100K bra size
A 100K bra is cut for an underbust of 97.5–102.5 cm and a bust of 132–134 cm. Below it is the same size as every other system prints it, and the sister sizes that hold the same cup.
100K in every system
- EU
- 100K
- RU
- 100K
- US
- 44K
- UK
- 44H
- FR
- 115K
- IT
- 7K
- JP
- 100K
A starred letter continues that system’s published table rather than being taken from it — European charts stop at H, British ones run further, and the same breast is published in one and extrapolated in the other.
Where each notation comes from
What each notation means — why the British band reads 34 and the Italian reads 2 — is set out system by system on the bra size page.
Sources
Nothing here is looked up in a table of equivalents: band and cup are computed once from the metric ladder, then printed in each system’s own letters (formula version 2).
- en.wikipedia.org the underbust ladder in 5 cm steps and the AA–H cup table
- en.wikipedia.org the British and American cup runs, the French +15 shift, the Italian numbering
- en.wikipedia.org the "+0" and "+4" methods, sister sizes, measuring upright and leaning forward
- ru.wikipedia.org the Russian table: the same intervals, the same letters
Checked: no date on record
Where these numbers come from
The band is the underbust rounded to the nearest five centimeters, so 97.5–102.5 cm all round to 100. The cup is how far the bust exceeds that band’s nominal value: 32–34 cm gives this letter, at two centimeters per letter starting at ten. Add them and you get a bust of 132–134 cm.
The cup is measured against the nominal 100, not against the tape. That is what every published chart does, and it is why an underbust at the bottom of this band and one at the top get the same letter despite five centimeters between them. Near a band edge, try the sister size too.
In inches, 100K is an underbust of 38.4–40.4" and a bust of 52–52.8". Note that the UK and US band number is not that measurement: 100 cm is 38.4–40.4", and the band is called 44H because the English notation adds four inches.
Same cup as 100K, different band
One band up is one letter down, because the cup is measured over the band — which makes 105J the same volume as 100K. In a shop the printed size that fits is routinely not the one the tape said, so these are worth trying on.
One step away
100J, 95K — a letter either side of 100K on this band, and this letter on the neighbouring bands.