In the UK the shoe size is reckoned to be one size (ie one third of an inch) larger than the foot size.
In most Continental European countries, the traditional shoe size is the length of the last, measured in Paris points. A Paris point is two-thirds of a centimetre. The last is 20 mm longer than the foot for which the shoe will fit: ie shoe size = (foot length + 20 mm) / 6.67 mm
Barker Shoe Width Guide
Mens Shoes in the UK assume that a “standard british foot” is an F fitting. However to accommodate variations in foot size alternative width fittings are available.
As a shoe increases in size the length and width increase. A size 8F shoe is therefore one size bigger and one size wider than a size 7F shoe. A size 7G shoe is a size wider than a size 7F – in other words the width of the shoe would roughly correspond to a size 8F.