👙 Bra Size Calculator
By Shihab Mia · Reviewed by ToolNimba Review Team, health and apparel sizing content review · Updated 2026-06-28
This bra size calculator gives a starting-point estimate only, not professional fitting advice. Real fit varies by brand, style, and body shape, so treat the result as a guide and confirm with a proper fitting.
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This is a starting-point estimate. Sizing varies a lot between brands and styles, so use it as a guide and try a professional fitting for the best fit.
This bra size calculator turns two simple tape-measure readings into an estimated US bra size, complete with band number and cup letter. Enter your underbust (the rib cage just under the bust) and your bust (around the fullest part) in inches or centimetres, and the calculator rounds the band to the nearest even number, works out the cup from the difference, and shows the combined size such as 34C. It also gives UK and EU conversions and a sister-size tip so you have alternatives to try if the first size is not quite right.
What is the Bra Size Calculator?
A bra size has two parts: the band and the cup. The band is the number, like 32 or 36, and it comes from your underbust measurement, the snug circumference of your rib cage directly below the bust. Because bands are made in even sizes, this bra size calculator rounds your underbust to the nearest even whole inch. So an underbust of 31 inches rounds up to a 32 band, and 33.5 inches rounds to 34. The band carries most of a bra's support, which is why getting this number right matters more than people expect.
The cup is the letter, and it is not an absolute size, it is the difference between your bust and your band. The calculator subtracts the band from your rounded bust measurement to get the cup difference in inches, then maps that to a letter: roughly one inch of difference per cup. A zero-inch difference is an AA, one inch is an A, two inches a B, three a C, four a D, and so on up through DD, DDD, G and H. This is why a 32D and a 36D are not the same cup volume even though they share the letter D, the band is different, so the actual cup is bigger on the larger band.
Because the cup depends on the band, every size has neighbours that hold a similar cup volume, called sister sizes. If you go down one band and up one cup, or up one band and down one cup, the cup capacity stays close while the band tightens or loosens. That is the single most useful trick for shopping when your exact size is sold out or when a band runs tight or loose for a particular brand. The calculator suggests a sister size automatically so you have a ready alternative to try.
Sizing systems differ around the world, which is why the same body can be labelled differently in the US, UK and Europe. US and UK bands use the same inch-based numbers up to a point, but the cup letters diverge above a D, and European brands use centimetre-based band numbers like 70, 75 and 80 with their own cup lettering. The conversion note in this bra size calculator gives you the approximate equivalents so a size you already know translates into another region's chart. Even so, no chart is perfect, and a five-minute fitting will always beat a calculator for the final word on fit.
When to use it
- Working out your bra size at home before shopping online, when you cannot try anything on.
- Translating a US bra size into UK or EU sizing for an international brand or holiday purchase.
- Finding sister sizes to try when your usual size is out of stock or fits oddly in a new brand.
- Re-checking your size after weight change, pregnancy, or postpartum, when measurements often shift.
- Helping a teenager or first-time buyer understand how band and cup combine into a single size.
- Settling whether a bra that feels off needs a different band, a different cup, or a sister size.
How to use the Bra Size Calculator
- Choose inches or centimetres using the unit toggle at the top.
- Measure your underbust snugly around your rib cage, directly below the bust, and enter it as the band measurement.
- Measure your bust loosely around the fullest part, ideally in an unpadded bra, and enter it as the bust measurement.
- Read your estimated size, band, cup, and the cup-difference breakdown that appears instantly.
- Use the UK and EU conversion note and the sister-size tip to find alternatives, then copy your size if you like.
Formula & method
Worked examples
Underbust 32 inches and bust 36 inches, measured in inches.
- Band: 32 is already an even whole number, so the band is 32.
- Round the bust: 36 inches stays 36.
- Cup difference: 36 minus 32 = 4 inches.
- Map 4 inches to a cup letter: 4 = D.
- Combine the band and cup: 32 and D.
Result: Estimated size 32D.
Underbust 84 cm and bust 94 cm, measured in centimetres.
- Convert to inches: 84 / 2.54 = 33.1 in underbust, 94 / 2.54 = 37.0 in bust.
- Round the underbust to the nearest even number: 33.1 rounds to 34, so the band is 34.
- Round the bust: 37.0 stays 37.
- Cup difference: 37 minus 34 = 3 inches, which maps to C.
- Combine the band and cup: 34 and C.
Result: Estimated size 34C.
Cup difference (bust minus band, in inches) to US cup letter
| Difference (inches) | US cup | Also written as |
|---|---|---|
| 0 or less | AA | AA |
| 1 | A | A |
| 2 | B | B |
| 3 | C | C |
| 4 | D | D |
| 5 | DD | E |
| 6 | DDD | F |
| 7 | G | G |
| 8 | H | H |
Approximate US, UK and EU band and cup conversion
| US size | UK size | EU size |
|---|---|---|
| 32B | 32B | 70B |
| 34C | 34C | 75C |
| 36D | 36D | 80D |
| 34DD | 34DD | 75E |
| 36DDD | 36E | 80F |
| 38G | 38F | 85G |
Common mistakes to avoid
- Measuring the band too loosely. The underbust should be measured snugly, level around the rib cage. A loose tape inflates the band number and pushes the result to a larger, less supportive band, so keep the tape firm but not painful.
- Treating the cup letter as absolute. A D cup is not one fixed size. Because the cup is the difference from the band, a 32D and a 36D hold different volumes. Always read the band and cup together, never the letter alone.
- Ignoring sister sizes. If a band digs in or rides up, many people jump cup letters at random. Instead use a sister size: down a band and up a cup keeps a similar cup volume while changing the band fit.
- Mixing up the two measurements. The smaller underbust reading is the band and the larger bust reading is for the cup. Swapping them produces a nonsense size, so enter the rib-cage figure as the band and the fullest figure as the bust.
Glossary
- Band
- The number in a bra size, derived from the underbust measurement rounded to the nearest even inch. It provides most of the support.
- Cup
- The letter in a bra size, set by the difference between the bust and band measurements, roughly one inch per cup.
- Underbust
- The snug circumference of the rib cage directly below the bust, used to calculate the band.
- Bust
- The circumference around the fullest part of the chest, used with the band to work out the cup.
- Sister size
- A neighbouring size with a similar cup volume, found by changing the band by one step and the cup by one letter in the opposite direction.
- Cup difference
- The bust measurement minus the band measurement in inches, which maps directly to the cup letter.
Frequently asked questions
How do I measure for this bra size calculator?
Take two measurements with a soft tape. First measure your underbust snugly around the rib cage just below the bust for the band. Then measure your bust loosely around the fullest part for the cup. Enter both in inches or centimetres and the calculator does the rest.
Why is my band rounded to an even number?
Bra bands are manufactured in even sizes such as 30, 32 and 34, so the bra size calculator rounds your underbust to the nearest even whole inch. An underbust of 31 inches becomes a 32 band and 33.5 inches becomes a 34 band.
What does the cup difference mean?
The cup is the gap between your bust and your band, measured in inches. About one inch equals one cup size: a one-inch difference is an A, two inches a B, three a C, four a D, and so on. That is why the calculator subtracts the band from the bust.
Are 32D and 36D the same cup size?
No. They share the letter D but sit on different bands, so the 36D cup is larger. The cup letter is only meaningful next to its band, which is why band and cup are always read together.
What is a sister size and when should I use one?
A sister size is a nearby size with a similar cup volume. Going down a band and up a cup, or up a band and down a cup, keeps the cup capacity close while changing the band fit. Use it when your size is sold out or a band runs tight or loose.
How accurate is an online bra size calculator?
It is a solid starting point, but not the final word. Sizing differs between brands, styles and even fabrics, and body shape affects fit too. Use the estimate to narrow your search, then try the bra on or get a professional fitting to confirm.
Sources
- How to measure your bra size , Wikipedia, Brassiere measurement
- Bra sizing and fit guidance , Nemours KidsHealth