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Heat Index Calculator

How hot it really feels when humidity is added to the air temperature, with the National Weather Service formula and danger level. At 90°F and 60% humidity it feels like 100°F.

Air temperature (°F)

Relative humidity (%)

Dew point (°F)

Heat index chart

Find the air temperature along the top and the relative humidity down the side. Blank cells are beyond the range of the formula.

Heat index in °F by air temperature and relative humidity
RH ↓ / °F →	80	82	84	86	88	90	92	94	96	98	100	102	104	106	108	110
40%	80	81	83	85	88	91	94	97	101	105	109	114	119	124	130	136
45%	80	82	84	87	89	92	96	100	104	109	114	119	124	130	137
50%	81	83	85	88	91	95	99	103	108	113	118	124	131
55%	81	84	86	89	93	97	101	106	112	117	124	130
60%	82	84	88	91	95	100	105	110	116	123	129	137
65%	82	85	89	93	98	103	108	114	121	128	136
70%	83	86	90	95	100	106	112	119	126	134
75%	84	88	92	97	103	109	116	124	132
80%	84	89	94	100	106	113	121	129
85%	85	90	96	102	110	117	126	135
90%	86	92	98	105	113	122	131
95%	88	94	101	109	117	127	137
100%	89	96	104	112	121	132

Heat index in °C by air temperature and relative humidity
RH ↓ / °C →	26	28	30	32	34	36	38	40	42
40%	26	28	30	32	35	39	43	48	54
45%	26	28	30	33	37	41	46	51	57
50%	26	28	31	34	38	43	49	55
55%	26	29	32	36	40	46	52
60%	26	29	33	37	42	48	55
65%	26	30	34	39	44	51
70%	26	31	35	40	47	54
75%	27	31	36	42	49	58
80%	27	32	38	44	52
85%	27	33	39	47	55
90%	27	34	41	49
95%	27	35	43	51
100%	27	36	44	54

Extreme danger (125°F / 52°C and up): Heat stroke is highly likely.
Danger (103°F / 39°C and up): Heat cramps or heat exhaustion are likely, and heat stroke is possible with long exposure or activity.
Extreme caution (90°F / 32°C and up): Heat stroke, heat cramps or heat exhaustion are possible with long exposure or activity.
Caution (80°F / 27°C and up): Fatigue is possible with long exposure or activity.

How the heat index is calculated

Sweat cools you as it evaporates. In humid air it evaporates slowly, so your body keeps more heat and the air feels hotter than it is. The heat index puts a number on that.

The National Weather Service uses the Rothfusz regression, fitted to Steadman’s 1979 model of how warm a person feels:

HI = −42.379 + 2.04901523·T + 10.14333127·RH − 0.22475541·T·RH − 0.00683783·T² − 0.05481717·RH² + 0.00122874·T²·RH + 0.00085282·T·RH² − 0.00000199·T²·RH²

T is the air temperature in °F and RH the relative humidity in %. When the result is under 80°F a simpler formula is used, and small corrections are made for very dry air (under 13% humidity) and very humid air (over 85% at 80–87°F).

Formula and danger levels from the US National Weather Service heat index equation (Rothfusz, 1990). Dew point is converted to humidity with the Magnus formula. A guide only: heat illness can start below these values for children, older people, people with health conditions and anyone working hard in the sun.

About the Heat Index Calculator

The Heat Index Calculator shows how hot it really feels when humidity is added to the air temperature. Enter the temperature in °F or °C and either the relative humidity or the dew point, and it works out the heat index with the same formula the US National Weather Service uses, along with the NWS danger level: caution, extreme caution, danger or extreme danger. The full heat index chart is on the page in both °F and °C.

How to use it

1 Choose °F or °C.
2 Enter the air temperature.
3 Enter the relative humidity, or switch to dew point if that is what your forecast shows.
4 Read the feels-like temperature and the danger level, and copy a link to share it.

Frequently asked questions

What is the heat index?

The heat index, or apparent temperature, is how hot it feels when relative humidity is combined with the air temperature. Humid air slows the evaporation of sweat, so your body cools less and the heat feels worse. At 90°F and 60% humidity it feels like 100°F.

What heat index is dangerous?

The National Weather Service uses four levels: caution at 80–90°F, extreme caution at 90–103°F, danger at 103–124°F and extreme danger at 125°F and above. Heat stroke becomes possible in the extreme caution range with long exposure or hard work, and likely in the danger range.

Is the heat index measured in the shade?

Yes. The heat index assumes shade and a light breeze. Standing in full sunshine can raise it by up to 15°F (about 8°C), so the real risk outdoors at midday can be one level higher than the chart shows.

How do I work out the heat index from the dew point?

Switch the calculator to Dew point and enter it with the air temperature. It converts the dew point to relative humidity with the Magnus formula and then uses the NWS heat index formula. A dew point above 65°F (18°C) feels muggy; above 70°F (21°C) is oppressive.

What is the heat index in Celsius?

Choose °C and enter the temperature in Celsius; the calculator converts it, applies the formula and gives the answer in °C and °F. As a guide, 32°C at 60% humidity feels like about 37°C, and 35°C at 60% feels like about 45°C.

Why is the heat index lower than the temperature?

In very dry air sweat evaporates quickly and cools you well, so the heat index can be a little below the air temperature. It is most useful from 80°F (27°C) upwards; below that humidity makes little difference.

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