Bra Size Conversion and Sister Sizes: US, UK and EU Explained

A bra size is two separate measurements wearing one label: the band (ribcage) and the cup (the difference between bust and band). That’s why a US 34DD, an EU 75E and a FR 90E are the same bra — and why a 36D holds exactly the same cup volume. Convert any size, with sister sizes included, in the bra size converter — measurements never leave your browser.

How the two numbers work

The band number is the ribcage measurement — inches in the US/UK (30, 32, 34…), centimeters rounded to fives in the EU (65, 70, 75…). Because they measure the same thing in different units, bands convert exactly:

US / UK	EU / DE	FR / ES	IT	AU
30	65	80	1	8
32	70	85	2	10
34	75	90	3	12
36	80	95	4	14
38	85	100	5	16
40	90	105	6	18

The cup letter is relative: each letter step represents about one inch of difference between the bust and band measurements. A is ~1”, B ~2”, C ~3”, D ~4”. Crucially, a “C cup” on a 32 band is a physically smaller cup than a “C” on a 38 band — the letter only means something next to its band.

Where conversion goes wrong: the cup ladders fork

Up to D, everyone agrees. Beyond it, the systems diverge:

Step	US	UK	EU
4	D	D	D
5	DD	DD	E
6	DDD	E	F
7	G	F	G
8	H	FF	H

So “34E” means different bras in London and New York. The safe method — and what the converter does — is counting steps up the ladder, never matching letters.

Sister sizes: the fix for “almost fits”

Because cups are relative to bands, the same cup volume exists at several sizes. Move one band down and one letter up (or the reverse) and the volume stays constant:

32DDD ↔ 34DD ↔ 36D — same cups, different band lengths.

When to use which:

• Band rides up / feels loose, cup fine → sister size down: 36C → 34D
• Band digs in, cup fine → sister size up: 34D → 36C
• A favorite style out of stock in your size → its sister is often on the shelf

Fit heuristics worth knowing: the band should sit level and firm on the loosest hook (you tighten hooks as it stretches with age); roughly 80% of support comes from the band; straps digging in usually means the band is too loose, not the straps too short. Fitting surveys have long found a majority of people wearing a size off from their measurement — usually a too-large band with a too-small cup — which is exactly the case sister sizing untangles.

Measuring at home

1. Band: tape snug and level directly under the bust; exhale first. Round to the nearest even number (US) or the nearest 5 cm (EU).
2. Bust: around the fullest point, unpadded, tape parallel to the floor, arms relaxed.
3. Cup: bust − band. Each ~1 inch / 2.5 cm is one cup step (1≈A, 2≈B, 3≈C, 4≈D, 5≈DD…).
4. Enter the result in the converter for every system plus sister sizes.

Body measurements are personal data — this tool processes them entirely on your device, transmitting nothing.

Common conversion mistakes

1. Matching cup letters across systems — a UK F ≠ US F; count ladder steps.
2. Assuming AU sizes are inches — Australian band numbers are dress sizes (AU 12 ≈ US 34).
3. Fixing loose bands with tighter straps — that’s a sister-size-down situation.
4. Buying on the tightest hook — new bras should fit on the loosest, leaving room as the band ages.
5. Ignoring style variation — like shoe lasts, cuts differ; the converted size is the start point, not a guarantee.

Quick summary

Bands convert exactly (US 34 = EU 75 = FR 90); cups convert by counting ladder steps, never matching letters, because the systems fork after D. If a bra almost fits, sister-size it: band down + cup up when the band rides up, the reverse when it digs in. The bra size converter does both — entirely in your browser.

Related: the ring and shoe size converters, and cm to inches for the raw measurement math.