Free heat index calculator
Enter the air temperature and relative humidity to get the heat index — the real feels-like temperature — in °F and °C, with its NWS risk category, updated live, as you type.
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NWS Rothfusz heat index regression. Valid for T ≥ 80°F and RH ≥ 40%.
Results are estimates. Consult a professional.
What the heat index tells you
The heat index is the "feels-like" temperature in hot weather — how hot the air actually feels to your body once humidity is factored in. This heat index calculator takes two readings you already have, the air temperature in °F and the relative humidity in percent, and returns a single apparent temperature plus the National Weather Service risk category that goes with it.
The number matters because the thermometer alone undersells the danger on a muggy day. At 90 °F and 70% humidity the air feels like 106 °F, not 90 °F. That gap is why a humid afternoon sends people to the emergency room while a hotter but bone-dry one does not — and it is the gap the heat index is built to capture.
Why humidity makes heat dangerous
Your body has one main way to shed heat when the air is hot: sweat evaporating off your skin. Each gram of sweat that turns to vapor carries a large amount of heat away with it. Evaporative cooling is what keeps your core temperature near 98.6 °F on a scorching day.
Humidity is what shuts that system down. Humid air is already crowded with water vapor, so there is little room for more. Your sweat beads up and drips instead of evaporating, and the cooling stops. The air temperature has not changed, but your body can no longer keep up — so the same 95 °F feels mild at 20% humidity and life-threatening at 80%.
How the heat index is calculated
The heat index comes from a polynomial regression that meteorologist Lans Rothfusz fit in 1990. It approximates a much more complex 1979 model of human heat balance — sweat rate, skin resistance, clothing, blood flow — with a single equation in two variables a weather station already measures: air temperature and relative humidity.
Two corrections refine the edges of the valid range. When humidity is very low (under 13%) and the air is 80–112 °F, a small amount is subtracted, because dry heat feels slightly less oppressive. When humidity is very high (over 85%) and the air is a milder 80–87 °F, a small amount is added. The calculator applies both adjustments automatically, exactly as the NWS does.
The equation and its low- and high-humidity adjustments are the National Weather Service implementation of Rothfusz, L.P. (1990), "The Heat Index 'Equation'," NWS Technical Attachment SR 90-23.Air temperature and humidity: the two inputs explained
The heat index needs only two numbers, and each one does a distinct job. Getting them right is the whole game — the equation is only as good as the readings you feed it.
Air temperature (°F)
This is the ordinary dry-bulb temperature from a thermometer or weather app, entered in Fahrenheit. The heat index is only meaningful in heat: the categories begin at 80 °F, and the full regression is designed for that range and above. Below 80 °F the result tracks the air temperature closely because there is little heat stress to amplify.
Relative humidity (%)
This is the share of water vapor the air is holding relative to the most it could hold at that temperature, reported as a percentage by weather apps and home hygrometers. Enter the current value, not the daily high — humidity often falls through the afternoon as the air warms. Higher humidity means less evaporative cooling, which pushes the heat index up fast.
A worked example using the heat index calculator
Dan checks the forecast before a midday shift outdoors: the air temperature is 90 °F and the relative humidity is 70%. Ninety degrees sounds manageable, but the humidity is high, so he wants the real feels-like figure before deciding how often the crew should break.
Step 1 — Enter the two readings
Air temperature 90 °F, relative humidity 70%. Because 90 °F is at or above 80 °F, the calculator uses the full Rothfusz regression rather than the simple form.
Step 2 — Apply the Rothfusz regression
Feeding T = 90 and R = 70 into the nine-term equation gives a heat index of 105.9 °F, which is 41.1 °C. Neither adjustment applies, because humidity is between 13% and 85%.
Step 3 — Read the risk category
105.9 °F lands in the danger band (103–124 °F), where heat cramps and heat exhaustion are likely and heatstroke is possible with prolonged exposure or activity. The plain 90 °F reading gave no hint of that — the 70% humidity added 16 °F of apparent heat on its own.
The NWS heat index risk categories
The National Weather Service sorts the heat index into four warning bands, each tied to the heat illness it makes likely for someone active or exposed for a sustained period. The thresholds below are the exact °F breakpoints this calculator uses, with the Celsius equivalents in parentheses.
| Heat index | Category | What it means |
|---|---|---|
| 80–89 °F (27–32 °C) | Caution | Fatigue possible with prolonged exposure and activity. |
| 90–102 °F (32–39 °C) | Extreme caution | Heat cramps and heat exhaustion possible; heatstroke with continued exposure. |
| 103–124 °F (39–51 °C) | Danger | Heat cramps and heat exhaustion likely; heatstroke possible. |
| 125 °F and above (52 °C+) | Extreme danger | Heatstroke highly likely. |
Categories and effects from the U.S. National Weather Service heat index. Below 80 °F the calculator reports no heat-stress category. These bands assume shade; full sun pushes the real risk a category higher.
Category boundaries and the associated heat-illness effects are taken from the National Weather Service heat-index safety guidance.Heat index vs. wet-bulb temperature
Both numbers come from temperature and humidity, and both warn about heat — but they answer different questions, and confusing them leads people to underrate true danger.
- Heat index — a perceived, "feels-like" temperature for an average person in the shade. It is a comfort-and-risk estimate, not a hard physical limit, and it can read well above the true air temperature.
- Wet-bulb temperature — a physical temperature: the lowest a wet thermometer can reach by evaporation, and the ceiling on how much the body can cool itself. A sustained wet-bulb near 95 °F (35 °C) is the survivability limit, beyond which even a healthy person at rest overheats.
Use the heat index for everyday comfort and exposure decisions, and the wet-bulb temperature calculator for the absolute survivability threshold. For the cold-weather equivalent of "feels-like," see the wind chill calculator.
Why wind is not part of the heat index
Wind chill folds wind speed in because moving air strips warmth from the skin faster — in the cold, wind is the danger. Heat is different. Below body temperature, a breeze still helps by speeding evaporation, but the effect is small and swamped by humidity. Above body temperature, around 98–99 °F, wind can make things worse: hot air blowing over the skin adds heat instead of removing it, the way a convection oven cooks faster than a still one.
Because wind's effect on perceived heat is small, variable, and sometimes reversed, the NWS leaves it out and defines the heat index for light wind only. The trade-off is a formula that needs just two easily measured inputs and stays reliable across the conditions that matter most — at the cost of being conservative when a hot wind is actually blowing.
Heat index terms defined
How accurate is this heat index calculator?
The calculator uses the exact National Weather Service implementation of the Rothfusz regression, including the low- and high-humidity adjustments, so its output matches an official NWS heat index value to the decimal. The regression itself is accurate to within about ±1.3 °F of the detailed model it approximates across its valid range.
Two practical limits are worth knowing. First, the full regression is built for hot conditions; below 80 °F the calculator switches to the simpler form, which tracks the air temperature closely and reports no heat-stress category. Second, the heat index is a guide for the general population — vulnerable people reach danger at lower readings. Treat the result as a solid estimate of how hot it feels, and let it inform precautions rather than replace official heat warnings.
Frequently asked questions about the free heat index calculator
About this heat index calculator
This calculator runs entirely in your browser. Nothing you type is sent anywhere — the heat index and its NWS risk category are computed locally from the National Weather Service Rothfusz regression as you adjust the air temperature and humidity, so you can see exactly how much humidity is adding to the real feels-like temperature.