In the US and the UK, there is a unit called barleycorn that defines the shoe-sizes.
1 barleycorn is equal to 1/3 inch, (0.33 inch); so, 3 barleycorns = 1 inch.
In the UK, the child shoe sizing starts with Size 0, which is taken as 4 inches (4”) long i.e. 12 barleycorns.
So, Size 1 will be 13 barleycorns, (4.33”), Size 2 = 14 barleycorns (4.66”), and so on.
The kids’ sizes run till Size 13 (25 barleycorns or 8.33”), and then adults’ Size 1 starts (same for both men and women).
Broadly, the shoe sizing for kids in the US is the same as that of the UK, except that in the US, the kids’ shoe sizes start at Size 1 instead of Size 0.
So, for the US, size 1 = 4” or 12 barleycorns, size 2 = 4.33” or 13 barleycorns, etc.
That is why the US sizes should be UK + 1, which means if you wear UK 8, your feet should fit US 9.
But things are very complicated in the US sizing because they have different calculations for women’s sizes and some manufacturers have their own starting points (instead of size 1).
So, while the shoe size increments happen in the same length terms (barleycorns) for both the UK and the US, because of other variables the equation, “US = UK + 1” may not always come true.
European sizes use a different unit of measurement called the Paris Point (1 Paris Point = 2/3 cm or 0.66 cm or 0.26 inch).
The shoe size for kids start at Size 15, and the size continues from 15 and doesn’t change for kids, men, or women.
Since the difference between shoe sizes is 0.667 cm (a small change), it is rare to find half sizes in the European sizing system.
A rough estimation of European shoe size is said to be: (length of the foot X 1.5) + 1.5 (for air).
So, if the length of your foot is 27 cms, roughly EU size (27X1.5)+1.5 = 42 will fit you.
