The formula
How it works
The heat index is how hot it really feels when humidity is added to the air temperature. Humid air stops sweat evaporating, so your body cannot cool itself as well — which is why a muggy 32 °C feels far hotter than a dry one.
FAQ
Why does humidity make heat feel worse?
You cool down by sweating, and sweat only cools you as it evaporates. When the air is already full of moisture, evaporation slows, so heat builds up and the same temperature feels hotter and more dangerous.
When is the heat index used?
It is meant for warm conditions — roughly 27 °C (80 °F) and above. Below that, humidity has little effect on comfort, so the calculator just returns the air temperature.
What heat index level is considered dangerous?
Roughly above 39 °C (103 °F) is classed as a danger zone where heat cramps or heat exhaustion become likely with continued exposure, and above 51 °C (125 °F) is extreme danger with a high risk of heat stroke. Values in the 32–39 °C range already call for caution during prolonged outdoor activity.
Is heat index the same as the “real feel” or “apparent temperature” shown in weather apps?
They are closely related but not always identical — many weather apps blend the NWS heat index with a wind-chill-style adjustment and other factors like sun exposure, so the displayed “feels like” value can differ slightly from a heat index calculated from temperature and humidity alone.
Does wind change the heat index?
No, the standard heat index formula only uses air temperature and relative humidity. Wind is not a factor here — strong wind on a hot, humid day does little to cool the body because sweat evaporation is already limited by the moisture in the air.
Why does the heat index use a different formula below 80°F?
The full Rothfusz regression was fitted to data from hot, humid conditions and becomes unreliable at cooler temperatures. Below about 27 °C (80 °F) humidity barely changes how the temperature feels, so a simple averaging formula is used instead.
How can I reduce my risk when the heat index is high?
Limit strenuous activity during peak heat, drink water regularly rather than waiting until thirsty, wear light and breathable clothing, and seek shade or air conditioning when possible. These steps matter most when the heat index is much higher than the actual air temperature.
About the heat index calculator
This calculator finds the heat index — often called the “feels-like” temperature — from the air temperature and the relative humidity. On a hot day the number on the thermometer only tells half the story, because humidity has a huge effect on how your body copes with heat. The heat index combines the two into one figure that better reflects the risk of overheating, which is why weather services quote it in summer.
How to use it
Choose Celsius or Fahrenheit, then enter the air temperature and the relative humidity as a percentage. The calculator returns the heat index. For example, 32 °C air at 70% humidity feels like roughly 41 °C. The effect grows quickly as humidity climbs: the same temperature at 40% humidity feels much milder. Use the current humidity from a forecast or a hygrometer for the most accurate result.
The formula
The heat index comes from the Rothfusz regression, a long equation of the form with nine terms that mix the temperature and the humidity . It is calculated in Fahrenheit; for Celsius input the value is converted first and the answer converted back. Below about 80 °F a simpler averaging formula is used instead, because the full equation is only fitted for hot conditions.
Where it is used
Weather services issue heat advisories and excessive-heat warnings based on the heat index, not the raw temperature, because it better predicts heat illness. Employers use it in occupational-safety rules to schedule breaks and hydration for outdoor and factory work. Sports leagues, schools and event organisers use it to decide when to modify or cancel activity, and it guides public-health messaging during heatwaves.