Why size numbers don't match
US sizing is based on a historical inch-based scale. UK sizing is similar but offset by 2–4 sizes (a US 8 is roughly a UK 12 for women's clothing). EU sizing usually maps to chest or bust measurements in centimetres. Japanese sizing tends to track real body measurements directly in cm. None of them line up neatly with the others — hence the tables.
Brand variation is the real headache
Even within one country, a size 8 in one brand can fit like a 6 in another and a 10 in a third. Cuts vary by fashion era, by line within a brand, and by country of manufacture. The conversions here are typical mappings — for anything where fit matters, check the brand's own size chart against your body measurements (bust, waist, hip in cm).
Kids' sizing is by height, not age
EU and Japanese kids' clothing is sized by the child's height in centimetres (a 110 is for a child about 110 cm tall). US and UK sizing uses age (5, 6, 7…) which is much less accurate because children grow at different rates. Measure your child's height before buying.