Heat Index Safety Calculator
Heat plus humidity feels hotter than the thermometer shows. Enter the air temperature and relative humidity — the heat index, the safety band and practical guidance appear instantly.
Guidance
How to use
- Choose metric or imperial units.
- Enter the current air temperature and relative humidity.
- Read the feels like temperature and follow the guidance for your safety band.
Note: The formula is most accurate at 80°F (27°C) and above. Humidity below 40% reduces the feels like temperature slightly; humidity above 85% pushes it higher. This is an estimate.
Tips
- Drink water before you feel thirsty and rest in shade regularly.
- Outdoor workers should plan the heaviest tasks for cooler morning hours.
- Check on elderly people, children and anyone working alone in the heat.
- Never leave children or pets in a parked vehicle.
Frequently Asked Questions (FAQ)
What is the heat index?
The heat index is the temperature your body feels when air temperature and humidity are combined; high humidity slows sweat evaporation and makes heat more dangerous.
How is the heat index calculated?
This tool uses the standard Rothfusz regression formula, the same equation used by national weather services, with adjustments for very low or very high humidity. It is most accurate at 80°F (27°C) and above.
What heat index level is dangerous?
From 104°F (40°C) upward is the danger zone and 125°F (52°C) and above is extreme danger; seek shade, water and medical help if heat exhaustion signs appear.
What inputs does the calculator need?
Choose metric or imperial units, then enter the air temperature and relative humidity — the feels-like temperature and safety guidance appear instantly.