Heat Index and Wind Chill Calculator
See how hot or cold it really feels from the temperature, humidity and wind.
Wind chill needs a wind above 4.8 km/h (3 mph). The US heat index assumes a light breeze, so wind doesn't change it.
How your feels-like temperature is worked out
Humid heat stops sweat cooling you. Wind strips warmth from skin. Each index puts a number on that.
- 1
Into Fahrenheit
32 °C × 1.8 + 32
= 89.6 °F - 2
Quick estimate
Steadman at 60%
= 91.1 °F - 3
Full NWS formula
≥ 80, so Rothfusz
= 98.7 °F - 4
Feels like
37 °C
Extreme caution
Read more: the formulas, the US heat categories and a Gulf summer example
Your body cools itself by sweating. In humid air, sweat dries slowly, so you stay hotter than the thermometer suggests. In cold weather, wind strips away the warm layer of air around your skin, so you lose heat faster. A feels-like temperature puts both effects into one number. By default this calculator follows the US National Weather Service: wind chill at 10 °C (50 °F) or colder with some wind, the heat index when it reaches 80 °F (about 27 °C), and the plain air temperature in between.
The formula
Heat index (US NWS, temperatures in °F):
first guess = 0.5 × (T + 61 + (T − 68) × 1.2 + RH × 0.094)
if the first guess is 80 °F or more, use the full formula:
HI = −42.379 + 2.04901523 T + 10.14333127 RH − 0.22475541 T RH − 0.00683783 T² − 0.05481717 RH² + 0.00122874 T² RH + 0.00085282 T RH² − 0.00000199 T² RH²
if RH is under 13% and T is 80–112 °F: subtract (13 − RH) ÷ 4 × √((17 − |T − 95|) ÷ 17)
if RH is over 85% and T is 80–87 °F: add (RH − 85) ÷ 10 × (87 − T) ÷ 5
Wind chill (US and Canada, T in °C, V = wind in km/h):
WC = 13.12 + 0.6215 T − 11.37 V^0.16 + 0.3965 T V^0.16
Humidex (Canada): T + 0.5555 × (e − 10), e = 6.11 × e^(5417.753 × (1 ÷ 273.16 − 1 ÷ (273.15 + dew point))) hPa
Apparent temperature (Australia): T + 0.33 e − 0.70 × wind (m/s) − 4, e = RH ÷ 100 × 6.105 × e^(17.27 T ÷ (237.7 + T)) hPa
The full heat index formula is the Rothfusz regression, a fit to Steadman's 1979 tables. The NWS uses the simple first guess below 80 °F, where the full formula doesn't fit well. Wind chill is the 2001 index the US and Canada agreed on, measured with the wind at 10 m and for a face in the open. When you enter the dew point, humidity comes from the Magnus formula (17.625, 243.04 °C).
Worked example
32 °C with 60% humidity and a 10 km/h wind (the starting values above):
- In Fahrenheit: 32 × 1.8 + 32 = 89.6 °F.
- First guess: 0.5 × (89.6 + 61 + 21.6 × 1.2 + 60 × 0.094) = 0.5 × (89.6 + 61 + 25.92 + 5.64) = 0.5 × 182.16 = 91.08 °F. That is 80 °F or more, so the full formula is used.
- Full formula, term by term: −42.379 + 183.5918 + 608.5999 − 1,208.2851 − 54.8952 − 197.3418 + 591.8713 + 275.0856 − 57.5137 = 98.73 °F. The humidity is between 13% and 85%, so no adjustment.
- Back to Celsius: (98.73 − 32) ÷ 1.8 = 37.07, so it feels like about 37 °C (99 °F), 5 °C hotter than the air.
- 98.7 °F is between 90 and 103 °F, so the NWS category is Extreme caution.
- Wind chill doesn't apply: 32 °C is above 10 °C.
- Humidex: the dew point is 23.27 °C, so e = 28.96 hPa and humidex = 32 + 0.5555 × (28.96 − 10) = 32 + 10.53 = 42.53, about 43.
- Australian apparent temperature: e = 0.6 × 47.38 = 28.43 hPa and the wind is 10 ÷ 3.6 = 2.78 m/s, so 32 + 0.33 × 28.43 − 0.70 × 2.78 − 4 = 32 + 9.38 − 1.94 − 4 = 35.44 °C.
A Gulf summer afternoon
On the Gulf coast in July or August, 42 °C (107.6 °F) with 50% humidity is not unusual. The first guess is 110.41 °F, so the full formula gives about 143 °F, or 62 °C, which is in the NWS Extreme danger band. That is beyond the top of the NWS chart (it stops at about 137 °F), so treat it as "extreme" rather than an exact figure. Humidex for the same air is about 60, and the Australian apparent temperature is about 49.5 °C with a 10 km/h breeze.
US National Weather Service categories
| Heat index (°F) | Heat index (°C) | NWS category |
|---|---|---|
| 80–90 | 27–32 | Caution |
| 90–103 | 32–39 | Extreme caution |
| 103–124 | 39–51 | Danger |
| 125 and over | 52 and over | Extreme danger |
The higher the category, the sooner long spells outside or hard work in the heat can make people unwell. The heat index is worked out for shade. The NWS notes that direct sun can raise it further.
Category tables checked against our knowledge, not yet against the NWS / Environment Canada pages.
Staying safe in the heat and cold
- Drink water regularly when it's hot, before you feel thirsty.
- Rest in the shade or somewhere cool, and plan hard work or exercise for the cooler early morning or evening.
- Wear light, loose, light-coloured clothes and a hat in the sun.
- Check on older neighbours, young children and anyone working outdoors. Never leave people or pets in a parked car.
- In the cold, cover exposed skin, wear layers, keep dry and get out of the wind.
- Follow local heat and cold warnings, which are based on more than one number.
Thinking about how much to drink? Try the water intake calculator. Pilots can find the dew point and cloud base with the cloud base calculator.
General information, not medical advice. The indexes are estimates for an average adult in the shade; sun, effort, clothing, age and health all change how heat and cold affect you. Formulas: US National Weather Service heat index, the 2001 US–Canada wind chill index, Environment Canada humidex and the Australian Bureau of Meteorology apparent temperature.
Common questions
What is the heat index?
The heat index is how hot it feels when humidity is added to the air temperature, in the shade with a light wind. Damp air slows down how fast sweat dries, so your body cools less well. The US National Weather Service works it out from the temperature and relative humidity. At 32 °C (90 °F) and 60% humidity it feels like about 37 °C (99 °F).
What is wind chill?
Wind chill is how cold it feels on bare skin when the wind blows away the thin layer of warm air next to your body. The US and Canada use the same formula from 2001. It only applies at 10 °C (50 °F) or colder with wind above 4.8 km/h (3 mph). At −10 °C with a 20 km/h wind it feels like about −18 °C.
What are the US National Weather Service heat index categories?
The US National Weather Service uses four: Caution at 80–90 °F (27–32 °C), Extreme caution at 90–103 °F (32–39 °C), Danger at 103–124 °F (39–51 °C) and Extreme danger at 125 °F (52 °C) and above. The higher the category, the sooner heat can cause problems with long exposure or hard work, especially in direct sun.
What does 42 °C with 50% humidity feel like?
The NWS formula gives about 62 °C (143 °F). That is past the top of the NWS heat index chart, so take it as "extreme danger" rather than an exact number. Summer days on the Gulf coast can reach this, which is why outdoor work rules and midday breaks exist in many Gulf countries.
What is the difference between heat index and humidex?
Both turn heat and humidity into one number. The heat index is the US version, worked out in °F from relative humidity. Humidex is the Canadian version, worked out in °C from the dew point. Humidex usually reads a few degrees higher than the heat index in °C in hot, humid weather. Neither includes wind or sun.
Why do weather apps show a different feels-like temperature?
Apps use different formulas. Some use the US heat index and wind chill, the Australian Bureau of Meteorology uses its apparent temperature (which includes wind in warm weather too), and others use their own models that add sunshine. Switch the method in Advanced mode to compare them.
Related calculators
Made by Zulu Calculator, part of AlphaZulu LLC. Formulas are shown in full and rule figures are checked against official sources; see how we build calculators.