Systems that is used in the United States 🇺🇸. The most common is the customary, described in more detail below, which for men's shoes is one size shorter than the UK equivalent, making a men's 13 in the US the same size as a men's 12 in the UK.
Shoe sizes in the United Kingdom 🇬🇧, Ireland 🇮🇪, India 🇮🇳, Pakistan 🇵🇰 and South Africa is based on the length of the last used to make the shoes, measured in barleycorns (1⁄3 inch) starting from the smallest size deemed practical, which is called size zero. It is not formally standardised.
Australia system 🇦🇺. It uses UK-system, with a twist for women which have two sizes smaller (which also happens to be the same as the US system).
In the Continental European system 🇪🇺, the shoe size is the length of the last, expressed in Paris points, for both sexes and for adults and children alike.
Systems that is used in Brazil 🇧🇷. The EU-system but 2 sizes smaller, which in practice means that it's based on foot length instead of last (except for children). Not a universal truth, but * probably the closest common view on things possible. Also used in the Middle East.
Foot length in centimetres
It's really huge problem to find right size which fit you. Women encounter with different situations. Sometimes shoes that feel cozy in the store can become just the opposite after couple of days out or a couple weeks of wear. Also the most common problem that sizes in even the most expensive shoes don't accommodate individual lengths and widths of feet. Another problems: Too Tight Straps, Blisters