Heat Index Calculator
Enter the air temperature and the relative humidity. You get the heat index, how hot it feels, and the National Weather Service danger category.
Your numbers
How we calculate it
The heat index combines the temperature and the humidity into the temperature it feels like in the shade. The calculator follows the National Weather Service method. It first uses a simple formula, 0.5 × (T + 61 + (T − 68) × 1.2 + RH × 0.094), with T in °F. If the average of that and T is 80 °F or more, it switches to the Rothfusz regression, a long polynomial in T and RH, with two small corrections for very dry air (RH under 13% between 80 and 112 °F) and very humid air (RH over 85% between 80 and 87 °F).
For example, 90 °F with 70% humidity has a heat index of about 106 °F, and 100 °F with 40% humidity is about 109 °F. The category is taken from the usual chart: caution up to 90, extreme caution to 103, danger to 124 and extreme danger from 125.
Assumptions and limits
- Weather figures are estimates for general information. Check your local weather service for warnings and advice.
- The heat index is meant for shade and light wind. Direct sun can add up to about 15 °F, and the values are for a healthy adult, so children, older people and anyone working hard are at risk sooner.
- Below 80 °F the heat index is close to the air temperature and the chart does not apply. Temperatures from 40 °F to 140 °F (4.4 °C to 60 °C) are accepted.
Frequently asked questions
What is the heat index?
It is the temperature the body feels when humidity is added to the air temperature. High humidity slows the evaporation of sweat, so the body cools less well and the air feels hotter.
What is a dangerous heat index?
On the National Weather Service chart, a heat index of about 103 °F to 124 °F is in the danger category and 125 °F or more is extreme danger. Even a lower heat index can be harmful during hard exercise.
How is the heat index different from the temperature?
The temperature is what a thermometer reads. The heat index adds the humidity to say how hot it feels, so 90 °F at 70% humidity feels like about 106 °F.
Related calculators
- Wind Chill CalculatorEnter the air temperature and the wind speed.
- Dew Point CalculatorChoose what to find.
- Temperature ConverterEnter a temperature in any scale and see it in Celsius, Fahrenheit and Kelvin at once.
- Calories Burned CalculatorChoose an activity, enter your weight and how long you did it.
Worked example (a fixed example using the starting numbers, it does not change as you type)
Inputs: Units = US (°F and mph); Air temperature (°F) = 90; Relative humidity (%) = 70.
- Heat index 105.9 °F
- Category Danger: heat cramps and heat exhaustion are likely, and heat stroke is possible
- Also in °C 41.1 °C
Embed this calculator
Add this calculator to your own page. Paste the code where you want it. The small link keeps credit to the source.