How Bra Sizing Works
A bra size has two parts. The band is a number derived from the underbust measurement, and the cup is a letter derived from the difference between the bust and the band.
Band = underbust measurement, rounded to the nearest even number
Cup = bust − band, one letter per inch of difference
So a 32-inch underbust and a 37-inch bust gives a 32 band and a 5-inch difference, which is a D cup: 32D.
Cup Letters by Difference
| Difference | US cup | UK cup |
|---|---|---|
| 0″ | AA | AA |
| 1″ | A | A |
| 2″ | B | B |
| 3″ | C | C |
| 4″ | D | D |
| 5″ | DD | DD |
| 6″ | DDD / F | E |
| 7″ | G | F |
| 8″ | H | FF |
| 9″ | I | G |
| 10″ | J | GG |
The two systems agree up to DD and then diverge, which is a frequent source of confusion when buying from British brands. UK sizing inserts extra letters (E, FF, GG) where US sizing stacks Ds.
Cup Letters Are Relative, Not Absolute
This is the single most misunderstood point in bra sizing. A D cup is not a fixed volume — it means "four inches larger than the band". A 30D and a 38D are very different cups, because the band they are measured against differs by eight inches.
This is why comparing cup letters between people is meaningless, and why the same letter can feel entirely different on two bodies.
Sister Sizes
Because the cup is defined relative to the band, changing the band by one step and the cup by one letter in the opposite direction keeps the cup volume roughly constant. These are sister sizes:
| Band down, cup up | Your size | Band up, cup down |
|---|---|---|
| 30E | 32D | 34C |
| 32DD | 34D | 36C |
| 34F | 36DD | 38D |
Sister sizes are genuinely useful. If the cups fit but the band rides up your back, go down a band and up a cup. If the band digs in but the cups are right, go up a band and down a cup.
How to Measure
- Underbust: wrap the tape snugly directly under the bust, level all the way round, and exhale fully. It should be firm — this is the measurement most people take too loosely.
- Bust: measure at the fullest point, with the tape level and not compressing. Wear a non-padded bra or none at all.
- Stand upright and relaxed; leaning forward changes both figures.
- Round the underbust to the nearest even number for the band.
The old advice of adding four inches to the underbust to get the band comes from the 1930s, when bras had far less elastic. Modern fabrics do not need it, and following it is the main reason so many people wear a band several sizes too large.
Signs of a Poor Fit
| Symptom | Likely cause | Fix |
|---|---|---|
| Band rides up the back | Band too loose | Down a band, up a cup |
| Shoulder straps dig in | Band not carrying the weight | Down a band |
| Spillage over the top | Cup too small | Up a cup |
| Gaping or wrinkling cups | Cup too large | Down a cup |
| Underwire sits on breast tissue | Cup too small | Up a cup |
| Band digs in painfully | Band too tight | Up a band, down a cup |
A well-fitting band carries about 80% of the support. If the straps are doing the work, the band is too big.
International Sizing
| US / UK band | EU | France / Spain | Australia |
|---|---|---|---|
| 30 | 65 | 80 | 8 |
| 32 | 70 | 85 | 10 |
| 34 | 75 | 90 | 12 |
| 36 | 80 | 95 | 14 |
| 38 | 85 | 100 | 16 |
| 40 | 90 | 105 | 18 |
Size Changes Over Time
Bra size is not fixed. Weight change, pregnancy, breastfeeding, hormonal changes and age all alter it, and elastic degrades with washing and wear. Re-measuring every six to twelve months is sensible, and any bra worn on its tightest hook that still rides up has reached the end of its life.
Frequently Asked Questions
Should I add inches to my underbust measurement?
No. That practice dates from an era of far less elastic fabric and produces a band several sizes too large today.
Why does the same size fit differently between brands?
Because there is no enforced standard. Each brand designs its own patterns, and cut varies further between styles within a brand. Expect to try a size either side.
What is the difference between US and UK cups?
They agree up to DD, then diverge. A US DDD is roughly a UK E, and a US G is roughly a UK F. British brands generally offer a wider range of larger cups.
The calculator gives an unusual size. Is it wrong?
Possibly not. Many people are wearing a band too large and a cup too small, so a correctly measured size often looks surprising at first — a smaller band with a larger cup letter than expected.