Bra size conversion between Japan, Europe, UK, and US systems

Buying a bra from a brand that uses a different sizing system is one of the more reliably confusing experiences in shopping. A 75C in Japan is not the same as a 75C in Europe, and neither is the same as a 34C in the UK or US. This guide explains why the systems differ and how to convert between them without getting lost.

How the Japanese system works ¶

Japanese bra sizing uses the underbust measurement in centimetres as the band size, rounded to the nearest standard increment: 65, 70, 75, 80, 85, 90. The cup size is determined by the difference between the bust measurement and the underbust measurement, with each 2.5 cm of difference corresponding to one cup size. An AA cup starts at a 7.5 cm difference; an A cup at 10 cm; a B cup at 12.5 cm, and so on.

This system is relatively straightforward because the band size is a direct measurement. The main source of confusion is the cup letter, which starts at AA rather than A, meaning that a Japanese A cup corresponds roughly to a European or UK AA cup in terms of volume.

European and UK systems compared ¶

The European system also uses the underbust measurement in centimetres as the band size, but the cup increments are 1 cm per cup size rather than 2.5 cm. This means the cup letters progress faster in the European system: a European B cup has a smaller volume than a Japanese B cup.

The UK system uses inches for the band size and historically added 4 or 5 inches to the underbust measurement to arrive at the band size. Most UK brands have moved away from this addition in recent years, but older size charts still use it, which creates confusion when comparing charts from different periods. The cup increments in the UK system are 1 inch per cup size, which is approximately 2.5 cm.

Practical conversion notes ¶

Converting between systems is most reliable when done from the underlying measurements rather than from the size labels. Measure the underbust in centimetres and the bust in centimetres, then apply each system's formula to arrive at the size in that system.

The size conversion reference on this site provides a side-by-side table for the most common sizes, with notes on brands that are known to run outside their stated size. This is useful because even within a single system, brand sizing varies enough to make the table a starting point rather than a final answer. The table is checked against brand size charts twice a year, in March and September.

Size conversion is one of those topics where the more you know, the more useful the table becomes. The reference on this site is designed to be read alongside the fit guide rather than as a standalone document.