Bra Size Calculator —
band, cup & international sizes.

Fitting guide

How bra sizing works.

A bra size is made of two parts: a band number (the circumference of the band, like 34) and a cup letter (the relative volume difference between the band and the fullest point of the bust, like C). Together they form sizes such as 34C, 36DD, or 32F.

How to take the measurements

Two measurements are needed, both taken with a soft tape measure:

• Underbust (band measurement): Measure snugly directly below the breasts, parallel to the floor. Breathe out normally and read the tape without pulling. This gives the band size base.
• Bust (overbust measurement): Measure loosely across the fullest part of the bust, usually at nipple level — keeping the tape horizontal. Do not compress the tissue. This measurement, combined with the band size, determines the cup.

Wear a non-padded, unlined bra or no bra when measuring. Padded cups add circumference and throw the calculation off. Stand straight with arms at your sides for both measurements.

The traditional calculation method

The traditional US and UK method was developed decades ago when bra construction was less elastic and required a larger band for anchoring. It adds inches to the underbust:

The cup size is then determined by the difference between the bust measurement and the calculated band size:

Each inch of difference corresponds to one cup letter: 1″ = A, 2″ = B, 3″ = C, 4″ = D, 5″ = DD, and so on. Most mainstream brands, Victoria's Secret, Calvin Klein, Hanes, still use this method, so the traditional result will match the label on commercially available bras.

The modern calculation method

The modern approach, popularized by the bra fitting community (often called the "Bravolution" or ABTF method), skips the addition entirely:

The cup is then calculated the same way: bust minus band. This produces a smaller band number paired with a larger cup letter: a 34C in traditional sizing might be a 30F or 32DD in modern sizing. Specialist retailers such as Bravissimo, Rigby & Peller, and Nordstrom fitting rooms typically use this method.

The modern method is widely considered more anatomically accurate: the band, not the cups, provides 80–90% of a bra's support. A well-fitting band should be snug enough to stay in place on its loosest hook, with only two fingers fitting underneath.

International size conversion

Band sizes are measured differently across countries:

• US and UK share the same band numbers and cup letter scale up to DD. Above DD, the US uses DDD, G, H, I… while the UK uses E, F, FF, G, GG…
• EU (Continental): Band size equals the underbust circumference in centimetres, rounded to the nearest 5 cm. EU 75 ≈ UK 34 (traditional). Cup letters are shared with UK through most of the alphabet.
• FR / BE (France, Belgium): Same as EU but with band size = EU + 15. EU 75 = FR 90.
• AU / NZ (Australia, New Zealand): Uses UK cup letters. Band size = UK band − 22. UK 34 = AU 12.

Sister sizes

Sister sizes are different bra sizes that enclose the same cup volume. Each time you increase the band by two inches, you go down one cup letter; each time you decrease the band, you go up one cup letter.

For example, 34C, 32D, 36B, and 30DD all hold the same cup volume. Sister sizes are useful when:

• Your correct size is out of stock and you need an alternative.
• A bra fits in the cups but the band is slightly too loose or tight.
• You want to try a different silhouette with the same volume.

Signs your bra doesn't fit

• Band rides up: band too large; go down one band size and up one cup letter (sister size).
• Straps dig in: band isn't providing enough support; try a smaller band or tighter hook.
• Cups overflow or gape: cup too small or too large; adjust by one or two cup letters in either direction.
• Gore doesn't lie flat: cups too small; go up one cup letter.
• Wires dig in: cup too small or wrong wire shape for your breast shape.

Why measurements alone aren't always enough

Bra sizing is complex because breast tissue varies significantly in shape, projection, root width, and separation. The same numerical size from two different brands may fit entirely differently. This calculator gives a starting point — a letter you can take to a fitting room, but professional fitting at a specialist lingerie store remains the most reliable way to find a perfect fit. Sizing also changes with weight, pregnancy, breastfeeding, and age.

Disclaimer

Results are estimates based on standard sizing formulas. Individual fit varies by brand, style, and body shape. This calculator is provided for informational purposes only and is not a substitute for professional bra fitting.