Heat Index Calculator
Find the heat index from temperature and relative humidity or dew point with the official NWS formula, plus dew point and the NWS caution and danger category.
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About this toolWhat is the Heat Index Calculator?
Humid heat feels hotter than dry heat because sweat cannot evaporate as easily. The heat index puts a number on that: the temperature that the combination of heat and humidity feels like to your body. Enter the air temperature and the relative humidity, or the dew point if that is what your forecast gives, and the Heat Index Calculator returns the feels-like temperature and the US National Weather Service risk category.
At 92°F and 60% relative humidity, for example, the heat index is 105°F, which falls in the NWS Danger band, where heat cramps and heat exhaustion are likely with prolonged exposure or exertion. The calculator also gives the dew point, the more direct measure of how muggy it feels, and a table showing how the heat index at your temperature climbs with humidity.
It works in Fahrenheit or Celsius, follows the NWS algorithm exactly including its two humidity adjustments, and shows each step of the working.
- Exact NWS Weather Prediction Center algorithm with both adjustments
- Relative humidity or dew point input, °F or °C
- Dew point output using the Magnus formula
- NWS Caution, Extreme Caution, Danger and Extreme Danger bands
- Humidity table at your temperature
- Step-by-step working, shareable link and copy-ready summary
How to use the Heat Index Calculator
- Choose °F or °C. Switching converts the numbers already entered.
- Enter the air temperature, measured in the shade as weather stations do.
- Choose whether you have relative humidity or dew point, and enter it.
- Read the heat index, its NWS category and the effects described for that band.
- Check the dew point and the humidity table, and open How this was calculated to see the formula used.
Worked examples
A humid summer afternoon
Input
92°F, 60% relative humidity
Output
Heat index 105°F (40°C), Danger; dew point 76°F
The Rothfusz regression applies because conditions are above 80°F. A 76°F dew point is oppressive, so sweat evaporates poorly.
From a Celsius forecast with a dew point
Input
33°C air temperature, 24°C dew point
Output
RH 59%, heat index 39.3°C (102.7°F), Extreme Caution
The Magnus formula turns the dew point into relative humidity before the NWS formula runs. One more degree of dew point would push it into the Danger band.
Hot but very dry
Input
104°F, 10% relative humidity
Output
Heat index 98°F, with a low-humidity adjustment of -0.5°F
In desert air, sweat evaporates well and the heat index can sit below the air temperature. Dehydration is still a serious risk.
Background and insight
The heat index comes from Robert Steadman's 1979 model of how the body exchanges heat with its surroundings. The National Weather Service computes it with a regression fitted to Steadman's tables by Lans Rothfusz in 1990. The NWS Weather Prediction Center describes the procedure: first compute a simple formula, HI = 0.5 x (T + 61 + (T - 68) x 1.2 + RH x 0.094). If that result averaged with the temperature is below 80°F, use it. Otherwise apply the full nine-term Rothfusz regression, then subtract an adjustment when humidity is below 13% at 80 to 112°F, or add one when humidity is above 85% at 80 to 87°F.
The NWS groups heat index values into four categories: Caution at 80 to 90°F, Extreme Caution at 90 to 103°F, Danger at 103 to 124°F and Extreme Danger at 125°F or more. The values assume shade and a light breeze. The NWS notes that direct sunshine can raise the heat index by up to 15°F, and people doing hard physical work, older adults, young children and those on some medications are at risk at lower values.
Dew point is calculated with the Magnus formula using the Alduchov and Eskridge (1996) constants a = 17.625 and b = 243.04°C, accurate to a small fraction of a degree across everyday temperatures. The same formula runs backwards when you enter a dew point, to recover relative humidity.
Common use cases
Planning outdoor work
Decide when to schedule breaks, shade and water for crews, events or sports practice.
Exercise and races
Judge whether to slow down, move a run to the morning or take extra fluids.
Reading forecasts
Turn a forecast temperature and dew point into the number that matters for comfort and safety.
Home weather stations
Check the heat index your station reports against the NWS method.
Teaching meteorology
Show students how humidity changes perceived temperature and how the NWS formula works.
Tips and limitations
- The heat index is for shade. In full sun, add up to 15°F (8°C) to judge exposure.
- Below about 80°F (27°C) the heat index is close to the air temperature and is rarely a safety concern.
- Very high results, above about 137°F, are beyond the data the formula was fitted to; treat them as "extremely dangerous" rather than a precise number.
- Dew point is the better measure of mugginess: under 55°F feels dry, 65 to 70°F feels sticky, above 70°F is oppressive.
- This is an estimate for an average adult, not medical advice. Heat illness can occur below these thresholds during heavy exertion.
Frequently asked questions
How is the heat index calculated?
The NWS first computes a simple formula, 0.5 x (T + 61 + (T - 68) x 1.2 + RH x 0.094). If the average of that and the temperature reaches 80°F, it uses the Rothfusz regression, a nine-term equation in temperature (°F) and relative humidity (%), with small adjustments for very dry or very humid air.
What heat index is dangerous?
The NWS classes 103 to 124°F as Danger, where heat cramps or heat exhaustion are likely and heat stroke is possible, and 125°F or higher as Extreme Danger. 90 to 103°F is Extreme Caution. Risk starts lower for hard work, direct sun and vulnerable people.
What is the difference between heat index and temperature?
Temperature measures the air. The heat index estimates how hot it feels when humidity limits the body's ability to cool by sweating. At 90°F, 50% humidity feels like 95°F and 70% feels like 106°F.
Should I use dew point or relative humidity?
Either gives the same heat index. Relative humidity changes through the day as the air warms and cools, while dew point stays steadier and reflects how much moisture is in the air, so forecasters often prefer it. Enter whichever you have.
Does the heat index account for sun?
No. The values assume shade and a light wind. The NWS says full sunshine can increase the heat index by up to 15°F.
What is the heat index in Celsius?
The formula works in Fahrenheit, so the calculator converts your Celsius inputs, applies it and converts back. The danger thresholds are about 27°C (caution), 32°C (extreme caution), 39°C (danger) and 52°C (extreme danger).
Is there an in-depth guide to the Heat Index Calculator?
Yes. “Heat Index and Wind Chill: How Feels-Like Temperatures Work” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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