Heat Index Calculator
Free online heat index calculator. Enter air temperature and relative humidity to get the "feels-like" apparent temperature using the NWS Rothfusz formula. Supports Fahrenheit and Celsius with a color-coded heat safety level. 100% client-side.
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What is Heat Index Calculator?
The heat index is the apparent "feels-like" temperature that tells you how hot it actually feels when relative humidity is combined with the air temperature. Because the human body cools itself by sweating, high humidity slows evaporation and makes a given temperature feel far hotter. This free online heat index calculator uses the U.S. National Weather Service Rothfusz regression with the standard low- and high-humidity adjustments, and it classifies every result into a color-coded safety level. Switch between Fahrenheit and Celsius, enter a shade temperature and relative humidity, and get an instant apparent temperature plus a built-in reference table. Everything runs 100% client-side in your browser.
Key Benefits
- Reports the true apparent temperature so you can judge real heat stress
- Uses the official NWS Rothfusz formula with humidity corrections
- Color-coded safety level from caution up to extreme danger
- Switch between Fahrenheit and Celsius with live value conversion
- Private and offline-capable; no weather data leaves your device
Common Use Cases
- •Planning outdoor work, sports, or events during hot and humid weather
- •Deciding when to cancel or reschedule activities to prevent heat illness
- •Checking conditions for vulnerable people such as children and the elderly
- •Comparing the "feels-like" temperature reported by different weather apps
- •Teaching or learning how humidity affects perceived temperature
How to Use the Heat Index Calculator
- Pick a unit: Toggle between Fahrenheit (°F) and Celsius (°C). If you switch after typing a temperature, the value is converted automatically.
- Enter the air temperature: Type the ambient (shade) temperature measured in the open air, not in direct sun or inside a vehicle.
- Enter relative humidity: Provide the relative humidity as a percentage between 0 and 100. Many weather apps report this alongside the temperature.
- Read the result: The large value is the apparent "feels-like" temperature, with a color-coded safety level. Copy the summary or consult the safety table for guidance.
Key Features
- NWS Rothfusz regression with low- and high-humidity adjustments
- Switch between Fahrenheit and Celsius with live conversion
- Color-coded heat safety level (caution to extreme danger)
- Built-in reference table of heat index safety thresholds
- Copy a one-line summary for reports, logs, or social posts