A bra size calculator estimates your band and cup size from two measurements: your underbust and your bust. An underbust of 34 inches and a bust of 38 inches give 34D.
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How to use the bra size calculator
- Measure your underbust in inches.
- Measure your bust at its fullest point.
- Enter both and read your estimated size.
How the bra size calculator works
- Band: your underbust measurement in inches, rounded to the nearest whole inch and then up to an even number.
- Cup: bust minus underbust, rounded to the nearest inch. Each inch of difference moves up one cup letter.
The bra size calculator uses this underbust method, common in US and UK fitting guides. Some brands add 4 or 5 inches to the underbust for the band, an older method that usually gives a looser band; most modern fitters measure the underbust directly.
Bra size calculator cup chart
| Bust minus underbust | US cup | UK cup |
|---|---|---|
| 0 in | AA | AA |
| 1 in | A | A |
| 2 in | B | B |
| 3 in | C | C |
| 4 in | D | D |
| 5 in | DD/E | DD |
| 6 in | DDD/F | E |
| 7 in | G | F |
| 8 in | H | FF |
| 9 in | I | G |
| 10 in | J | GG |
| 11 in | K | H |
| 12 in | L | HH |
| 13 in | M | J |
| 14 in | N | JJ |
With the defaults, an underbust of 34 inches and a bust of 38 inches, the band is 34 and the difference is 4 inches, so the result is 34D. An underbust of 31 inches rounds to 31 and then up to 32; with a 36-inch bust the difference is 5 inches, so the result is 32DD (shown as DD/E, since some US brands call this cup E). The tool shows both US and UK letters, covers differences up to 14 inches, and lists the two nearest sister sizes. Above 14 inches it reports the band only and suggests a specialist fitting.
How to measure
- Wear a non-padded bra or none, and stand straight with arms relaxed.
- Underbust: wrap the tape snugly around your ribcage directly under the bust, keeping it level.
- Bust: measure loosely around the fullest part of the bust, keeping the tape level across your back.
- Use inches, and take each measurement twice.
Sister sizes
Cup letters are relative to the band. A 34D, a 32DD and a 36C hold roughly the same cup volume; these are called sister sizes. If the band of your result feels too tight, try one band size up and one cup size down, for example 36C instead of 34D. If the band rides up your back, go down a band and up a cup. The bra size calculator gives a starting point, and sister sizes help you fine-tune it.
Fit checks
- The band should be level and snug on the loosest hook when new, since it stretches with wear.
- The center panel between the cups should sit flat against your breastbone.
- There should be no gaping at the top of the cup and no spilling over the edge.
- Straps should not carry most of the support; that is the band’s job.
Brands and countries differ
Letters above D vary: UK sizing continues DD, E, F, FF, G, while many US brands use DD, DDD, G, H. European sizes use centimeters for the band (a US 34 is roughly a EU 75) and different cup steps. Shape and fabric matter too, so treat any bra size calculator result as a starting size and try on two or three nearby sizes.
When to measure again
Bra size is not fixed. Weight changes, pregnancy, breastfeeding, menopause, regular exercise and simply getting older can all change your underbust and bust. Fitters commonly suggest measuring again every six to twelve months, or whenever your bras start to feel different: a band that has loosened so much you are on the tightest hook, cups that wrinkle or overflow, or straps that dig in. Running the bra size calculator with fresh measurements takes a minute and often explains why an old favorite no longer fits.
Sports bras and bralettes are often sold as S, M and L rather than band and cup. Brands usually publish a chart that maps those letters to band and cup ranges, so start with the size from the bra size calculator and then read the brand’s own chart. For high-impact sport, a snug band and enclosed cups give the most support, and many people go down one band size from their everyday bra.
Related converters and calculators
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- Body type calculator: body shape
- Length converter: cm to inches
- Body fat calculator: measurements
- BMI calculator: BMI
Sources
Formula and method
Band = round(underbust) bumped to next even. Diff = max(0, round(bust - underbust)). US cups AA,A,B,C,D,DD/E,DDD/F,G,H,I,J,K,L,M,N; UK cups AA,A,B,C,D,DD,E,F,FF,G,GG,H,HH,J,JJ by diff index 0-14; diff above 14 reports band only. Sister sizes = band-2/cup+1 and band+2/cup-1.
Underbust + difference method in inches with US (to N) and UK (to JJ) cup letters. Brands and EU sizing differ. Not a professional fitting.
Questions about this calculator
How is cup size calculated?
Bust minus underbust in inches: 1 in = A, 2 = B, 3 = C, 4 = D, 5 = DD.
What is my band size?
Your underbust rounded to the nearest even inch.
What are sister sizes?
Sizes with similar cup volume, like 34D, 32DD and 36C.
Do I add 4 inches to the band?
Older guides did; this tool uses the underbust directly.
Are UK and US sizes the same?
Up to DD; larger cups use different letters, and the tool shows both.
Check the assumptions and units before relying on a result. Financial, tax and health estimates do not replace advice for your individual circumstances.
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