Heat Index Calculator
Humidity is why 95°F in Phoenix feels survivable and 95°F in Houston feels hostile: sweat evaporates slower in moist air, so your body's cooling system underperforms exactly when it matters. The heat index puts one number on that — and this calculator runs the full NOAA Rothfusz regression, the same formula the National Weather Service publishes, not the rough doubling shortcuts.
The risk band under the result translates the number into what to actually do: caution means hydrate and pace yourself, extreme caution means cut midday effort, danger and extreme danger mean heat illness becomes likely without intervention. The delta stat shows how many degrees humidity added — the number most people find genuinely surprising. Note the figure is for shade with light wind; full sun can push the effective temperature up another 15°F.
How to use
- Pick °F or °C, then enter the air temperature and relative humidity from any weather report.
- Read the feels-like heat index in the big result, with the official NOAA risk band spelled out below it.
- Check the delta to see what humidity added, and share the figure in one tap for the group chat.
Frequently asked questions
What is the heat index?
The temperature a body perceives when humidity is combined with air temperature, because reduced sweat evaporation impairs cooling. 95°F at 60% humidity computes to a heat index of about 114°F — the air is 95, but your cooling system is working as if it were 114.
How is the heat index calculated?
This page uses the NOAA Rothfusz regression — a multi-term polynomial of temperature and relative humidity — plus the official corrections for very dry (RH<13%) and very humid (RH>85%) conditions. It applies only above 80°F (27°C); below that the air temperature itself is the standard feels-like figure.
What heat index is dangerous?
NOAA bands: 80-90°F caution (fatigue with prolonged exposure), 90-102°F extreme caution (heat cramps and heat stroke possible), 103-124°F danger (likely without precautions), 125°F+ extreme danger (heat stroke imminent). The calculator prints your band with the result.
Why does the heat index assume shade?
The NOAA formula models a shaded, lightly-winded person so forecasts are comparable across locations. Direct sun adds up to 15°F (8°C) to the effective load — treat the shaded figure as the floor, not the worst case, and plan outdoor effort accordingly.