Pay attention, this gets complicated!
It all harks back to a time before fractions of inches were commonplace, to a time where agriculture was the main industry in the British countryside.
The unit used to gauge smaller measurements in footwear sizing was a 'barleycorn'. The average length of a grain of barley is approximately ⅓ of an inch - thus making three per inch - and equivalent to 8.46mm in today's decimal world.
Now a shoe or boot would be constructed on a carved wooden or metal foot-shaped object called a 'last' which to ensure the item fitted a certain foot size, was slightly bigger than the wearer's actual foot.
It was, and is, the number of the 'last' that denotes the UK shoe size. The calculation to work out what your foot size is too tricky to simplify, but we can list the UK size shoe or boot, based on your foot length - in millimetres. The chart below shows the conversion.