Weather and Garden: Wind Chill, Heat Index and Watering
The numbers behind the weather and a thriving garden — how wind chill and heat index work, what dew point tells you, when each becomes dangerous, and how to water and harvest rain.
Heat Index Calculator
Verified formula Updated Jun 2026 Private — runs on your device
Verified formula
Private
Heat index (feels like)
105.9
For general information only — not financial, tax, legal or medical advice. Verify before you rely on it.
How to use the Heat Index Calculator
The Heat Index Calculator works out your heat index (feels like) in an instant. Enter temperature and relative humidity and the result updates as you type — it is free, needs no sign-up, and runs entirely in your browser so your figures stay private.
- Enter the temperature.
- Enter the relative humidity.
- Read off your heat index (feels like) — the calculator updates automatically, with no button to press.
Formula
The Heat Index Calculator uses the formula:
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 ^ 2
Worked example
For example, with temperature of 90 °F and relative humidity of 7%, the heat index (feels like) is 105.9.
| Temperature | 90 °F |
|---|---|
| Relative humidity | 7% |
| Heat index (feels like) | 105.9 |
|---|
Results are estimates for educational use, not professional advice.
Frequently asked questions
It is how hot it feels when humidity is combined with air temperature. At 90°F and 70% humidity it feels around 106°F.
High humidity slows sweat evaporation, the body's main cooling method, so you feel hotter than the thermometer reads.
Temperature in Fahrenheit and humidity as a percentage. The formula is most accurate above about 80°F.
Above about 103°F it signals heat exhaustion risk, and over 124°F is extremely dangerous. Stay hydrated and cool.
The Heat Index Calculator uses the formula: 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 ^ 2. For example, with temperature of 90 °F and relative humidity of 7%, the heat index (feels like) is 105.9.
Enter the temperature. Enter the relative humidity. Read off your heat index (feels like) — the calculator updates automatically, with no button to press.
Related guides
Heat Index Table: Heat index (feels like) by Temperature
Reference table of heat index (feels like) for Heat Index across a range of temperature values — exact, engine-computed figures you can read off at a glance.
How to Calculate Heat Index: Formula, Steps & Examples
Learn how to calculate Heat Index — the formula explained step by step, with worked examples and a free calculator to check your answer.
Rainfall Volume Table: Water volume by Catchment area
Reference table of water volume for Rainfall Volume across a range of catchment area values — exact, engine-computed figures you can read off at a glance.