Wind chill calculator
Wind chill on the formula that replaced the famous one in 2001 — and an honest refusal outside the narrow range it is actually defined for, which almost no calculator enforces. Nothing you type here leaves your browser — it is worked out on your own device, so there is nothing to send and nothing to log.
The wind chill you grew up with was replaced in 2001
The original index came from Paul Siple and Charles Passel, working in the Antarctic between 1939 and 1941. They hung a plastic bottle of water outside and measured how long it took to freeze.
That gives a rate of heat loss, in kilocalories per square metre per hour. It is a real measurement of a real thing. It is not a temperature, and the step that turned it into one is where the trouble started — because water in a bottle loses heat quite differently from skin on a living face.
On 1 November 2001 the National Weather Service and the Meteorological Service of Canada replaced it with an index built from human facial cooling trials in wind tunnels. At strong winds the new figures are typically 10 to 15 °F warmer. A reading reported as −69 °F in 1985 would be −48 °F today, and that is the same weather.
It is only defined in a narrow range
At or below 10 °C, and above 4.8 km/h. Outside that the index does not apply: on a warm day moving air does not cool you the way the formula describes, and in still air there is nothing for it to describe.
Almost every wind chill calculator will give you a number anyway. This one will not, for the same reason it would be wrong to run a one-rep-max formula at forty repetitions — a formula outside the range its authors published is not a conservative estimate, it is an answer to a question nobody asked.
Two official formulas, and they agree
The American version works in Fahrenheit and miles per hour, the Canadian in Celsius and kilometres per hour, and their coefficients look nothing like each other. Swept across the whole valid range they never differ by more than 0.06 °C — measured at 0.0546, and stated as a bound that is actually true rather than the rounder number just underneath it.
They are the same curve, published twice with the constants rounded for different units. That is worth checking rather than assuming, and this page runs both every time.
What wind chill is not
It is not the temperature of anything. Wind does not make the air colder — a thermometer in the wind reads the same as one out of it. What it does is strip away the thin layer of warmed air against your skin, so you lose heat faster.
Which means wind chill tells you how fast you will cool, not how cold it is. Your car will not freeze any faster in a wind, and a puddle will not drop below the air temperature however hard it blows.