Heat Index Calculator
How hot it feels when humidity is added to the air temperature, by the National Weather Service’s own heat index procedure, in °F or °C.
Result
Heat index: 105.9 °FEstimate
For a person in shade with a light breeze. Full sun can add up to 15 °F (8 °C).
- Feels hotter than the air by
- 15.9 °FEstimate
- Formula used
- The Rothfusz regression
https://calcgallery.com/weather/heat-index-calculator/
Results come from the published equations applied to the values you enter. They are not electrical design, conductor sizing, laboratory safety or medical guidance. Weather indices describe a standard person in standard conditions and are not a forecast. Where codes apply, local rules vary and must be checked. Read the full disclaimer
How to use the Heat Index Calculator
- Enter the air temperature in °F or °C.
- Enter the relative humidity as a percentage.
- Read the heat index, which formula the Weather Service procedure used, and any adjustment it applied.
The procedure
The heat index is defined by the National Weather Service in Technical Attachment SR 90-23, Rothfusz’s regression of Steadman’s apparent-temperature tables. The page follows it step by step, working in °F and percent as the equation does:
- Work out the simple formula, 0.5 × (T + 61 + 1.2 × (T − 68) + 0.094 × RH).
- If its average with T is below 80 °F, that is the heat index.
- Otherwise use the full regression, a nine-term polynomial in T and RH.
- Lower it for very dry air, or raise it for very humid air, when conditions fall inside those corrections’ windows.
Shade, not sun
The index assumes a person in the shade with a light breeze. Direct sunshine can raise the heat felt by as much as 15 °F. Celsius entries are converted to Fahrenheit exactly before the calculation and the result converted back, so either system gives the same answer.
With your numbers
- T = 90 °F, RH = 70%
- The Rothfusz regression: 105.92 °F
Quick reference
For a person in shade with a light breeze. Full sun can add up to 15 °F (8 °C).
| Example | Heat index |
|---|---|
| 90 °F, 70% | 105.9 °F |
| 100 °F, 40% | 109.3 °F |
| 80 °F, 40% | 79.6 °F |
| 75 °F, 50% | 74.6 °F |
| 95 °F, 10%, very dry | 89.4 °F |
| 85 °F, 90%, very humid | 101.8 °F |
| 32 °C, 70% | 40.4 °C |
Common questions about the Heat Index Calculator
- Why does humidity make it feel hotter?
- The body cools itself by sweating, and sweat cools only as it evaporates. In humid air it evaporates more slowly, so less heat is carried away and the body warms more than the air temperature alone would suggest. The heat index puts a number on that: 90 °F at 70% humidity feels like about 106 °F.
- Why does the page sometimes use a simpler formula?
- That is the Weather Service's own procedure. The full regression was fitted to hot, humid conditions and misbehaves in mild weather, so a simple formula is worked out first. Only if its average with the air temperature reaches 80 °F is the full regression used. At 80 °F and 40% humidity the simple formula gives 79.6 °F.
- What are the two adjustments?
- Corrections the Weather Service applies to the regression at its edges. In very dry air, under 13% humidity, between 80 and 112 °F, the index is lowered. In very humid air, over 85%, between 80 and 87 °F, it is raised. At 95 °F and 10% humidity the dry correction brings the index down to about 89.5 °F.
- Is there a danger level?
- The Weather Service publishes risk categories for the heat index, and this page does not reproduce them. Heat illness also depends on activity, clothing, sun, acclimatisation and health, which no index captures. Follow local heat warnings.