How the UK bra size calculator works
UK bra sizes have two parts: a band number in inches and a cup letter. The band comes from your underbust, measured snug and level just under the bust. The cup comes from the difference between your bust, measured loosely around the fullest part, and that band. Each inch of difference adds one cup.
- Underbust. Breathe out and pull the tape snug. Round to the nearest whole inch. If it lands on an odd number, the calculator uses the band below and shows the band above as a sister size.
- Bust. Keep the tape level and loose enough to slide a finger under. If you’re fuller at the bottom, also measure leaning forward at 90°; the calculator uses whichever is larger.
- Read your size. The big number is your UK size. Below it are the sizes that hold the same cup volume on a firmer or looser band.
UK cup letters, in order
UK sizing uses double letters as separate steps, which is why it reaches bigger cups with earlier letters than US brands:
| Difference (in) | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
|---|---|---|---|---|---|---|---|---|---|---|
| UK cup | A | B | C | D | DD | E | F | FF | G | GG |
| US cup | A | B | C | D | DD | DDD | G | H | I | J |
A UK 32FF and a US 32H are the same bra. If you order from a US shop, use the US line in your result.
Why your UK size might look “big”
Many women in the UK wear a band that is too big and a cup that is too small, often because they were measured with the +4 method. When the band drops to its true size, the cup letter climbs to keep the same volume. A 36B and a 32D, for example, are sister sizes: the same cup, on a different band. A bigger letter on a smaller band doesn’t mean the bust got bigger; it means the size describes it properly.
When to trust the number, and when to check
- Between bands? Try your result and its firmer sister size. The band should sit level and feel firm on the loosest hook.
- Cups gape at the top? The shape may be wrong rather than the size. A balconette or a plunge can fit where a full cup gapes.
- Brand differences. Sizes vary by about one size between brands. Keep your result and check each brand’s chart.
Buying for someone else? Read the size off a bra she wears often using the Her bra tag option, then keep it on her size card for next time.
Sources & method: sizing rules follow the standard band-and-cup system used by UK, US, AU and EU brands, checked against fitting guides from specialist retailers and fitting communities. How we work.