Heat Index & Wet-Bulb Temperature Calculator

Find out how hot it really feels, and how dangerous it is, from temperature and humidity or dew point. Works in Fahrenheit and Celsius.

Air temperature (°F)Relative humidity (%)

Heat index (feels like)

105.2°F

40.7°C

Wet-bulb temperature

79.9°F

26.6°C

Dew point

73.5°F

Danger

Heat cramps or heat exhaustion are likely, and heat stroke is possible with prolonged exposure or activity.

Moderate heat stress for physical work outdoors.

Heat index uses the US National Weather Service equation (shade, light wind). Full sun can add up to about 15°F (8°C). Wet-bulb temperature uses Stull (2011) at sea-level pressure. For information only; follow local heat warnings.

What is the heat index?

The heat index, sometimes called the “feels like” or apparent temperature, combines air temperature and relative humidity to show how hot it feels to the human body. When the air is humid, sweat evaporates more slowly, so the body cools less well and the heat feels more intense. At 90°F (32°C) with 60% humidity, for example, it feels like about 100°F (38°C).

This calculator uses the same equation as the US National Weather Service: the Rothfusz regression with NOAA’s adjustments for very dry and very humid air, and a simpler formula for cooler conditions. It assumes shade and a light wind; standing in full sun can make it feel up to about 15°F (8°C) hotter.

Heat index chart

Heat index (°F) by temperature and relative humidity
Air temperature	30%	40%	50%	60%	70%	80%
86°F (30°C)	84	85	88	91	95	100
90°F (32°C)	88	91	95	100	106	113
95°F (35°C)	94	99	105	113	123	134
100°F (38°C)	102	109	118	129	143	158
105°F (41°C)	112	121	134	149	166	187

Heat index danger levels

US National Weather Service heat index categories
Heat index	Level	Possible effects
80–90°F (27–32°C)	Caution	Fatigue with prolonged exposure or activity
90–103°F (32–39°C)	Extreme caution	Heat cramps or heat exhaustion possible
103–124°F (39–51°C)	Danger	Heat cramps or exhaustion likely; heat stroke possible
125°F+ (52°C+)	Extreme danger	Heat stroke highly likely

What is wet-bulb temperature?

Wet-bulb temperature is the lowest temperature air can be cooled to by evaporating water into it. It is a direct measure of how well sweat can cool you. When the wet-bulb temperature approaches skin temperature, sweating stops working. A wet-bulb of 35°C has long been described as the theoretical limit of human survival, but laboratory research suggests that even young, healthy people struggle to stay cool above about 31°C. This calculator uses the Stull (2011) formula, which is accurate to within about 0.3°C on average at sea-level pressure.

How to calculate heat index from dew point

If your weather report gives the dew point instead of relative humidity, choose “Dew point” above. The calculator converts it to relative humidity with the Magnus formula and then applies the heat index equation. A dew point above about 70°F (21°C) feels muggy; above 75°F (24°C) it feels oppressive.

Staying safe in extreme heat

Drink water regularly, schedule hard work and exercise for the cooler early morning, rest in shade or air conditioning, and check on older neighbours and anyone living alone. Watch for warning signs of heat exhaustion (heavy sweating, dizziness, nausea, headache) and treat confusion or hot, dry skin as a medical emergency. Long, deadly heatwaves are often caused by a stalled heat dome, and they are more likely in El Niño years.

Frequently asked questions

How is the heat index calculated?

With the US National Weather Service equation (the Rothfusz regression), which combines air temperature and relative humidity, plus adjustments for very dry or very humid air.

What heat index is dangerous?

The NWS rates 103°F (39°C) and above as Danger, and 125°F (52°C) and above as Extreme danger. Heat exhaustion is possible from about 90°F (32°C).

What is the difference between heat index and wet-bulb temperature?

The heat index estimates how hot it feels; the wet-bulb temperature measures how well evaporation (sweat) can cool you, which is more directly linked to the body’s physical limits.

What wet-bulb temperature is deadly?

35°C is often cited as the theoretical survival limit, but research suggests healthy people can overheat at wet-bulb temperatures above about 31°C.

Can I calculate heat index in Celsius?

Yes. Choose °C above; the calculator converts internally and shows results in both units.