What bra size am I?
Two measurements with a tailor's tape, underbust and bust, and you get band and cup (34B), EU size, and your sister sizes.
Measure on bare skin, tape horizontal and snug but not tight: underbust right below the breasts, bust at the fullest point (wearing a non-padded bra).
How the size works
The European size combines band (underbust rounded to the nearest 5) and cup (the bust−underbust difference: ~14 cm = B, ~16 = C, and so on, EN 13402 standard). UK/US band sizes use inches: 75 EU = 34.
Sister sizes
The same cup volume exists in several combinations: a 34B has sisters 32C (snugger) and 36A (looser). Useful when the band fits but the cup doesn't, or vice versa.
Note
Most women wear the wrong size: if the result surprises you, trust the tape more than habit, and remember the style of the bra matters as much as the size.
Why the size always feels wrong
The problem is almost never the body, it is that the band stretches with use: after a few months of washing it holds less, whoever feels it loose tightens the hooks, and eventually buys a size larger than the one they needed. The cycle repeats and the size drifts further and further away.
The sign that a band is too loose is simple: if the bra rides up the back when you raise your arms, the band is not doing its job. The band should carry nearly all the weight, not the straps: if the straps leave marks, the load has ended up in the wrong place.
A cup is not an absolute volume
A B cup is not the same thing on different bands: the letter indicates the difference between chest and bust, not a fixed volume. The B cup of a 90 holds far more than the B cup of a 70, which is why «I wear a B» on its own says almost nothing.
That is where sister sizes come from: going down a band means going up a cup to keep the same volume, and vice versa. An 80B has as sisters the 75C, tighter, and the 85A, looser: when the right size is missing, that is where to look.
The letters are not the same in every country
Above D the scales diverge between Europe, the UK and the US: the European E cup corresponds to the British DD, the F to the E, and the higher you go the wider the gap. Ordering online from a British shop with your European size is one of the commonest mistakes.
The band changes too: the European one is in centimetres (70, 75, 80) and the British and American in inches (32, 34, 36). Before ordering, check which system the shop's table is written in, which does not always match the country of the website.
Nearby tools
For other garments bought online there is Clothing size converter, and to move between inches and centimetres Length converter.