Wind Chill / Heat Index Calculator
Calculate wind chill and heat index
Wind Chill / Heat Index Calculator tool
Wind Chill / Heat Index Calculator: key facts
- What it does
- Calculate wind chill and heat index
- Category
- Other Calculators
- Cost
- Free, with no account, sign-up, or install.
- Your data
- Runs entirely in your browser — the files and text you enter are never uploaded to a server.
- Last reviewed
- . Report an incorrect result.
What the Wind Chill / Heat Index Calculator does
The thermometer rarely tells the whole story — wind makes the cold bite harder, and humidity makes the heat more dangerous. This calculator computes the "feels like" temperature using the official NOAA wind chill formula in cold conditions and the Rothfusz heat index in hot, humid ones, and adds a safety warning for the result.
People use it to dress for a winter walk, judge whether outdoor exercise is safe on a humid afternoon, or understand a weather warning. It automatically applies the right formula based on the temperature, wind, and humidity you enter, and translates the number into a plain-language risk level from "safe" to "extreme danger".
Using the Wind Chill / Heat Index Calculator, step by step
- Enter the temperature and choose Fahrenheit or Celsius.
- Enter the wind speed (for cold conditions) and the relative humidity (for hot conditions).
- Read the "feels like" temperature.
- Heed the safety warning, which describes the frostbite or heat-illness risk at that level.
Two official formulas for two problems
Cold and heat make air feel different from the thermometer reading for unrelated physical reasons, so two separate equations are used. Wind chill applies the North American formula adopted in 2001, which models how moving air strips the insulating boundary layer of warm air from exposed skin, accelerating heat loss.
Heat index uses the Rothfusz regression, which approximates apparent temperature from air temperature and relative humidity. High humidity impairs evaporative cooling: sweat is the body's main mechanism for shedding heat, and when the air is already near saturation it cannot evaporate efficiently, so the same temperature feels considerably hotter. The tool applies the standard low-humidity correction where conditions call for it.
Both formulas have restricted domains. Wind chill is defined for temperatures at or below 50°F with wind above about 3 mph; heat index is meaningful above roughly 80°F. Outside those ranges the equations return values, but they are not describing anything real.
- The defaults — 32°F with a 15 mph wind — give a wind chill of 21.6°F, roughly ten degrees below the air temperature.
- 95°F at 50% relative humidity gives a heat index of 105.2°F.
- The same 95°F at 20% humidity feels close to the actual temperature, which is why dry heat is more tolerable than the number suggests.
What makes this one worth using
- It uses the official formulas — the NOAA wind chill equation and the Rothfusz heat index regression — rather than a rough approximation.
- It applies the correct formula automatically: wind chill in the cold, heat index in the heat, and the actual temperature in between.
- It pairs the result with tiered safety warnings, so the number translates into real-world risk and precautions.
- All processing happens on your device, free to use.
Reading apparent temperature for safety
Both figures describe risk, not just comfort. Wind chill is used because it predicts how quickly exposed skin freezes: around −20°F of wind chill, frostbite can develop in about half an hour, and at lower values in minutes. Covering exposed skin matters more than the layer count when wind is the driver.
On the heat side, an index above roughly 103°F brings a real risk of heat exhaustion and heatstroke with prolonged exertion, and above 125°F the risk is extreme. Humidity is why athletes and outdoor workers are affected at temperatures that would be manageable in dry air. Hydration, shade, and reducing exertion during peak hours are the practical responses.
Neither model describes an individual. Both assume a typical adult, in shade, walking; direct sunlight can add up to 15°F to the heat index, and children, older adults, and people with cardiovascular conditions or on certain medications are affected at milder readings than the tables imply. Treat these as an official guide to conditions rather than a personal safety threshold, and follow local weather warnings.
Frequently Asked Questions
What is the difference between wind chill and heat index?
Wind chill is how cold it feels when wind strips heat from your skin, used in cold weather. Heat index is how hot it feels when humidity limits your body's ability to cool by sweating, used in hot weather. This tool computes whichever applies to your conditions.
When does wind chill apply?
The wind chill formula is valid only when the temperature is at or below 50°F and the wind is at least 3 mph. Outside those conditions wind chill is not meaningful, so the calculator switches to the heat index or the actual temperature instead.
When does the heat index apply?
The heat index is calculated when the temperature is at or above 80°F and relative humidity is at least 40%. Below those thresholds humidity has little effect on perceived temperature, so the calculator reports the actual temperature.
Why does humidity make heat more dangerous?
Your body cools itself by evaporating sweat. When humidity is high, sweat evaporates poorly, so cooling is less effective and the heat feels worse and stresses the body more — which is exactly what the heat index quantifies.
