Heat Index Calculator
Combine temperature and humidity to find the actual heat stress level.
Heat Index Calculator
On a humid summer day, 95°F can feel like 115°F. That difference is the heat index — the temperature your body perceives when humidity prevents sweat from evaporating properly. This calculator tells you what outdoor conditions truly feel like so you can stay safe.
What Is the Heat Index?
The heat index (also called "apparent temperature" or "feels like" temperature) combines air temperature and relative humidity to determine how hot it actually feels to the human body. High humidity reduces sweat evaporation — your body's primary cooling mechanism — making you feel much hotter than the thermometer shows.
How to Use This Calculator
- Enter the air temperature in °F or °C.
- Enter the relative humidity as a percentage.
- Click Calculate to see the heat index and risk level.
Heat Index Formula (NWS Rothfusz)
The National Weather Service uses the Rothfusz regression equation:
HI = −42.379 + 2.04901523T + 10.14333127R − 0.22475541TR − 6.83783×10⁻³T² − 5.481717×10⁻²R² + 1.22874×10⁻³T²R + 8.5282×10⁻⁴TR² − 1.99×10⁻⁶T²R²
- T = Temperature in °F
- R = Relative humidity (%)
Example Calculation
Temperature: 96°F, Humidity: 65%
- Applying the Rothfusz formula...
- Heat Index ≈ 121°F
- Risk level: Extreme Danger — heat stroke likely
Heat Index Risk Levels
- 80–90°F: Caution — fatigue possible with prolonged exposure
- 90–103°F: Extreme Caution — heat cramps/exhaustion possible
- 103–124°F: Danger — heat cramps/exhaustion likely; heatstroke possible
- 125°F+: Extreme Danger — heatstroke imminent
Common Mistakes to Avoid
- Exercising in extreme heat index conditions — your body generates internal heat during exercise, compounding the external heat burden significantly.
- Assuming shade helps equally on humid days — shade helps with radiant sun heat, but doesn't reduce the humidity effect on your body's cooling system.
- Ignoring the heat index for children and elderly — these groups are far more vulnerable to heat illness; their safe thresholds are lower.
Frequently Asked Questions
What is a dangerous heat index?
A heat index above 103°F is considered dangerous by the NWS, with heat cramps and exhaustion likely and heatstroke possible. Above 125°F, heatstroke is imminent without intervention.
How do I stay safe in high heat index conditions?
Stay in air-conditioned spaces, drink water before you feel thirsty, wear loose light-colored clothing, avoid alcohol and caffeine, and limit outdoor activity to early morning or after sunset.
What is the difference between heat index and temperature?
Temperature measures actual air warmth. Heat index adjusts for humidity to show how hot it feels. At 90°F with 90% humidity, the heat index can exceed 115°F.
Why does humidity make heat feel worse?
Your body cools itself by sweating. When humidity is high, sweat evaporates slowly — so the cooling effect is reduced. Your body works harder and overheats more quickly.
Does the heat index apply at night?
Yes. Nighttime heat index can still be dangerously high in humid regions, preventing the body from recovering from daytime heat exposure — a key factor in heat-wave mortality.
Conclusion
The heat index is the number that truly matters on hot, humid days — not just the air temperature. Use this calculator to assess actual conditions before outdoor activity, and take heat illness risk seriously, especially for vulnerable individuals.
Related: Wind Chill Calculator | Dew Point Calculator | BMI Calculator
Important Note
Heat index values are for shady, light wind conditions. Exposure to full sunshine can increase values by up to 15°F.
