Heat Index Calculator
Calculate the apparent temperature (feels like) from temperature and humidity.
About Heat Index Calculator
Heat index combines temperature and humidity to show how hot it actually feels. High humidity reduces sweat evaporation.
$$HI = f(T, RH)$$
How to use this calculator
- Enter the air temperature in degrees Fahrenheit.
- Enter the relative humidity as a percentage.
- Click calculate to see the apparent temperature and the step-by-step result.
- Use the result to judge how hot conditions may feel to your body.
The formula explained
$$ \text{Heat Index} = -8.784695 + 1.61139411T + 2.338549H - 0.14611605TH - 0.012308094T^2 - 0.016424828H^2 + 0.002211732T^2H + 0.00072546TH^2 - 0.000003582T^2H^2 $$
- \(T\) = air temperature in degrees Fahrenheit
- \(H\) = relative humidity in percent
- \(\text{Heat Index}\) = apparent temperature, or how hot it feels
Step by step method
- The calculator takes the actual air temperature and the humidity you provide.
- It applies the heat index formula, which estimates how hot it feels by combining the effects of heat and moisture in the air.
- It then shows the apparent temperature so you can compare it with the actual temperature.
- Higher humidity usually makes the heat index feel higher than the air temperature alone.
Worked example
Suppose the air temperature is 90 degrees Fahrenheit and the relative humidity is 70 percent.
- Use the inputs \(T = 90\) and \(H = 70\).
- Plug them into the formula to get a heat index of about \(105.1\) degrees Fahrenheit.
- That means the air feels about 15 degrees hotter than the measured temperature.
Answer. The heat index is about 105.1 degrees Fahrenheit.
Tips and common mistakes
- Use degrees Fahrenheit, since this formula is based on Fahrenheit inputs.
- Make sure humidity is a percent, not a decimal.
- The heat index is an estimate, so treat it as a guide rather than an exact body temperature reading.
- Very low humidity or mild temperatures can make the result close to the actual air temperature.
Frequently asked questions
What does the heat index tell me?+
It estimates how hot the weather feels to people, not just the air temperature. It combines temperature and humidity because humid air slows the body's ability to cool itself through sweat.
Do I need to use Fahrenheit or Celsius?+
This calculator uses the standard heat index formula in Fahrenheit. If you have a Celsius temperature, convert it before entering it.
Why does humidity matter so much?+
When humidity is high, sweat does not evaporate as easily. That makes the same air temperature feel hotter and more tiring.
Is the heat index the same as actual temperature?+
No, it is an apparent temperature. The actual temperature is what the thermometer reads, while the heat index estimates how the weather feels.
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