama/heatindex. The calculator shows the level under each result.
Is heat index for shade or sun?
Shade. The NWS says its heat index values are for shady places, and that direct sunlight can raise the heat index by up to 15°F. The calculator gives the shade value.