Heat Index Calculator turns an air temperature and a relative humidity reading into the National Weather Service's "feels like" temperature using the Rothfusz regression. Type a temperature and humidity percentage, switch between US (°F) and metric (°C) units, and the tool shows the heat index in both scales alongside its NWS risk category — Caution, Extreme Caution, Danger, or Extreme Danger. Humidity slows how effectively sweat evaporates, so a muggy 90°F afternoon can feel noticeably hotter than a dry one at the same reading. It is useful for outdoor workers, coaches, event planners, and anyone deciding whether conditions call for extra water breaks or shade. Everything runs locally in your browser, and the result is an estimate — verify current conditions before making outdoor safety decisions.
Features
Computes the NWS Rothfusz-regression heat index from temperature and humidity
Shows the feels-like temperature in both °F and °C at once
Flags the NWS risk category: Caution, Extreme Caution, Danger, or Extreme Danger
Toggle between US (°F) and metric (°C) input units instantly
Warns when the reading falls in the Danger or Extreme Danger range
Copy a plain-text summary of the temperature, humidity, and result
Persists your last inputs so you can tweak and recheck quickly
Runs entirely in your browser — no weather data is uploaded
How to use the Heat Index Calculator
Pick US (°F) or metric (°C) units
Enter the air temperature
Enter the relative humidity as a percentage
Read the feels-like heat index and its risk category
Copy the summary if you need to share or log it
Example
Input
90°F, 70% RH
Output
Feels like: 105°F (Danger)
The Rothfusz regression pushes the feels-like reading well above the actual temperature once humidity climbs.
Common errors & troubleshooting
The result looks unexpectedly close to the raw temperature. — Below about 80°F the heat index formula returns a value close to the actual temperature — this is expected, not an error. The Rothfusz regression only kicks in for warmer, more humid conditions.
Humidity was typed as a decimal like 0.7 instead of a percentage. — Enter relative humidity as a whole percentage between 0 and 100 (e.g. 70 for 70%), not a fraction.
Switching units gave a different-looking number than expected. — The temperature input is reinterpreted in the new unit, so re-enter the reading in the selected system (°F or °C) rather than assuming the tool converts a leftover number.
The heat index seems too extreme to be real. — The formula is validated for typical summer ranges; very unusual temperature/humidity combinations can produce less reliable estimates, so treat the number as a guide, not a precise forecast.
Frequently asked questions
What formula does the Heat Index Calculator use?
It uses the National Weather Service's Rothfusz regression, the standard equation for computing the heat index from air temperature in °F and relative humidity as a percentage, with low- and high-humidity adjustment terms applied at the edges of its range.
How is heat index different from the actual temperature?
Heat index is a "feels like" estimate that accounts for humidity slowing sweat evaporation. At the same air temperature, higher humidity always produces a higher heat index because the body cools less efficiently.
What do the Heat Index Calculator's risk categories mean?
Caution (80-90°F) suggests fatigue is possible with prolonged exposure; Extreme Caution (90-105°F) raises the risk of heat cramps or exhaustion; Danger (105-125°F) and Extreme Danger (125°F+) indicate a high chance of heat stroke with continued activity.
Can I use the heat index calculator with metric units?
Yes. Switch the unit toggle to metric to enter temperature in °C — the tool converts internally and still shows the result in both °F and °C.
Does the Heat Index Calculator upload my weather readings?
No. All calculations happen locally in your browser. The temperature and humidity you enter are not sent anywhere.
Is the heat index reading accurate enough for safety planning?
It is a reliable estimate based on the NWS formula, but treat it as a guide alongside official local forecasts and heat advisories rather than a substitute for them, especially near the Danger thresholds.
Related tools
Wind Chill Calculator — Calculate the wind chill “feels like” temperature from temperature and wind.
Dew Point Calculator — Calculate the dew point from temperature and relative humidity.
Density Calculator — Solve density, mass or volume from the density formula ρ = m/V.
Speed Calculator — Solve speed, distance or time and convert between mph, km/h and m/s.