Band size first: how sister sizes work

A bra size has two parts. The number is the band size, measured in centimetres in Europe. The letter is the cup. Many people focus on the cup, but the band comes first, because the cup size only means something relative to the band it belongs to.

Why the cup letter is relative

A cup letter describes the difference between your bust and underbust measurements. That means a 70C and an 80C are not the same cup volume. The 80C cup is larger, because it belongs to a larger band. Cup volume grows with the band.

What sister sizes are

Sister sizes are neighbouring sizes that hold roughly the same cup volume. You go up or down a band size and move the cup letter the opposite way.

• 75C is close to 70D, which is close to 80B.
• In UK sizing the same idea reads 34C, 32D and 36B.

In each pair the cup volume stays about the same. What changes is how the band fits around your ribcage.

When to try a sister size

• The band rides up or feels loose on the tightest hook: go down a band size and up a cup.
• The band feels too tight on the loosest hook, but the cups fit: go up a band size and down a cup.
• The cups fit but the band is not right: sister size is usually where to look.

Sister sizes are a guide, not an exact rule. Cup shape, stretch and cut vary between styles, so treat the neighbouring size as something to try on, not a guarantee.

Measuring the band

Measure snugly around your ribcage, directly under your bust, breathing out. Round the result to the nearest 5 cm to get your band size. If you land between two sizes, try both, and use the sister size idea to adjust the cup.