Heat Index
This free heat index calculator finds how hot it feels from air temperature and relative humidity, using the US National Weather Service's official formula. Nothing you enter is sent anywhere or stored.
How to Use It
- Enter the air temperature.
- Enter the relative humidity.
- The heat index updates instantly, no submit button.
How It Works
Sweat evaporating off skin is the body's main way of shedding heat, and humid air slows that evaporation down, since it already holds more moisture and has less room to absorb more. The National Weather Service's heat index formula, a regression fitted to skin heat transfer data, combines air temperature and relative humidity into a single "feels like" number that reflects that slowdown.
Formula
heat index = −42.379 + 2.049T + 10.143RH − 0.225TRH − 0.0068T² − 0.0548RH² + 0.00123T²RH + 0.00085TRH² − 0.000002T²RH²
A worked example: at 90°F with 70% relative humidity, the formula gives a heat index of about 105.9°F, well into the range where heat related illness becomes a real risk. At 100°F with 50% relative humidity, it climbs further to about 118.3°F. Below roughly 80°F, a simpler formula applies instead, since the full regression was only fitted for hot, humid conditions.
Everything runs client-side in your browser. Nothing you enter is sent anywhere or stored.
Frequently Asked Questions
What does heat index actually measure?
Heat index measures how hot it actually feels once humidity is factored in, not just the air temperature a thermometer reads. Sweat evaporating off skin is the body's main way of cooling down, and humid air already holds more moisture, so it can absorb less sweat, slowing evaporation and making the same air temperature feel hotter and more dangerous.
Why does heat index only fully kick in around 80°F and up?
Below that range, humidity doesn't add much of a meaningful heat burden on top of the actual temperature, so this calculator uses a simpler estimate rather than the full regression formula, which was only fitted and validated for hot, humid conditions.
Why can very low humidity make a hot day feel slightly less extreme?
Very dry air lets sweat evaporate quickly, which is an effective cooling mechanism, so the full heat index formula includes a downward correction for low humidity between 80°F and 112°F. It does not mean a hot, dry day is comfortable, only that it feels closer to the actual air temperature than an equally hot, humid day would.
Can you give a real-world example?
A humid 90°F day with 70% relative humidity has a heat index of about 106°F, solidly into the range the National Weather Service flags for increased risk of heat related illness. A dry 90°F day with the same thermometer reading feels far less extreme, since evaporation can still do most of its job.