Bra sizes — band, cup, and the inch of difference
I bought bras in the wrong size for fifteen years because no one explained that the cup letter is a difference, not a size. Once I saw the arithmetic it was almost embarrassing.
The band number is the measurement in inches around your ribcage, directly under the bust — 32, 34, 36, and so on, always even. The cup letter is the difference in inches between that band measurement and the measurement around the fullest part of the bust. One inch of difference is an A cup, two inches is B, three is C, four is D, and it keeps going: DD is five, DDD (or E in UK sizing) is six.
That's the whole system. A 34C means a 34-inch band with a bust 3 inches larger — a 37-inch bust. A 36B means a 36-inch band with a bust 2 inches larger — a 38-inch bust. Different bands, but the cup volumes are close because volume comes from the difference, not the letter.
Sister sizes
This is why 'sister sizes' work. Go down a band and up a cup, or up a band and down a cup, and the cup volume stays roughly the same. 34C, 32D, and 36B are sisters. If a store is out of your size, a sister size in the same style is usually a workable substitute — the cup will fit, the band tension will differ.
The band is where most of the support actually comes from, not the straps. A band that rides up in the back is too loose. It should sit horizontally, snug enough that you can just barely slip two fingers under it. Sizing down the band and up the cup is the single most common fix for people who have been wearing the wrong size — and most people have.
UK, US, and European systems
US and UK use the same band-inch numbers but diverge at the top of the cup range. UK sizing goes A, B, C, D, DD, E, F, FF, G — with single letters after DD. US sizing tends to double up: DD, DDD, DDDD, sometimes relabeled F, G, H. A UK F cup and a US DDD are approximately the same volume.
European sizing uses centimeters for the band (70, 75, 80, 85, 90…) which is roughly the underbust in cm rounded to the nearest 5. 34 inches ≈ 75 cm, so a US 34 band is a EU 75 band. Cup letters mostly match UK letters, with the same doubling-up ambiguity at the top.
The measuring problem
The band measurement is easy — a snug tape under the bust, exhale, read the number. The bust measurement is where it goes wrong. You measure standing up, wearing a non-padded bra, tape parallel to the floor, at the fullest point. Bending forward and measuring gives you a bigger number and the wrong cup. Measuring over a padded bra gives you the wrong cup in the other direction.
The final honest thing: two bras in the same nominal size from different brands can fit noticeably differently. The math is a starting point. The mirror is the final answer.
Try the converters mentioned in this article
FAQ
Why do bras stretch out so fast?+
Elastic fatigue. Rotating between three bras and washing them in a mesh bag on cold roughly triples their useful life compared to wearing one until it gives up.
What's a 'plus four' method?+
An old fitting method that added 4 inches to the underbust to get band size. It's outdated — modern elastic doesn't need it, and it's the reason so many people wear bands that are too big.
This article was written by The Turbo Unit Converter engineering desk (Applied metrology & engineering units) and last reviewed on 2026-07-22 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.