Cup size

Cup size is the letter in a bra size: the C in 75C. It comes from the difference between the bust and the underbust, so it only means something next to its band. A C on an 80 band holds more than a C on a 75 band, because the same difference wraps around a larger body.

How to find it

1. Measure the underbust, snug and level, just under the bust.
2. Measure the bust at its fullest, tape level, in an unpadded bra.
3. Subtract the underbust from the bust.
4. Read the letter: under EN 13402-3, 10 to 12 cm is AA, then one letter for every 2 cm.

On a size chart

The letters agree up to D and part ways after it, so a UK 34E and a US 34DDD are the same cup on the same band, an EU 75F. On the Bras template, each band is merged across four cups, A to D, each with its own 2 cm range of bust.

In Ruban Size

On a bra chart, Find my size asks for Underbust and Bust and reads the table as bands and cups, merged cells included. When the chart's sizes aren't bra sizes (S, M and L for bralettes, say), the answer adds "Your bra size from these measurements (EN 13402)", worked out with the rule above. The Bras template is a starting point to fill with your own values.