What is the Heat Index Calculator?
Find the feels-like temperature on hot, humid days with this free heat index calculator. Enter air temperature and relative humidity in metric (°C, % RH) or imperial (°F, % RH) to get the heat index using the official NWS Rothfusz regression. See heat risk levels from Caution to Extreme Danger, outdoor safety guidance, and how much humidity raises perceived temperature. Results update instantly in your browser with no signup.
How to use the Heat Index Calculator
- Choose metric (°C and % RH) or imperial (°F and % RH).
- Enter air temperature and relative humidity.
- Heat index appears instantly — the apparent temperature your body feels in shade.
- Review heat risk level and health guidance for your conditions.
- Try presets such as 32°C at 70% humidity or 100°F at 75% RH for summer heat scenarios.
Common use cases
- Checking how humid summer weather will feel before outdoor sports or events
- Planning yard work or construction shifts during heat waves
- Teaching how relative humidity affects human comfort and heat illness risk
- Comparing °C and °F heat index readings from weather apps
- Pairing with the wind chill calculator for year-round apparent temperature reference
Frequently asked questions
- What is the heat index?
- The heat index (apparent temperature) describes how hot it feels when humidity is factored in. High humidity slows sweat evaporation, so your body cannot cool as efficiently and the air feels hotter than the thermometer reading.
- Which formula does this calculator use?
- It uses the NWS Rothfusz regression with low- and high-humidity adjustments, the same approach behind U.S. heat index charts. Temperatures below 80°F (27°C) return the actual air temperature.
- What do the heat risk levels mean?
- NWS categories: Caution (80–90°F), Extreme Caution (90–103°F), Danger (103–125°F), and Extreme Danger (125°F+). Higher levels mean greater risk of heat cramps, exhaustion, and heat stroke with exposure or exertion.
- Is heat index the same as wind chill?
- No. Heat index applies to hot, humid weather. Wind chill applies to cold, windy weather and describes heat loss from exposed skin. They address opposite weather extremes.
- Does this account for sun or wind?
- The standard heat index assumes shade and light wind. Full sun can make it feel up to 15°F hotter; stronger wind can lower perceived temperature slightly. This tool uses the official shade heat index.
