Heat Index Calculator
The "feels like" temperature from heat and humidity, using the NWS Rothfusz formula.
Results update as you type.
About this calculator
The heat index, or apparent temperature, is how hot it really feels when humidity is combined with the air temperature, and this heat index calculator computes it from those two inputs. The body cools itself by evaporating sweat, but humid air is already close to saturated so sweat evaporates slowly, the cooling falters, and it feels hotter than the thermometer says. The tool uses the U.S. National Weather Service Rothfusz regression, a multi-term fit of temperature and relative humidity, together with its standard low- and high-humidity corrections.
As a worked example, 90 °F air at 60% relative humidity carries a heat index of about 100 °F — ten degrees hotter than the reading — and pushing the humidity to 80% drives it past 110 °F. The calculator returns the feels-like temperature, how much hotter that is than the air, and a humidity table so you can see how quickly the danger rises as the air grows muggier at a fixed temperature.
The regression is most accurate around 80 °F (27 °C) and above with meaningful humidity; below about 80 °F it reports no heat stress because humidity has little effect on comfort in cool air. It also grades the reading into the NWS danger bands — caution, extreme caution, danger and extreme danger — so a heat index above 103 °F signals a real risk of heat cramps or exhaustion, and above 125 °F a high risk of heat stroke, which is vital for planning outdoor work, sport and travel in hot weather.
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. High humidity slows sweat evaporation, so the body sheds heat less easily and feels hotter than the thermometer reads.
What formula does this use?
The NWS Rothfusz regression: HI = −42.379 + 2.04901523T + 10.14333127R − 0.22475541TR − … (with T in °F and R in %), plus corrections for very low or very high humidity. It is most accurate at about 80 °F and warmer.
What do the danger levels mean?
Roughly: 80–90 °F is caution, 90–103 °F extreme caution, 103–124 °F danger (heat cramps or exhaustion likely) and 125 °F or above extreme danger with a high risk of heat stroke.
Why does humidity make heat feel worse?
Sweating cools you only when the sweat evaporates. In humid air, already near saturation, evaporation slows, so the body sheds heat poorly and the same 90 °F feels far hotter at 80% humidity than at 30%.
Why does the calculator say "no heat stress" on a warm day?
The heat index is only defined for hot conditions — roughly 80 °F (27 °C) and up. Below that, humidity barely changes how the air feels, so the tool reports the plain air temperature rather than an apparent one.
How is heat index different from wind chill?
They are opposite corrections: heat index adds apparent warmth from humidity in hot weather, while wind chill subtracts apparent warmth from wind in cold weather. Heat index needs temperature and humidity; wind chill needs temperature and wind speed.
Does the heat index assume shade or sun?
It is calculated for shade. Direct sunlight can add as much as 15 °F to the felt temperature, so the real feels-like value outdoors in full sun is higher than the figure this calculator returns.
API — use this calculator from code
Call this calculator as a free JSON endpoint — no key required. Send the field values below as query parameters or JSON. Read the full API docs →
Endpoint
GET https://calculator.free/api/v1/heat-index/curl
curl "https://calculator.free/api/v1/heat-index/?units=imperial&temp=90&humidity=60"JavaScript fetch()
const r = await fetch(
"https://calculator.free/api/v1/heat-index/?" + new URLSearchParams({
"units": "imperial",
"temp": "90",
"humidity": "60"
}));
const data = await r.json();
console.log(data.results);Results are estimates for general guidance only, not financial, medical or tax advice.