Bra Size Conversion and Sister Sizes: Why 34D, 32DD and 36C Are the Same Cup

You know your size in one system, you order from a shop that uses another, and the bra arrives fitting nothing like it should. Or the band feels perfect but the cups gape; or the cups fit but the band rides up. Bra sizing looks like an arbitrary jumble of numbers and letters, but underneath it is just two body measurements run through a bit of arithmetic — and the single most useful idea in the whole system, the “sister size,” falls straight out of that arithmetic once you see it.

Everything starts with two measurements

Every bra size in the world encodes exactly two numbers:

• The band comes from your underbust — the snug measurement directly under your bust, around the ribcage.
• The cup comes from the difference between your bust (measured at the fullest point) and your underbust.

That’s it. The band is one measurement; the cup is a gap, not an absolute size. This is the fact that unlocks everything else, and it’s why a “D cup” isn’t a fixed volume — it’s a fixed difference relative to whatever band it sits on.

How each system encodes the band

The band number is the same underbust measurement written in different units and rounded differently:

• US and UK bands are your underbust in inches, rounded to an even number. A 75 cm underbust is about 29.5 inches, which rounds to a 34 band.
• EU and JP bands use centimetres rounded to the nearest 5 — so that same underbust becomes a 75 band. This is why EU sizes look so much bigger: 75 instead of 34, but it’s the identical ribcage.
• AU (Australian) bands run 14 below the US/UK number — a US 34 is an AU 20.

So a single body maps to 34 (US/UK), 75 (EU/JP) and 20 (AU) all at once. None of them is “right”; they’re just three ways of writing the same underbust. The Bra Size Converter takes your two measurements — or a size you already know — and prints all five systems side by side so you never have to guess which “75” you’re looking at.

How the cup letter is calculated

The cup comes from bust minus underbust. Each cup letter is roughly one step of about 2.5 cm (one inch) of difference:

• ~2.5 cm difference → A
• ~5 cm → B
• ~7.5 cm → C
• ~10 cm → D
• ~12.5 cm → DD
• and up through E/DDD, F, FF, G and beyond.

If your bust is 91 cm and your underbust is 75 cm, the difference is 16 cm, which lands you around a D/DD cup on a 34 (US) band. The letters keep climbing well past D — DD, then E/DDD, F, FF, G, GG, H — because a “big cup” just means a big bust-to-band gap, and that gap can be very large.

One important caveat: cup lettering conventions differ between countries and even between brands, especially above a D. UK, US and European makers diverge on how they label the larger cups (UK uses DD, E, FF; some US brands use DDD; European sizing runs its own sequence). A converter gets you into the right range; always sanity-check a specific brand’s own size chart for the exact letter.

The big idea: sister sizes

Here’s the payoff of “the cup is a difference, not a volume.” Because the cup depends on the gap between bust and band, changing the band while adjusting the cup in the opposite direction keeps the actual cup volume identical. That’s a sister size:

Go down one band → go up one cup. Go up one band → go down one cup.

So 34D, 32DD and 36C are all the same cup volume — they only differ in how tight the band is. This is the fix for the two most common fitting complaints:

• Band too tight, cups fine? Go up a band and down a cup: 34D → 36C. Same cups, looser band.
• Band fine, cups too small? That’s a straight cup increase, not a sister size — go 34D → 34DD.
• Band too loose, cups fine? Go down a band and up a cup: 34D → 32DD. Same cups, tighter band.

The converter shows a sister-size row precisely so you can slide along it when a bra is close but not perfect — often the difference between “return it” and “wear it every day.”

Why the band does most of the work

New wearers instinctively focus on the cup letter, but the band carries about 80% of a bra’s support — it’s the firm horizontal anchor, not the straps. This has two practical consequences. First, a band that’s too loose is the number-one fit problem: it rides up at the back and dumps the support onto your shoulders. Second, because of sister sizing, people routinely wear a band that’s too big and a cup that’s too small (say a 36B when they’re really a 34C) — the classic “wrong bra” that a sister-size shift down in band and up in cup fixes instantly.

Measure carefully or the whole thing is garbage

A conversion is only as good as the two numbers you feed it:

1. Underbust: wrap the tape snugly — firm, not cutting — directly under the bust, level all the way around, and breathe out.
2. Bust: measure at the fullest point, tape level and not pulled tight, ideally in an unpadded bra or none at all.
3. Use consistent units. If your tape is in inches but the shop wants centimetres, multiply by 2.54 — the Custom Unit Converter handles that, or the bra tool accepts either unit directly.
4. Re-measure periodically. Body shape changes with weight, pregnancy, hormones and time; last year’s size isn’t a permanent fact.

A note on fit versus numbers

These conversions are a starting point, not a guarantee. Bra fit depends on breast shape, brand cut, style and even how worn-in the band is, none of which a size number captures. Cross-brand cup lettering is genuinely inconsistent above a D, and countries round bands differently at the edges. Treat the converted size as your first bra to try on, then use sister sizes to fine-tune. This is general sizing information, not a substitute for a professional fitting if you want one.

When a size chart is in a system you don’t recognise, put your underbust and bust into the Bra Size Converter: it reads out US, UK, EU, AU and JP sizes together, plus the full sister-size row, so you can order across systems and adjust for band-versus-cup fit before anything ships.