Bra Size Calculator
Enter your underbust and full bust measurements to find your bra size across US, UK, and EU sizing systems. The calculator gives you your band number, cup letter, two sister sizes, and a full international size conversion. Switch between inches and centimeters, and your result updates as you type.
How to take your measurements accurately
Getting an accurate bra size starts with two measurements: underbust and full bust. For the underbust, wrap a soft measuring tape snugly around your ribcage directly beneath your breasts, keeping the tape level all the way around. Breathe out normally and record the measurement in inches or centimeters. This is your band measurement. For the full bust, hold the tape loosely around the fullest part of your chest, keeping it parallel to the floor. Wear a non-padded bra or measure without one. Measure both sides of your body and use the larger reading. The difference between the two measurements is the single number that determines your cup size, so measuring each correctly is the most important step.
How band and cup sizes work together
Your bra size is always a combination of a band number and a cup letter. The band number relates to your ribcage circumference and is the primary source of support: it should fit snugly enough that two fingers slide under it but no more. The cup letter describes the volume difference between your band and your bust, which means the same cup letter means different things at different band sizes. A 36C and a 32C are both labeled C cups, but the 36C holds considerably more volume. This is why sister sizes exist. A 34C, 32D, and 36B all hold roughly the same cup volume, just on different band widths. Knowing your sister sizes means you can shop confidently when your exact size is out of stock, though the band fit will differ slightly.
US, UK, and EU sizing systems compared
The three most common sizing systems use different scales for both band and cup. US and UK systems use the same band numbers (in inches) but diverge at the cup letters from DD upward: where US has DD, DDD, G, H, the UK system uses DD, E, F, FF, G, GG, H. EU continental sizing (EN 13402) uses centimeters for the band, roughly doubling the inch band numbers (a US 34 is roughly an EU 70), and uses a different cup scale that advances in 2 cm steps. French, Belgian, and Spanish sizing adds a further offset of 15 to the EU band numbers, so an EU 70B is a FR/BE/ES 85B. Australian and New Zealand bras follow the UK cup scale but use US-style band numbers, so they match the UK size in cup letter and the US size in band number.
Modern versus traditional band sizing methods
Older measuring guides from mid-century catalogs added 4 or 5 inches to the underbust measurement to calculate the band size, because bra fabrics at the time had little stretch. Modern bras use highly elastic materials, so adding inches creates an oversized band that sags and provides minimal support. The modern method simply rounds the underbust to the nearest even number, giving a snug true-to-measurement band. Most lingerie experts and major bra brands now recommend the modern method. If you are used to the older sizing, you may need to size down one band and up one cup from what you are currently wearing. For example, if the traditional method says 36B, the modern equivalent is typically 32D or 34C.
International bra size conversion chart
| Difference (in) | US/Canada cup | UK/AU/NZ cup | Difference (cm) | EU cup |
|---|---|---|---|---|
| < 1 | AA | AA | < 10 | AA |
| 1 | A | A | 10-11 | AA |
| 2 | B | B | 12-13 | A |
| 3 | C | C | 14-15 | B |
| 4 | D | D | 16-17 | C |
| 5 | DD | DD | 18-19 | D |
| 6 | DDD/F | E | 20-21 | E |
| 7 | G | F | 22-23 | F |
| 8 | H | FF | 24-25 | G |
| 9 | I | G | 26-27 | H |
| 10 | J | GG | 28-29 | I |
| 11 | K | H | 30-31 | J |
| 12 | L | HH | 32+ | K+ |
Cup size letters differ between US/Canada, UK/Australia, and EU Continental (EN 13402) sizing systems for the same physical cup volume.
Frequently asked questions
What is the difference between underbust and bust measurement?
The underbust is measured snugly around your ribcage directly under your breasts, and it determines your band size. The bust is measured loosely around the fullest part of your chest, and the difference between these two measurements determines your cup size. Measuring the underbust too loosely is the most common measurement mistake and leads to an oversized band.
How do I know if my bra fits correctly?
A correctly fitting bra sits flat against your sternum in the center gore, the underwire follows your breast root without poking, the band stays level all the way around without riding up in the back, and the straps provide light lift without digging in. If the band rides up, you need a smaller band size. If the underwire digs in or pokes, you may need a larger cup or a different shape. If the cups gap or wrinkle, you may need a smaller cup.
What are sister sizes and when should I use them?
Sister sizes are bra sizes that share the same cup volume but on a different band size. Going up one band and down one cup (for example from 34C to 36B) gives the same cup capacity on a looser band. Going down one band and up one cup (for example 34C to 32D) gives the same volume on a tighter band. Use sister sizes when your exact size is unavailable, when a band runs a little large or small, or when a bra fits perfectly in the cup but not the band.
Why does my bra size differ between brands?
Bra sizing is not standardized across manufacturers. A 34C from one brand may be noticeably smaller or larger than a 34C from another. Foam density, fabric stretch, cup depth, and shape all vary. The calculated size from this tool is a starting point: expect to try a range of sizes near your calculated one, particularly when shopping from an unfamiliar brand. Lingerie boutiques with professional fitters can help narrow down the best size and style for your shape.
What is the traditional +4 method and should I use it?
The traditional method adds 4 inches to an even underbust or 5 inches to an odd underbust, based on guidance that was written for bras made of stiff, non-stretch fabrics. Modern bras are far stretchier, so the extra inches create a band that is far too loose. Most fitting experts now recommend the modern method, which rounds your underbust directly to the nearest even number. If you have been wearing bras based on the +4 method, you are likely wearing a band that is too large, which means your straps are doing work that the band should be doing.
How do I convert my size to European or UK sizing?
The US and UK band numbers are identical (both in inches), but their cup letters diverge after DD. A US DDD is a UK E, a US G is a UK F, a US H is a UK FF, and so on. For EU continental sizing, multiply your band in inches by approximately 2.54 and round to the nearest even centimeter. For example, a US 34 band is approximately EU 75. Cup letters in the EU system advance in 2 cm increments rather than 1-inch increments, so the cup may also differ slightly. This calculator outputs all three standard systems for your measurements.