What this calculator does
Heat index is the "feels like" temperature that combines actual air temperature with relative humidity. High humidity slows the evaporation of sweat, which is the body's main way of shedding heat, so the same air temperature can feel noticeably hotter and place more strain on the body as humidity rises.
This heat index calculator uses the NOAA Rothfusz regression, the standard formula used by weather services to publish heat index and heat advisory figures, with the low-humidity and high-humidity corrections NOAA applies at the edges of its normal range.
The formula
The temperature is converted to Fahrenheit, since the published regression is defined in that unit, and combined with relative humidity through the Rothfusz polynomial. Below about 27°C (80°F) a simpler linear approximation is used instead, since the full regression is only validated at higher temperatures. Small corrections are added at very low humidity above 80°F and at very high humidity in the high 80s°F, matching NOAA's published method, before converting the result back to Celsius.
| Term | Meaning |
|---|---|
| Heat index | The apparent, or "feels like", temperature once humidity is accounted for. |
| Relative humidity | How much water vapour the air is holding compared to the maximum it could hold at that temperature. |
| Rothfusz regression | The polynomial fit to a more complex thermodynamic model that NOAA uses to publish heat index values quickly. |
The inputs explained
| Field | What to enter |
|---|---|
| Air temperature (°C) | The actual, measured air temperature in the shade, not in direct sun. |
| Relative humidity (%) | The relative humidity as a percentage, from a weather station or forecast. |
When to use it
Reading a heat warning
Weather services issue heat advisories based on heat index, not raw temperature, so converting a forecast temperature and humidity yourself shows how close conditions are to an advisory threshold.
Planning outdoor work or sport
Heat index above roughly 32°C (90°F) is when caution guidance for outdoor exertion typically starts, well before the raw air temperature alone would suggest a problem.
Comparing a humid day against a dry day
Two locations can report the same air temperature yet feel very different; heat index puts a single comparable number on that difference.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How heat index changes with humidity at 32°C
The same 32°C air temperature across a range of relative humidity levels.
| Relative humidity | Heat index ("feels like") | Heat stress category |
|---|---|---|
| 30% | 30.8 °C | caution, fatigue possible with prolonged exposure |
| 40% | 32.3 °C | extreme caution, heat cramps or exhaustion possible |
| 50% | 34.4 °C | extreme caution, heat cramps or exhaustion possible |
| 60% | 37.1 °C | extreme caution, heat cramps or exhaustion possible |
| 70% | 40.4 °C | extreme caution, heat cramps or exhaustion possible |
| 80% | 44.4 °C | danger, heat exhaustion likely and heatstroke possible |
How heat index changes with temperature at 60% humidity
A range of air temperatures at a constant 60% relative humidity.
| Air temperature | Heat index ("feels like") | Difference from actual air temperature |
|---|---|---|
| 26°C | 26.2 °C | +0.2 °C |
| 28°C | 29.4 °C | +1.4 °C |
| 30°C | 32.8 °C | +2.8 °C |
| 32°C | 37.1 °C | +5.1 °C |
| 34°C | 42.2 °C | +8.2 °C |
| 36°C | 48.1 °C | +12.1 °C |
Questions
Is heat index the same as the "real feel" or "feels like" temperature on weather apps?
It is closely related but not always identical. Some apps blend heat index with wind chill and other factors into a single "feels like" figure, while heat index specifically is the NOAA formula combining just temperature and humidity.
Why does the formula switch to a simpler calculation at lower temperatures?
The full Rothfusz regression was fitted to conditions above about 27°C (80°F) and can behave oddly outside that range, so NOAA specifies a simpler linear formula for cooler, less humid conditions where heat stress is less of a concern anyway.
Does heat index apply in the shade or in direct sun?
The published heat index formula assumes shaded conditions. Direct sun exposure can add the equivalent of several more degrees of felt heat on top of the calculated heat index.
What heat index is considered dangerous?
US National Weather Service guidance generally treats a heat index of 39 to 51°C (103 to 124°F) as a danger zone with likely heat exhaustion, and above that as extreme danger with heatstroke highly likely, though vulnerable individuals can be affected at lower levels.
For the humidity inputs themselves, see the relative humidity calculator and the wet-bulb temperature calculator.