Heat Index Calculator
Heat index (apparent temperature) from temperature and humidity using the NWS Rothfusz regression with both adjustments, plus the NWS risk category.
Heat Index Calculator: with the default inputs, heat index is 105.9 °F.
Shade temperature. The heat index assumes shade — full sun adds up to 15 °F.
From the forecast, a hygrometer, or the dew point calculator.
What the combination of heat and humidity feels like in the shade.
- In both scales
- 106 °F (41 °C)
- NWS risk category
- Danger
- What that means
- Heat cramps and heat exhaustion likely; heat stroke possible with prolonged exposure or exertion.
- Degrees above the air temperature
- 15.9In Fahrenheit degrees.
- In full sun, roughly
- 121 °FThe NWS notes full sunshine can add up to 15 °F to the heat index.
- Formula applies?
- Yes — at or above 80 °F and 40% humidity.
Assumptions
- Uses the NWS Rothfusz regression including both the low-humidity and high-humidity adjustments; accuracy is about ±1.3 °F.
- Fitted for T ≥ 80 °F and RH ≥ 40%; below that the NWS simple-average fallback is used and flagged.
- Shade value for a walking adult in light clothing; full sun can add up to 15 °F, and heavy clothing or hard work adds more.
- Not a substitute for wet-bulb globe temperature where work-rest cycles are being planned.
| Relative humidity | Heat index (°F) | Category |
|---|---|---|
| 40% | 91 | Extreme Caution |
| 50% | 95 | Extreme Caution |
| 60% | 100 | Extreme Caution |
| 70% | 106 | Danger |
| 80% | 113 | Danger |
| 90% | 122 | Danger |
| 100% | 132 | Extreme Danger |
NWS categories: Caution 80–90 °F, Extreme Caution 90–103 °F, Danger 103–125 °F, Extreme Danger above 125 °F.
How this is worked out
The formula
NWS Rothfusz regression (T in °F, R in % relative humidity):
HI = −42.379 + 2.04901523·T + 10.14333127·R − 0.22475541·T·R
− 0.00683783·T² − 0.05481717·R² + 0.00122874·T²·R
+ 0.00085282·T·R² − 0.00000199·T²·R²
Adjustments:
R < 13% and 80 ≤ T ≤ 112: subtract ((13 − R)/4) × √((17 − |T − 95|)/17)
R > 85% and 80 ≤ T ≤ 87: add ((R − 85)/10) × ((87 − T)/5)
If the simple estimate ½·(T + 61 + (T − 68)·1.2 + R·0.094) averaged with T is
below 80 °F, that value is used instead.Open How it’s calculated above to see this worked through with your own numbers.
What you enter
- Air temperature
- Shade temperature. The heat index assumes shade — full sun adds up to 15 °F.in °F, °C, K · from 30 to 140 · defaults to 90
- Relative humidity
- From the forecast, a hygrometer, or the dew point calculator.a percentage · from 0 to 100 · defaults to 70
What you get back
- Heat indexmain answer
- What the combination of heat and humidity feels like in the shade.
- In both scales
- NWS risk category
- What that means
- Degrees above the air temperature
- In Fahrenheit degrees.
- In full sun, roughly
- The NWS notes full sunshine can add up to 15 °F to the heat index.
- Formula applies?
What this assumes
- Uses the NWS Rothfusz regression including both the low-humidity and high-humidity adjustments; accuracy is about ±1.3 °F.
- Fitted for T ≥ 80 °F and RH ≥ 40%; below that the NWS simple-average fallback is used and flagged.
- Shade value for a walking adult in light clothing; full sun can add up to 15 °F, and heavy clothing or hard work adds more.
- Not a substitute for wet-bulb globe temperature where work-rest cycles are being planned.
About this calculator
The heat index answers a specific question: at this temperature and this humidity, what shade temperature at a reference humidity would feel the same? It exists because sweating is how humans dump heat, and sweat that cannot evaporate does nothing. At 95 °F and 20% humidity you are uncomfortable; at 95 °F and 70% humidity you are in danger.
The formula
This uses the NWS's Rothfusz regression — the same polynomial behind the heat index chart on weather.gov — including both published adjustments, which most online calculators skip:
- Below 13% humidity, between 80 and 112 °F, the index is reduced. Very dry air lets sweat evaporate faster than the base model assumes.
- Above 85% humidity, between 80 and 87 °F, it is increased.
The regression itself is a fit to Steadman's 1979 tables, with a stated accuracy of about ±1.3 °F. It is fitted for temperatures at or above 80 °F and humidity at or above 40%; outside that box the NWS uses a simple average instead, and this calculator says so when it does.
What the model assumes about you
Steadman's underlying model is very specific, and worth knowing before you argue with the number. It assumes an adult of about 5 ft 7 in and 147 lb, walking at 3.1 mph, in the shade, in a light breeze, wearing long trousers and a short-sleeved shirt, and not overheating. Depart from that and the number moves:
- Full sun can add up to 15 °F. The reported figure is a shade value.
- Still air makes it worse; a real breeze makes it better.
- Heavy clothing, protective gear or hard physical work all push effective heat stress well above the index. This is why OSHA and the military use wet-bulb globe temperature rather than the heat index for work planning.
Reading the results
The category follows the NWS bands: Caution at 80 °F, Extreme Caution at 90, Danger at 103, Extreme Danger at 125. Those describe risk with prolonged exposure or exertion, not standing still in the shade.
Two heat illnesses matter. Heat exhaustion is heavy sweating, cool clammy skin, nausea and weakness — get out of the heat, cool down, drink. Heat stroke is hot skin, often dry, with confusion or collapse; it is a medical emergency, and the difference between the two is measured in minutes.
Frequently asked questions
▸What is the heat index at 90 °F and 70% humidity?
About 106 °F — the Rothfusz regression gives 105.9 °F, which is squarely in the NWS Danger category. In full sun it would feel closer to 120 °F.
▸Why does humidity make heat worse?
You cool yourself by evaporating sweat, and evaporation slows as the air fills with water vapour. At high humidity the sweat runs off instead of evaporating, so it takes heat off your body and does nothing useful.
▸Is the heat index measured in the sun or the shade?
The shade. The NWS notes that full sunshine can add as much as 15 °F to the heat index, which is why a 100 °F index in the shade is a very different afternoon on an unshaded field.
▸How is the heat index different from humidex or wet-bulb globe temperature?
Humidex is the Canadian equivalent, built from the dew point rather than relative humidity, and reads a few degrees differently. Wet-bulb globe temperature adds sun and wind and is what OSHA and the military actually use for work-rest planning.
▸What heat index is dangerous?
The NWS calls 103–125 °F Danger and above 125 °F Extreme Danger, but heat illness starts well below that with exertion, in heavy clothing, or for people who are not acclimatised. The first two weeks of a heat wave are when most heat deaths occur, before people adapt.
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