Heat Index Calculator
Calculate how hot it feels — heat index from air temperature and relative humidity in °C and °F.
Feels like
50.34 °C
122.61 °F — heat index
The heat index combines temperature and humidity to show how hot it actually feels. The formula applies at or above 26.7 °C with at least 40% humidity — below that, the index equals the air temperature.
Last updated: January 2026
How this calculator is verified
Checked by True Calculator automated test suite on
- Formula verified against a published worked example in the automated test suite
- Edge cases (zero, negative, boundary and unit-mismatch inputs) covered by unit tests
The full verification method is on our how we verify page. Found an error? Tell us and we will re-check it.
When to Use This Calculator
Use this calculator during Indian summers, when heat advisories and 'feels like' figures dominate weather apps. Employers planning outdoor shifts for construction, delivery riders, farmers and street vendors use the index to set rest breaks and hydration schedules — the Ministry of Earth Sciences issues heat-wave guidance built on the same temperature–humidity logic. Schools use it to decide whether morning assemblies and sports days should move indoors. It is also the number behind every 'why does it feel like 50 degrees' conversation, giving a concrete, comparable figure instead of a vague impression. Even inside a home, knowing the index helps decide when to run the fan versus the air conditioner on muggy days.
How to Use This Calculator
- Step 1: Read the current air temperature in °C.
- Step 2: Read the relative humidity percentage, from a hygrometer or weather report.
- Step 3: Enter both and get the heat index — how hot the air actually feels on skin.
- Step 4: Note the 'applies' note: below 26.7 °C or 40% humidity, the index equals the air temperature.
Worked Example
On a hot Indian summer afternoon at 40 °C with 50% humidity, the heat index reaches 54.77 °C — far above the thermometer reading. Sweat cannot evaporate quickly in that much moisture, so the body feels the heat of a 55 °C day. At 35 °C and 70% humidity the index is 50.34 °C. Both comfortably exceed the 41 °C threshold where heat advisories recommend limiting outdoor work, and both sit in the 'danger' band for heat illness.
Tips and Common Mistakes
- •Above a heat index of 41 °C, schedule outdoor work for early morning or late evening.
- •Drink water before you feel thirsty — the body loses moisture faster than thirst signals it.
- •Use the index, not the thermometer, when deciding if it is safe to walk or exercise outdoors.
- ✗Do not use the heat index below 26.7 °C or below 40% humidity — the formula is not valid there.
- ✗Do not ignore humidity in dry but hot regions like Rajasthan; 40 °C at 20% humidity feels far less brutal than at 60%.
Frequently Asked Questions
What is the heat index?
The heat index ('feels like' temperature) combines air temperature and relative humidity to estimate how hot it feels. At 35 °C with 70% humidity, the heat index is about 50 °C, even though the thermometer reads 35 °C.
How is the heat index calculated?
It uses the Rothfusz regression on temperature in °F and humidity: HI = −42.379 + 2.049T + 10.143RH − 0.225T·RH − 0.00684T² − 0.0548RH² + 0.00123T²·RH + 0.000853T·RH² − 0.000002T²·RH², with adjustments at extremes.
When does the heat index apply?
Only at or above 26.7 °C (80 °F) with relative humidity of at least 40%. Below those thresholds the index equals the air temperature — this calculator tells you whether the formula applied.
Why does humidity make heat feel worse?
Sweat cools the body by evaporating. In humid air, evaporation slows, so the body cannot shed heat as fast. The heat index estimates this effect on the skin's perceived temperature.
How do I protect myself from high heat index?
At an index above 41 °C, limit outdoor work, take breaks in shade and drink water regularly — India's heat-wave advisories use this scale. Above 54 °C is classed as extremely dangerous for heatstroke.
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