Heat Index Calculator
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Heat index 107.6 °F (42 °C), danger, at 96 °F and 50% humidity
Danger (heat index 103–124 °F (39–51 °C)): Heat cramps or heat exhaustion likely, and heat stroke possible with prolonged exposure and/or physical activity. Drink water, rest in the shade and check on people at higher risk. Heat stroke is an emergency: call emergency services.
Heat index values assume shade and a light wind. NWS notes that full sunshine can raise them by up to 15 °F (about 8 °C).
How to use the heat index calculator
- Enter the air temperature measured in the shade, in °F or °C.
- Enter the relative humidity, or switch to dew point if that is what your forecast shows. The dew point uses the same unit as the temperature.
- Read the heat index, its NWS category and which version of the formula was used.
Heat index formula
Rothfusz: HI = −42.379 + 2.04901523T + 10.14333127RH − 0.22475541T·RH − 0.00683783T² − 0.05481717RH² + 0.00122874T²RH + 0.00085282T·RH² − 0.00000199T²RH²
If RH < 13% and 80 ≤ T ≤ 112 °F: subtract [(13 − RH) ÷ 4] × √[(17 − |T − 95|) ÷ 17]
If RH > 85% and 80 ≤ T ≤ 87 °F: add [(RH − 85) ÷ 10] × [(87 − T) ÷ 5]
T is the air temperature in °F and RH the relative humidity in percent. Following the NWS Weather Prediction Center's description, the calculator first works out the simple formula, which is based on Robert Steadman's 1979 research. If that gives 80 °F or more, it uses Lans Rothfusz's 1990 multiple regression instead, with one of the two corrections for very dry or very humid air. Celsius inputs are converted to °F, calculated, and converted back.
Worked example
At 96 °F and 50% relative humidity, the simple formula gives 0.5 × (96 + 61 + 28 × 1.2 + 50 × 0.094) = 97.65 °F. That is above 80, so the Rothfusz regression applies: HI = 107.6 °F, which rounds to 108 °F, matching the NWS chart. That is in the "danger" category. In metric terms, 35 °C at 50% humidity gives a heat index of 40.7 °C (105 °F).
Heat index chart (°F)
Computed with the calculator's engine. Blank cells are beyond 137 °F, where the NWS chart stops.
| Temperature | 40% RH | 50% RH | 60% RH | 70% RH | 80% RH | 90% RH | 100% RH |
|---|---|---|---|---|---|---|---|
| 80 °F | 80 | 81 | 82 | 83 | 84 | 86 | 89 |
| 84 °F | 83 | 85 | 88 | 90 | 94 | 98 | 104 |
| 88 °F | 88 | 91 | 95 | 100 | 106 | 113 | 121 |
| 92 °F | 94 | 99 | 105 | 112 | 121 | 131 | |
| 96 °F | 101 | 108 | 116 | 126 | |||
| 100 °F | 109 | 118 | 129 | ||||
| 104 °F | 119 | 131 | |||||
| 108 °F | 130 |
Heat index chart (°C)
| Temperature | 30% RH | 40% RH | 50% RH | 60% RH | 70% RH | 80% RH |
|---|---|---|---|---|---|---|
| 27 °C | 27 | 27 | 27 | 28 | 29 | 30 |
| 30 °C | 29 | 30 | 31 | 33 | 35 | 38 |
| 32 °C | 31 | 32 | 34 | 37 | 40 | 44 |
| 35 °C | 35 | 37 | 41 | 45 | 50 | 57 |
| 38 °C | 39 | 43 | 49 | 55 | ||
| 40 °C | 43 | 48 | 55 |
NWS heat index categories
| Category | Heat index | Possible effects with prolonged exposure or activity |
|---|---|---|
| Caution | 80–90 °F (27–32 °C) | Fatigue possible with prolonged exposure and/or physical activity. |
| Extreme caution | 90–103 °F (32–39 °C) | Heat stroke, heat cramps or heat exhaustion possible with prolonged exposure and/or physical activity. |
| Danger | 103–124 °F (39–51 °C) | Heat cramps or heat exhaustion likely, and heat stroke possible with prolonged exposure and/or physical activity. |
| Extreme danger | 125 °F (52 °C) or higher | Heat stroke highly likely. |
Heat index values were devised for shady, light-wind conditions. The NWS notes that full sunshine can raise them by up to 15 °F, and hard work or heavy clothing adds to the strain. Local heat warnings use thresholds that vary by region, so follow your forecast office's alerts.
Accuracy and the NWS chart
The regression is a statistical fit to Steadman's tables, and the NWS notes that it is not valid for extreme temperature and humidity beyond the range of Steadman's data, nor for heat index values below about 80 °F, which is why the simple formula is used there. In one small corner, between 80 and 87 °F with humidity above 85%, the printed NWS chart shows the regression without the humid-air adjustment. This calculator applies the adjustment as written on the equation page, so there it can read 1 to 2 °F higher: at 80 °F and 100% it gives 89.3 °F against the chart's 87 °F.
Staying safe in the heat
- Drink water regularly, before you feel thirsty, and limit alcohol and sugary drinks.
- Schedule hard work and exercise for the cooler early morning or evening, and take breaks in the shade or air conditioning.
- Never leave children or pets in a parked car: the temperature inside rises far above the outdoor reading.
- Heat stroke (confusion, hot skin, collapse) is a medical emergency: call emergency services and cool the person immediately.
The dew point calculator converts between dew point and relative humidity, and for cold weather the wind chill calculator does the same job in reverse.
Frequently asked questions
How is the heat index calculated?
The NWS first uses a simple formula, HI = 0.5 × [T + 61 + (T − 68) × 1.2 + RH × 0.094]. If that is 80 °F or more, it switches to the Rothfusz regression, with small adjustments for very dry or very humid air. At 96 °F and 50% humidity the result is about 107.6 °F, which the NWS chart shows as 108.
What heat index is dangerous?
The NWS categories are Caution at 80–90 °F, Extreme Caution at 90–103 °F, Danger at 103–124 °F and Extreme Danger at 125 °F or more. Heat exhaustion and heat stroke become likely in the Danger range with prolonged exposure or activity.
What is the heat index at 35 °C and 50% humidity?
About 40.7 °C (105 °F), in the NWS Danger category. At the same temperature with 60% humidity it rises to about 45.1 °C (113 °F).
Can I use dew point instead of relative humidity?
Yes. Switch the humidity input to dew point. The calculator converts it to relative humidity with the Magnus formula: 95 °F with a 75 °F dew point is about 52.7% humidity and a heat index of about 107 °F.
Does sunshine change the heat index?
Yes. Heat index values assume shade and a light wind. According to the NWS, full sunshine can raise them by up to 15 °F, so the real strain in direct sun can be much higher than the number shown.