Heat Index Calculator
Calculate heat index and apparent temperature in hot weather
Actual air temperature
Relative humidity percentage
Heat Index
34°C
Feels Hotter By
4°C
Extreme Caution
Heat cramps and exhaustion possible with prolonged exposure
Dew Point
74°F (23°C)
Very humid and uncomfortable
Actual Temperature
85°F (29°C)
Humidity: 70%
Heat Index Guidelines
How it works
The heat index is the “feels-like” temperature when humidity is added to heat. High humidity slows sweat evaporation, so your body cools less effectively and the air feels hotter than the thermometer reads. It combines temperature and relative humidity into one apparent temperature.
Apparent temperature
Heat index = f(air temperature, relative humidity) — rises sharply as both climb
- temperature
- actual air temperature
- humidity
- relative humidity (%)
Worked example
- Air temperature = 90 °F
- Relative humidity = 70%
- Combine via the heat-index model
Feels like ≈ 105 °F — into the “danger” band for heat illness.
Good to know
- The index assumes shade; direct sun can add up to ~15 °F to how hot it feels.
- Above ~103 °F apparent temperature, heat cramps and exhaustion become likely — hydrate and rest.
- It only matters above roughly 80 °F; in cold weather, wind chill is the relevant adjustment.
Related Calculators
Frequently Asked Questions
What is the heat index?
The heat index is the "feels like" temperature combining air temperature and relative humidity. It estimates how hot conditions feel to the human body, which cools itself by evaporating sweat — a process humidity slows down.
Why does humidity make it feel hotter?
Your body sheds heat mainly through sweat evaporation. High humidity means the air already holds a lot of moisture, so sweat evaporates slowly and cooling becomes inefficient. At 95°F, raising humidity from 30% to 70% pushes the heat index from about 96°F to over 120°F.
At what heat index do conditions become dangerous?
The US National Weather Service classifies 80-90°F as caution, 90-103°F as extreme caution, 103-124°F as danger, and 125°F+ as extreme danger. In the danger range, heat cramps and heat exhaustion are likely and heat stroke becomes possible with prolonged exposure or activity.
Does the heat index apply in direct sunlight?
Standard heat index values are computed for shady, light-wind conditions. Full sunshine can raise the effective heat index by up to about 15°F, so outdoor workers and athletes in direct sun face significantly more stress than the published number suggests.
What's the difference between heat index and wind chill?
They're opposite-season counterparts: heat index combines temperature with humidity to show how hot it feels, while wind chill combines cold temperatures with wind speed to show how cold it feels. Both measure apparent temperature rather than what the thermometer reads.