Wind Chill factor is an estimate of how cold it will feel based on a formula involving wind speed and air temperature. It was originally developed as a rudimentary way of determining how quickly frostbite could develop due to the passing flow of lower-temperature air over exposed skin.
This calculator uses the latest wind chill formula index developed by scientists and medical experts in the UK, Canada and US working for the Joint Action Group for Temperature Indices. Its formula is shown below with T as the temperature in Fahrenheit and V as the wind speed in mph (miles per hour):
wind chill = 35.74 + 0.6215×T - 35.75×V0.16 + 0.4275×T×V0.16
There are other models for determining wind chill such as the Australian apparent temperature but for most people, JAG/TI should work well.
As mentioned at the beginning wind chill should be taken as advice rather than a precise value, despite this calculator providing exact figures in Fahrenheit, Celsius, Kelvin and heat loss per meter2. The reason for this is all humans lose heat at different rates based on the size/shape of their bodies and multiple external factors that are not taken into account such as wind gusts.