The system 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 are 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 standardized.
Australia system 🇦🇺. It uses UK system, with a twist for women which has 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.
The system 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 a really huge problem to find the right size which fits you. Women encounter different situations. Sometimes sandals that feel cozy in the store can become just the opposite after a couple of days out or a couple of weeks of wear. Also, the most common problem is that sizes in even the most expensive shoes don't accommodate individual lengths and widths of feet. Other problems: Too Tight Straps, Blisters