Wind Chill & Heat Index Calculator
The Wind Chill & Heat Index Calculator turns a thermometer reading into what the air actually feels like.
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Below 50 °F it applies the US National Weather Service wind chill formula and reports how long exposed skin has before frostbite. At or above 80 °F it applies the NWS heat index and its risk category. In between it says so, rather than pretending a formula applies where it was never fitted.
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How to Use
Choose your units
Celsius and km/h, or Fahrenheit and mph. Switching converts the numbers you already typed rather than reinterpreting them, so 20 °C never silently becomes 20 °F.
Enter the conditions
Air temperature, wind speed and relative humidity. Only one of the two indices needs all three, but entering all three lets the calculator pick the right one and show you the dew point.
Read the feels-like temperature
The large figure is the applicable index. The badge underneath is the risk band, and in the cold it is joined by the number of minutes before frostbite on exposed skin.
Check the sensitivity table
The table holds your temperature fixed and varies the wind, or the humidity, across its full range. It shows how steep the curve is around your reading, which a single number cannot.
Frequently Asked Questions
Why does the calculator sometimes refuse to apply either formula?
Because neither one is defined there. Wind chill was fitted at or below 50 °F (10 °C) with wind above 3 mph (5 km/h); the heat index was fitted at or above 80 °F (27 °C). Between those ranges the honest feels-like temperature is the air temperature. Calculators that extrapolate anyway will happily tell you that a still 15 °C day feels like 12 °C, which the formula was never fitted to support.
Why does wind chill ignore humidity, and heat index ignore wind?
They model different mechanisms. Wind chill is about convective heat loss from bare skin, which the wind drives and humidity barely affects. The heat index is about sweat evaporation, which humidity governs — the wetter the air, the less your sweat can evaporate and the less it cools you. Each formula takes only the variables it was regressed against, so the calculator tells you which one it left out instead of quietly folding it in.
How accurate are the frostbite times?
They are the bands from the NWS wind chill chart — 30, 10 and 5 minutes — and they describe exposed skin on an average adult in dry conditions. They are a planning guide, not a guarantee. Wet skin, wind gusts above the sustained speed you entered, and individual circulation all shorten the time, so treat the band as the optimistic case.
What is the dew point for, if the humidity is right there?
Relative humidity is relative to the temperature, so the same percentage means very different things at different temperatures: 70 % at 15 °C is pleasant and 70 % at 32 °C is oppressive. The dew point is an absolute measure of how much water the air holds, which is why forecasters use it to describe mugginess. Above roughly 20 °C dew point most people notice; above 24 °C almost everyone does.