Heat Index Calculator
What the temperature feels like once humidity is taken into account — the "apparent temperature" weather services warn on.
Your body sheds heat by evaporating sweat, and humid air takes it up more slowly.
How the heat index calculator works
Your body sheds heat by evaporating sweat, and humid air takes it up more slowly. The heat index is how hot the air would have to be at a reference humidity to feel the way the actual conditions do.
The formula is the Rothfusz regression the US National Weather Service uses, a curve fitted to a detailed heat-balance model rather than a physical law. It is valid above roughly 27 °C (80 °F); below that it stops meaning anything, so this page says so instead of extrapolating.
The official figures assume shade and a light breeze. In full sun the effective heat index can be up to 8 °C higher.
Formula: Rothfusz regression on temperature (°F) and relative humidity, with low- and high-humidity adjustments
Worked examples
| Inputs | Feels like | Note |
|---|---|---|
| 90 °F at 70% humidity | 105.9 °F | feels like 106 °F — danger |
| 32 °C at 70% humidity | 40.4 °C | feels like about 41 °C |
| Hot but dry | 35.7 °C | feels close to the actual temperature |
FAQFrequently asked questions
What is the heat index?
The apparent temperature — how hot it feels once humidity is included, because humid air slows the evaporation of sweat.
Why is there no heat index on a cool day?
Because the formula is fitted only above about 27 °C. Below that humidity barely changes how hot you feel, so no meaningful figure exists.
Does sunshine change it?
Yes, substantially. The official figure assumes shade; direct sun can add up to 8 °C.
What heat index is dangerous?
Above about 39 °C (103 °F) heat exhaustion becomes likely and heat stroke is possible with exertion.
Is the heat index the same as wet-bulb temperature?
No. Wet-bulb is a directly measurable physical temperature; the heat index is a fitted comfort model.
Where these figures come from
- UK DESNZ / DEFRA — Greenhouse gas reporting: conversion factors — the flight, fuel and waste emission factors used here
- Scarborough et al. (2014), Climatic Change — Dietary greenhouse gas emissions — the per-diet daily emission figures
- US EPA — Technical Assistance Document for the Reporting of Daily Air Quality (AQI) — the PM2.5 breakpoints the air quality calculator uses
- UK Department for Energy Security and Net Zero — conversion factors — the official UK emission factors
Last checked: September 2026. Factors are taken from published government and peer-reviewed sources, each named on the page with its year.