Heat Index Calculator: How Hot Does It Really Feel?

Humid heat feels hotter than dry heat because sweat evaporates more slowly, and evaporation is how the body cools itself. The heat index puts a number on that: it’s the “feels like” temperature the National Weather Service (NWS) uses for its heat advisories. Enter the air temperature and relative humidity above to get the heat index and the NWS risk level that goes with it.

How the calculator works

It follows the method on the NWS Weather Prediction Center’s heat index equation page, with T in °F and RH in percent.

Step 1, the simple formula. For mild conditions the NWS uses a formula consistent with Steadman’s original heat index tables:

HI = 0.5 × {T + 61.0 + [(T − 68.0) × 1.2] + (RH × 0.094)}

Step 2, the Rothfusz regression. If step 1 gives 80 °F or more, the calculator switches to the regression by Lans P. Rothfusz (NWS Technical Attachment SR 90-23, 1990):

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

Step 3, the NWS adjustments:

• If RH is below 13% and T is between 80 and 112 °F, subtract [(13 − RH) ÷ 4] × √{[17 − |T − 95|] ÷ 17}.
• If RH is above 85% and T is between 80 and 87 °F, add [(RH − 85) ÷ 10] × [(87 − T) ÷ 5].

°C inputs are converted to °F first and the result is converted back. The output panel tells you which step produced your number.

Worked examples

The first three rows are the examples the NWS itself publishes; all rows match the NWS heat index calculator to 0.1 °F:

Air temperature	Relative humidity	Heat index	NWS level
96 °F	65%	121.0 °F (49.5 °C)	Danger
100 °F	55%	123.6 °F, rounds to 124 °F (50.9 °C)	Danger
100 °F	15%	95.8 °F, rounds to 96 °F (35.5 °C)	Extreme Caution
90 °F	60%	99.7 °F (37.6 °C)	Extreme Caution
85 °F	90%	101.8 °F (38.8 °C), high-humidity adjustment	Extreme Caution
105 °F	10%	99.1 °F (37.3 °C), low-humidity adjustment	Extreme Caution
78 °F	40%	77.4 °F (25.2 °C), simple formula	Below Caution

Notice the 100 °F rows: the same temperature feels like 124 °F in humid air but only 96 °F in desert-dry air.

NWS heat index risk levels

Heat index	Classification	Effect on the body (NWS)
80–90 °F	Caution	Fatigue possible with prolonged exposure and/or physical activity
90–103 °F	Extreme Caution	Heat stroke, heat cramps, or heat exhaustion possible with prolonged exposure and/or physical activity
103–124 °F	Danger	Heat cramps or heat exhaustion likely, and heat stroke possible with prolonged exposure and/or physical activity
125 °F or higher	Extreme Danger	Heat stroke highly likely

Important: These values are for shade and light wind. The NWS notes that full sunshine can raise the heat index by up to 15 °F, and that strong winds with very hot, dry air can also be extremely hazardous. Follow your local NWS heat advisories and warnings, and talk to your doctor about heat if you have a health condition.

Heat index indoors

The heat index isn’t only an outdoor number. Indoors you’re in the shade, so you can enter the temperature and humidity of a hot room to see how it feels. Humidity makes a big difference: at 90 °F, going from 50% to 60% relative humidity raises the heat index from about 94.6 °F to 99.7 °F.

• Know your indoor humidity. The EPA recommends keeping it below 60%, ideally 30–50%. Our guide to the ideal indoor humidity explains the range, and how to check humidity in your house covers hygrometers.
• Bring humidity down in summer. Air conditioners remove water as they cool; when that isn’t enough, see how to reduce humidity in the house.
• Don’t run a humidifier in a heat wave. Adding moisture to hot air raises the heat index. Turn the humidifier off once summer humidity is above the 30–50% range.

Related calculators

• The wet bulb calculator shows how far evaporation can cool the same air, which is what sweat and evaporative coolers rely on.
• The dew point calculator turns temperature and humidity into a dew point; a high summer dew point is a quick sign of muggy air.
• The absolute humidity calculator shows how many grams of water vapor each cubic meter of hot, humid air carries.

Limitations

The Rothfusz regression is a curve fit, with an error of about ±1.3 °F according to the NWS, and the NWS notes it isn’t valid for extreme combinations of temperature and humidity beyond the range of Steadman’s data. Below 80 °F the heat index stays close to the air temperature, and the NWS risk table doesn’t flag those values. The heat index also ignores sun, wind and how active you are; for outdoor work and sports the NWS points to the wet bulb globe temperature (WBGT), which includes them.