Heat Index Calculator
Enter the air temperature and relative humidity. The calculator returns the heat index — what hot, humid air actually feels like on exposed skin — using the National Weather Service Rothfusz regression, plus an NWS heat-risk band.
Heat index (felt temperature)
113.1 °F
- Feels hotter than air by
- 18.1 °F
- Heat index in other unit
- 45.1 °C
- Air temperature
- 95.0 °F
- Relative humidity
- 60%
- Risk band
- Danger — Heat cramps and heat exhaustion likely; heatstroke possible with prolonged exposure. Avoid outdoor exertion.
The US National Weather Service computes the heat index from air temperature and relative humidity using the Rothfusz regression: 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 in °F, RH in %). Below 80°F the formula falls back to a simpler Steadman estimator that essentially returns the air temperature.
How to use this calculator
Choose your unit system (°F or °C — humidity is always a percentage). Enter the actual outdoor air temperature and the relative humidity. The calculator returns the heat index in both unit systems, the gap between air temperature and felt temperature, and an NWS heat-risk band. The heat index is meaningful only when it is hot — the formula is calibrated for temperatures of about 80°F (27°C) and above. Below that, humidity has little perceived effect and the answer effectively equals the air temperature.
How the calculation works
When the air is humid, sweat evaporates more slowly, so your body cannot shed heat as efficiently — and you feel hotter than the thermometer reads. The US National Weather Service captures that effect with a multiple regression developed by Lans P. Rothfusz in 1990, fit to Steadman's earlier biometeorological model of human thermal comfort. The full formula is: 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, where T is air temperature in °F and RH is relative humidity in percent. Two small adjustments apply at the corners: a subtraction for very dry air (RH < 13%) at high temperatures and an addition for very humid air (RH > 85%) at moderate temperatures. Below about 80°F the NWS uses a simpler Steadman estimator that essentially returns the air temperature.
Worked example
Take an air temperature of 96°F and 65% relative humidity. Plugging into the Rothfusz equation: HI = -42.379 + 2.04901523·96 + 10.14333127·65 − 0.22475541·96·65 − 0.00683783·96² − 0.05481717·65² + 0.00122874·96²·65 + 0.00085282·96·65² − 0.00000199·96²·65² ≈ 121°F. So 96°F with 65% humidity feels like 121°F — a difference of 25 degrees, and well into the NWS Danger band where heat exhaustion is likely and heatstroke possible with prolonged exposure or exertion. The NWS publishes this exact example in its heat-index documentation.
Frequently asked questions
What is the heat index, exactly?
The heat index is the temperature your body "feels" when the air is hot and humid — specifically, how warm a healthy adult perceives the combination of air temperature and relative humidity in the shade. It is not the actual air temperature (a thermometer always reads the latter) and it does not change how fast water boils or how quickly something heats up in the sun. It is a human-perception quantity, designed to communicate heat-illness risk in numbers people already understand.
Why does humidity make it feel hotter?
Your body keeps itself cool by sweating; sweat absorbs heat as it evaporates off your skin. In dry air, sweat evaporates quickly and the cooling is efficient. In humid air, the surrounding air is already nearly saturated with water vapour, so sweat evaporates slowly (or not at all), and your built-in cooling system stalls. The hotter and stickier it is, the harder it is for your body to lose heat — which is why 95°F at 30% humidity feels very different from 95°F at 80% humidity.
When is the heat index formula valid?
The NWS Rothfusz regression is calibrated for air temperatures of about 80°F (27°C) or higher. Below that threshold the NWS uses a simpler Steadman estimator that essentially returns the air temperature — humidity has little perceived effect when the air itself is comfortable. The formula also assumes the subject is in the shade with a light wind; direct sunlight can add up to 15°F (8°C) to the felt temperature.
Is the heat index the same as "RealFeel" or "apparent temperature"?
Not quite. The heat index uses only air temperature and relative humidity. Apparent temperature (the Australian Bureau of Meteorology version) and proprietary indices like AccuWeather RealFeel combine more variables — wind speed, solar radiation, cloud cover — and produce different numbers. For pure heat-and-humidity discomfort the NWS heat index is the US standard; the WBGT (wet-bulb globe temperature) is the standard for occupational and athletic heat-stress thresholds.
What do the risk bands mean?
The NWS publishes four heat-index bands. Caution (80–90°F) — fatigue possible with prolonged exposure. Extreme Caution (90–103°F) — heat cramps and heat exhaustion possible. Danger (103–125°F) — heat cramps and heat exhaustion likely; heatstroke possible with prolonged exposure or exertion. Extreme Danger (125°F+) — heatstroke highly likely. The bands assume shade with a light breeze; in full sun the effective category can jump by one level.
Why does the answer differ slightly from what my weather app shows?
Most national weather services use the same Rothfusz formula, but apps may round differently or feed it slightly different inputs — a humidity reading from a nearby airport, or an hourly average versus the instantaneous value. Differences of one or two degrees are normal. The math here applies the NWS formula directly to the numbers you enter.
Does the heat index matter for anything besides comfort?
Yes. The NWS uses heat-index thresholds to issue Heat Advisories (typically heat index ≥ 100°F for two or more days) and Excessive Heat Warnings (typically heat index ≥ 105°F day with ≥ 80°F overnight). OSHA references the heat index in its outdoor-worker safety guidance, and many school athletics associations use it to trigger practice modifications. It is the closest single-number proxy most countries have for cardiovascular heat strain on the general public.