Calculating your heat index (feels like) is straightforward once you know the Heat Index formula and what each input means. This guide explains the method in plain language, walks through a manual calculation, and gives worked examples you can follow — then you can do it instantly with the Heat Index Calculator.
What is Heat Index?
The Heat Index calculation tells you your heat index (feels like) from a few simple inputs. The figure you are solving for here is the heat index (feels like).
The Heat Index formula
The core formula is:
Heat index (feels like) = -42.379 + 2.04901523 × Temperature + 10.14333127 × Relative humidity - 0.22475541 × Temperature × Relative humidity - 0.00683783 × Temperature ^ 2 - 0.05481717 × Relative humidity ^ 2 + 0.00122874 × Temperature ^ 2 × Relative humidity + 0.00085282 × Temperature × Relative humidity ^ 2 - 0.00000199 × Temperature ^ 2 × Relative humidity ^ 2Here is what each input means:
- Temperature — a value measured in °F. Example: 90 °F.
- Relative humidity — a percentage, such as an annual rate. Example: 7%.
How to calculate it step by step
- Write down the temperature (for example, 90 °F).
- Write down the relative humidity (for example, 7%).
- Apply the formula above to get your heat index (feels like).
- Double-check the result with the Heat Index Calculator.
Worked examples
Example 1
| Input / Output | Value |
|---|---|
| Temperature | 90 °F |
| Relative humidity | 7% |
| Heat index (feels like) | 105.9 |
With temperature of 90 °F and relative humidity of 7%, the heat index (feels like) works out to 105.9.
Example 2
With temperature of 180 °F and relative humidity of 7%, the heat index (feels like) works out to 937.4.
| Result | Value |
|---|---|
| Heat index (feels like) | 937.4 |
Example 3
With temperature of 45 °F and relative humidity of 7%, the heat index (feels like) works out to 111.9.
| Result | Value |
|---|---|
| Heat index (feels like) | 111.9 |
Tips for an accurate result
- Keep your units consistent — mixing, say, months with years or grams with kilograms is the most common source of error.
- Round only at the very end. Rounding inputs early can shift the final answer noticeably.
- Re-run the numbers whenever an input changes, rather than estimating from an old result.
Prefer not to do the maths by hand? — the Heat Index Calculator does it instantly, for free, with the formula and a worked example built in.
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