Hot and humid weather feels hotter than the thermometer says, because sweat evaporates more slowly. Enter the air temperature and relative humidity (or dew point) to get the heat index from the National Weather Service equation, the NWS caution or danger level, and how the feels-like changes with humidity.
Last updated:
Humidity input
%
Enter your values to see the result.
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How to use the Heat Index Calculator
Enter the air temperature and choose °C or °F.
Enter the relative humidity, or switch to Dew point if that is what your forecast shows.
Press Calculate heat index to see the feels-like temperature and the NWS risk level.
Check the table to see how much hotter it would feel at other humidity levels.
Plan accordingly: at Extreme caution and above, drink water, rest in shade and avoid hard work at midday.
Heat index chart (°F)
Air temp ↓ / RH →
40%
50%
60%
70%
80%
90%
80 °F (27 °C)
80
81
82
83
84
86
84 °F (29 °C)
83
85
88
90
94
98
88 °F (31 °C)
88
91
95
100
106
113
92 °F (33 °C)
94
99
105
112
121
131
96 °F (36 °C)
101
108
116
126
138
152
100 °F (38 °C)
109
118
129
143
158
176
104 °F (40 °C)
119
131
145
161
181
202
Values come from the same NWS equation the calculator uses. 90 °F (32 °C) at 60% humidity already feels like about 100 °F (38 °C).
Heat index risk levels
Heat index
NWS level
Possible effects with prolonged exposure or activity
80–90 °F (27–32 °C)
Caution
Fatigue
90–103 °F (32–39 °C)
Extreme caution
Heat cramps and heat exhaustion possible
103–124 °F (39–51 °C)
Danger
Heat cramps and heat exhaustion likely; heat stroke possible
125 °F (52 °C) and above
Extreme danger
Heat stroke highly likely
Signs of heat exhaustion include heavy sweating, weakness, dizziness, nausea and cool, clammy skin: move to a cool place and sip water. Hot skin, confusion or fainting can mean heat stroke, which is a medical emergency.
Frequently asked questions
How is the heat index calculated?
The US National Weather Service first computes a simple estimate, HI = 0.5 × [T + 61 + (T − 68) × 1.2 + RH × 0.094] in °F. If that averaged with T is 80 °F or more, it uses the Rothfusz regression HI = −42.379 + 2.04901523T + 10.14333127RH − 0.22475541T·RH − 0.00683783T² − 0.05481717RH² + 0.00122874T²RH + 0.00085282T·RH² − 0.00000199T²RH², with small corrections for very dry or very humid air.
What heat index is dangerous?
The NWS levels are Caution at 80–90 °F (27–32 °C), Extreme caution at 90–103 °F (32–39 °C), Danger at 103–124 °F (39–51 °C) and Extreme danger at 125 °F (52 °C) and above. Young children, older adults and people working outdoors are at higher risk.
Does the heat index include sunshine?
No. It assumes shade and a light wind. Direct sunlight can make it feel up to about 15 °F (8 °C) hotter than the heat index value.
Why is there no heat index below 80 °F?
The equation was fitted for hot conditions. Below about 80 °F (27 °C) humidity has little effect on comfort and the heat index is roughly equal to the air temperature, so it is shown for reference only.
What is the difference between heat index and humidex?
Both express how hot humid air feels. The heat index (used in the USA) is based on Steadman's model and the Rothfusz equation. Canada's humidex uses the dew point and a simpler formula, and its values are usually somewhat higher.
Can I use the dew point instead of humidity?
Yes. Choose Dew point and the calculator converts it to relative humidity with the Magnus formula, RH = 100 × e^(17.625·Td/(243.04+Td)) ÷ e^(17.625·T/(243.04+T)), then computes the heat index.
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