Heat index calculator
What the heat index means
The National Weather Service calls the heat index "a measure of how hot it really feels" when humidity is combined with the air temperature. These are the classes on its heat index chart, for long exposure or activity:
Source: NOAA / National Weather Service heat index chart.
Heat index at a glance
The same formula as the calculator, for a few common combinations (°F). Read across to see how much a few extra percent of humidity add.
| Temp \ RH | 40% | 50% | 60% | 70% | 80% |
|---|---|---|---|---|---|
| 80°F | 80 | 81 | 82 | 83 | 84 |
| 85°F | 84 | 86 | 89 | 93 | 97 |
| 90°F | 91 | 95 | 100 | 106 | 113 |
| 95°F | 99 | 105 | 113 | 123 | 134 |
| 100°F | 109 | 118 | 129 | 143 | 158 |
| 105°F | 121 | 134 | 149 | 166 | 187 |
Calculated with the NWS formula (shade, light wind). Colors follow the classes above.
Know the signs of heat illness
A high heat index is a warning for your body. The National Weather Service lists these signs and first steps:
Painful muscle cramps and spasms, usually in the legs and abdomen, and heavy sweating.
Press firmly on the cramping muscle or gently massage it, and give sips of water (stop if the person feels sick). Get immediate medical help if cramps last longer than 1 hour.
Heavy sweating, weakness, cool, pale and clammy skin, a fast weak pulse, cramps, dizziness, nausea or vomiting, headache, fainting.
Move to a cooler, preferably air-conditioned place, loosen clothing, use cool wet cloths or a cool bath and offer sips of water. Get immediate medical help if the person vomits or symptoms get worse or last longer than 1 hour.
Throbbing headache, confusion, slurred speech, nausea, dizziness, body temperature above 103°F, hot red skin (dry or damp), a rapid strong pulse, fainting or loss of consciousness.
Call 911 or get the person to a hospital immediately. Move them somewhere cooler and cool them with cloths or a bath. Do not give fluids.
Staying safe on hot, humid days
Tips from the National Weather Service page on staying safe during extreme heat. Want to track it at home? See our indoor temperature and humidity guide for heat waves.
Heat index, WBGT or HeatRisk?
The National Weather Service uses three heat numbers. They answer different questions:
Heat index
Temperature and humidity only, calculated for shady areas with a light wind. Quick to work out from any weather station.
Wet Bulb Globe Temperature
Adds wind speed, sun angle and cloud cover. The NWS calls it a "particularly effective indicator of heat stress" for outdoor workers and athletes.
NWS HeatRisk
A color scale from 0 (green, little to no risk) to 4 (magenta, extreme) for the next seven days. It is "supplementary to official NWS heat watches, warnings, and advisories".
None of them replaces your local alerts. Our guide to heat advisories and extreme heat warnings explains what the official alerts mean.
How the calculator works
We use the algorithm of the National Weather Service's Weather Prediction Center. First a simple formula by Steadman; when the result averaged with the temperature reaches 80°F, the more exact Rothfusz regression takes over, with two small corrections for very dry air (below 13% humidity, 80°F to 112°F) and very humid air (above 85% humidity, 80°F to 87°F). The NWS notes that the regression is not valid for extreme combinations outside Steadman's data, so treat very high results as "dangerous", not as an exact number.
Quick answers
Can the heat index be lower than the temperature?
Yes. In very dry air sweat evaporates easily, so the formula can give a lower number. With this calculator, 100°F at 10% humidity gives a heat index of 94°F.
Does the sun make it worse?
Yes. The heat index is made for shade and a light wind. The National Weather Service says full sunshine can raise heat index values by up to 15°F.
Where do I get the humidity number?
From your weather station, your local forecast or a weather app. A sensor in the sun or right next to a wall reads wrong; our guide to checking the outside temperature accurately explains good placement.
Why does the calculator stop at 40°F and 130°F?
The heat index is about hot weather: below 80°F it is not in the NWS heat classes at all. We accept 40°F to 130°F (about 4.4°C to 54.4°C) so typing errors do not turn into strange results.
Sources: NWS Weather Prediction Center: The Heat Index Equation · National Weather Service: Heat Index · NWS Amarillo: What is the heat index? · NWS: Heat-related illnesses · NWS: Staying safe during extreme heat · NWS Tulsa: Wet Bulb Globe Temperature · NWS HeatRisk
