Heat Index Calculator
This heat index calculator shows how hot it feels when humidity is added to the air temperature. Enter the temperature in Celsius and the relative humidity, and the heat index calculator returns the apparent temperature in °C and °F with a risk level. It uses the National Weather Service regression, so you can figure heat index the same way forecasters do.
Results
The Heat Index Calculator is part of our Cooling & Heating collection. Browse more cooling & heating to size, convert, and check your work alongside this tool.
What the heat index calculator shows
Sweat cools the body by evaporating, and humid air slows that down, so a humid day feels hotter than the thermometer says. The heat index calculator puts a number on that feeling: the apparent temperature that combines air temperature and relative humidity. Use it as a heat index converter from plain temperature to the "feels like" value, together with the heat stress risk that value carries.
How to compute heat index
To compute heat index, the National Weather Service uses the Rothfusz regression, with T in °F and RH in percent:
Below about 80 °F a simpler formula is used, and small corrections apply in very dry or very humid air. The heat index calculator handles these branches for you, so determining heat index takes one step instead of a long hand calculation.
Figure heat index: worked example
Air at 32 °C (89.6 °F) with 60 percent relative humidity. The heat index calculator gives:
So the day feels about five degrees hotter than the thermometer reads, in the extreme caution band where heat cramps and exhaustion become likely with prolonged activity.
How to use the heat index calculator celsius mode
Enter the air temperature in degrees Celsius and the relative humidity in percent. The heat index calculator converts the temperature to Fahrenheit internally, applies the regression, and returns the heat index in both °C and °F plus a risk level from caution to extreme danger. Use a shaded temperature reading, since direct sun can add several degrees to how hot it feels.
Reading the heat index calculator result
The NWS bands are: under 80 °F no added risk, 80 to 90 °F caution, 90 to 103 °F extreme caution, 103 to 124 °F danger, and 125 °F and above extreme danger. The heat index calculator assumes shade and light wind; full sun can raise the felt temperature by up to 15 °F. Use the risk level to plan breaks, water, and work hours rather than the raw temperature alone.
When to compute heat index
- Planning outdoor work shifts with the heat index calculator
- Setting rest and water breaks for sports and events
- Checking an indoor heat index in warehouses or kitchens
- Comparing forecast days by how hot they will feel
Common heat index calculator mistakes
- Using a sun reading. Measure air temperature in shade; the formula already assumes shade.
- Mixing up dew point and humidity. Enter relative humidity in percent, not the dew point.
- Ignoring exertion. Heavy work raises risk at any heat index level.
Tips for accurate results
- Take temperature and humidity at the same time and place.
- Add a safety margin in full sun or heavy clothing.
- Recheck during the day, since humidity often peaks in the morning.
Determining heat index: why humidity matters
The body sheds heat mainly by sweating, and sweat only cools when it evaporates. At high humidity the air already holds much of the water it can, so evaporation slows and core temperature climbs. That is why determining heat index uses humidity as heavily as temperature: at 35 °C, moving from 40 to 80 percent humidity pushes the felt temperature from about 37 °C to well over 50 °C. Buildings feel this too, and the heat load calculator sizes the cooling that keeps indoor conditions out of the danger bands.
Online heat index calculator vs WBGT
An online heat index calculator is quick and needs only two readings, but it is not the only heat stress measure. Wet bulb globe temperature (WBGT) adds sun and wind and is used by militaries and sports bodies for activity limits. Heat index is the right tool for general public warnings and daily planning; WBGT is better for strict occupational rules in direct sun.
Heat index calculator chart (°C)
| Air temp | 40% RH | 60% RH | 80% RH |
|---|---|---|---|
| 28 °C | 28 °C | 30 °C | 32 °C |
| 30 °C | 31 °C | 33 °C | 38 °C |
| 32 °C | 33 °C | 37 °C | 45 °C |
| 35 °C | 37 °C | 45 °C | 58 °C |
Values are rounded and assume shade.
Heat Index Calculator FAQ
What is the heat index?
It is the apparent temperature that combines air temperature and relative humidity, showing how hot it feels to the body. The heat index calculator returns it in °C and °F.
How do you figure heat index?
Use the NWS Rothfusz regression with temperature in °F and humidity in percent, then convert back if you work in Celsius. The calculator does both steps.
Does the heat index include sunlight?
No. It assumes shade and light wind. Direct sunlight can add up to about 8 °C (15 °F) to the felt temperature.
Is the heat index the same as the humidex?
No. Humidex is a Canadian index based on dew point. The two give similar trends but different numbers, so do not mix them.