Heat Index Calculator
Calculate heat index (feels like temperature) from temperature and relative humidity, or calculate temperature or humidity from heat index with step-by-step solutions.
Input
Result
Visualization
Step-by-Step Solution
Calculate Heat Index
Select a calculation type and enter values to see results with detailed step-by-step solutions.
Complete User Guide
What is Heat Index?
The Heat Index (also known as 'apparent temperature' or 'feels like temperature') is a measure of how hot it feels when relative humidity is factored in with the actual air temperature. It is an important indicator for heat-related health risks, especially during hot weather. The heat index is calculated using the Rothfusz equation, which takes into account both temperature and relative humidity to determine the perceived temperature.
How to Use This Calculator
- Select a calculation type from the dropdown (Heat Index, Temperature from Heat Index, Humidity from Heat Index, or Convert Temperature Units).
- Enter the required values (temperature, relative humidity, or heat index) with appropriate units.
- Select your preferred temperature unit (Fahrenheit or Celsius).
- Click 'Calculate' to see the results.
- Review the detailed results, risk category, visualizations, and step-by-step solution.
Calculation Types
Heat Index
Calculate heat index from temperature and relative humidity using the Rothfusz equation. This is the most common calculation.
Temperature from Heat Index
Calculate the actual temperature when you know the heat index and relative humidity. Uses iterative numerical methods.
Humidity from Heat Index
Calculate relative humidity when you know the heat index and temperature. Uses iterative numerical methods.
Convert Temperature Units
Convert between Fahrenheit and Celsius. Formula: °F = (°C × 9/5) + 32, °C = (°F - 32) × 5/9.
Heat Index Formula (Rothfusz Equation)
Main Formula:
HI = -42.379 + 2.04901523×T + 10.14333127×RH - 0.22475541×T×RH
- 6.83783×10⁻³×T² - 5.481717×10⁻²×RH² + 1.22874×10⁻³×T²×RH
+ 8.5282×10⁻⁴×T×RH² - 1.99×10⁻⁶×T²×RH²
Where:
- HI = Heat Index (°F)
- T = Temperature (°F)
- RH = Relative Humidity (%)
Adjustments:
- If RH < 13% and 80°F ≤ T ≤ 112°F: Subtract adjustment
- If RH > 85% and 80°F ≤ T ≤ 87°F: Add adjustment
Heat Index Risk Categories
| Heat Index (°F) | Category | Risk Level | Health Effects |
|---|---|---|---|
| 80-89 | Caution | Moderate | Fatigue possible with prolonged exposure |
| 90-102 | Extreme Caution | High | Sunstroke, heat cramps, heat exhaustion possible |
| 103-124 | Danger | Very High | Sunstroke, heat cramps, heat exhaustion likely; heat stroke possible |
| 125+ | Extreme Danger | Extreme | Heat stroke highly likely |
Important Notes
- Heat index is only valid for temperatures above 80°F (27°C) and relative humidity above 40%.
- Heat index calculations are based on the Rothfusz equation developed by the National Weather Service.
- The formula uses Fahrenheit for temperature; Celsius values are converted automatically.
- Direct sunlight can increase the heat index by up to 15°F (8°C).
- Wind can reduce the heat index by increasing evaporation.
- Heat index is most accurate in shaded, light wind conditions.
- Elderly, children, and those with health conditions are more susceptible to heat-related illnesses.
- Stay hydrated and take breaks in cool, shaded areas when heat index is high.
Common Applications
- Weather forecasting and heat warnings
- Outdoor activity planning and safety
- Athletic event planning and heat safety protocols
- Workplace safety in hot environments
- Agricultural and farming operations
- Understanding heat-related health risks
- Climate and weather analysis