What is the heat index?

The heat index (also called the "apparent temperature" or "feels-like" temperature) measures how hot it actually feels to the human body when relative humidity is factored in alongside the air temperature. High humidity slows evaporative cooling from sweat, making you feel hotter.

Which formula does this tool use?

This tool uses the NWS Rothfusz regression equation, the same one used by the National Weather Service. For conditions where the full equation is not valid (temperature below 80 °F or humidity below 40%), the simpler Steadman approximation is applied.

What are the NWS heat danger levels?

Caution (80–90 °F): fatigue possible with prolonged exposure. Extreme Caution (90–103 °F): heat cramps or exhaustion possible. Danger (103–124 °F): heat cramps and exhaustion likely, heat stroke possible. Extreme Danger (125 °F+): heat stroke highly likely.

Is the heat index accurate below 80 °F?

No. The Rothfusz equation is only reliable when the temperature is at or above 80 °F (27 °C) and relative humidity is 40% or higher. Below those thresholds this tool notes the result is "not applicable" because the body is not under meaningful heat stress.

Why does humidity make heat feel worse?

Your body cools itself by sweating. Sweat evaporates and removes heat. In high humidity, the air is already saturated with moisture, so sweat evaporates more slowly. Less evaporation means less cooling, so your body temperature rises faster.

Heat Index Calculator

Name: Heat Index Calculator
Availability: InStock
Author: Nham Vu

Enter air temperature and relative humidity to get the apparent "feels like" heat index with NWS danger level warnings.

Enter Conditions

Temperature Unit

Air Temperature (°F)

Relative Humidity (%)

0% Dry 50% 100% Humid

Enter temperature and humidity, then click
Calculate Heat Index.

Heat Index (Feels Like)

-- °F

-- °C

Air Temp

Humidity

--%

NWS Heat Index Scale

Caution

80–90 °F (27–32 °C)

Extreme Caution

90–103 °F (32–39 °C)

Danger

103–124 °F (39–51 °C)

Extreme Danger

125 °F+ (52 °C+)

Summary

Enter air temperature and relative humidity to get the apparent "feels like" heat index with NWS danger level warnings.

How it works

Enter the current air temperature in Fahrenheit or Celsius.
Enter the relative humidity as a percentage (0–100).
The tool applies the NWS Rothfusz multi-variable regression equation.
For low temperatures or humidity, a simpler Steadman formula is used instead.
The calculated heat index is displayed in both Fahrenheit and Celsius.
A color-coded NWS danger level (Caution through Extreme Danger) is shown with safety guidance.

Use cases

Check whether outdoor exercise is safe on a hot, humid day.
Help workers and supervisors assess heat-related illness risk on job sites.
Plan outdoor events such as sports, concerts, or festivals.
Understand heat index readings in weather forecasts and news reports.
Compare historical weather data across different cities and climates.
Educate students about the relationship between temperature and humidity.

Frequently Asked Questions

Last updated: 2026-05-23 · Reviewed by Nham Vu