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Heat Index Calculator

🌡️Heat Index Calculator

Best for T > 27°C / 80°F

40–100%

What is Heat Index Calculator?

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The heat index (also called apparent temperature or feels-like temperature in summer) estimates how hot it actually feels based on air temperature and relative humidity. High humidity prevents sweat from evaporating effectively, reducing the body's ability to cool itself. The Rothfusz (1990) regression equation used by NOAA and the NWS is the most widely accepted formula.

Calkulon makes complex calculations simple — built for students and everyday problem-solvers.

Formula

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f(x)HI = −42.379 + 2.04901523T + 10.14333127R − 0.22475541TR

Variable Legend

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SymbolNameUnitDescription
HI−42—−42

How to Heat Index Calculator

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  1. 1Based on Rothfusz (1990) regression of physiological studies
  2. 2HI = −42.379 + 2.04901523T + 10.14333127R − 0.22475541TR ...
  3. 3T = air temperature (°F), R = relative humidity (%)
  4. 4Applies when T ≥ 80°F (27°C) and RH ≥ 40%
  5. 5Accuracy: ±1.3°F under typical conditions

Worked Examples

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Example 1
Given:35°C, 60% humidity
Result:Heat index ≈ 42°C — feels significantly hotter

Health risk threshold

Example 2
Given:40°C, 80% humidity
Result:Heat index ≈ 60°C — extreme danger

Can be fatal without cooling

Real-World Applications

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Tracking personal health and fitness goals

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Guidance for healthcare conversations with your doctor

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Monitoring progress during a weight management programme

Heat Index Categories (NOAA)

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Feels-Like TemperatureCategoryHealth Effects
27–32°C (80–90°F)CautionFatigue with prolonged exposure
33–39°C (91–103°F)Extreme CautionPossible heat cramps/exhaustion
40–51°C (104–124°F)DangerHeat cramps/exhaustion likely; heat stroke possible
> 52°C (125°F)Extreme DangerHeat stroke highly likely

Frequently Asked Questions

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Q

What is Heat Index Calc?

A

The heat index (also called apparent temperature or feels-like temperature in summer) estimates how hot it actually feels based on air temperature and relative humidity. High humidity prevents sweat from evaporating effectively, reducing the body's ability to cool itself

Q

How accurate is the Heat Index Calc calculator?

A

The calculator uses the standard published formula for heat index calc. Results are accurate to the precision of the inputs you provide. For financial, medical, or legal decisions, always verify with a qualified professional.

Q

What units does the Heat Index Calc calculator use?

A

This calculator works with inches, Celsius, Fahrenheit, percentages. You can enter values in the units shown — the calculator handles all conversions internally.

Q

What formula does the Heat Index Calc calculator use?

A

The core formula is: HI = −42.379 + 2.04901523T + 10.14333127R − 0.22475541TR . Each step in the calculation is shown so you can verify the result manually.

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Pro Tip

Heat index assumes shaded conditions. Direct sunlight can add an additional 8–15°C to the apparent temperature. Drink water before you feel thirsty — thirst indicates you are already mildly dehydrated.

Formula-verified for precision
Reviewed October 2026
Our methodology

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