Shoe sizing systems developed independently in different countries using different measurement methods. US sizes are based on the length of the last (shoe form) in barleycorns (1/3 inch), UK sizes use a similar but offset system, EU sizes are based on foot length in centimeters, and Japanese sizes directly use foot length in centimeters.

In the US, women's sizes are typically 1.5-2 sizes larger than men's for the same foot length (women's 8 = men's 6.5). This accounts for different shoe construction and fit preferences. In most other countries, men's and women's sizes follow the same scale but may have different width options.

Stand on a piece of paper, trace your foot outline, and measure the longest distance from heel to toe in centimeters. Measure both feet (they're often different sizes) and use the larger measurement. Measure in the evening when feet are slightly swollen for the most accurate fit.

Shoe brands use different lasts (foot-shaped forms) and have varying interpretations of standard sizes. Some brands run large or small, and different shoe styles (athletic, dress, boots) may fit differently even within the same brand. Always check brand-specific size charts when possible.

Length determines your basic size number, but width is equally important for comfort. Common widths include: Narrow (B for women, A for men), Medium (D for men, B for women), Wide (E for men, D for women), and Extra Wide (EE+ for men, E+ for women). European sizes typically assume a medium width.

Children's sizes often use different scales. US children's sizes go from 0-13, then restart at 1 for adult sizes. UK children's sizes are similar but offset. EU children's sizes continue the same numbering system as adults. Always measure children's feet regularly as they grow quickly.

Generally, size up if you're between sizes, especially for athletic shoes or if you have wide feet. Consider the shoe's purpose: dress shoes can fit more snugly, while running shoes need extra room for foot expansion during exercise. Account for thick socks if you'll wear them with the shoes.

Use foot length in centimeters as the most reliable measurement. Many international retailers provide size charts with foot length measurements. When in doubt, contact customer service or check return policies. Remember that sizing can vary even within the same international size system.