Sister size

A sister size is a bra size with the same cup volume on a different band. Go one band down and one cup up, or one band up and one cup down: the sisters of a 75C (UK and US 34C) are 70D (32D) and 80B (36B). The cups hold about the same; the band is firmer or looser.

How to find one

1. Start from your size, band and cup.
2. For a firmer band: one band down, one cup up.
3. For a looser band: one band up, one cup down.
4. Count the cup in your own system's letters, which matters after D.

One band is 5 cm in EU, French and Japanese sizes, and 2 in UK, US and Australian numbers. It works because a cup size is a difference between bust and underbust, not a volume: drop a band and the cup gets a little smaller, go up a letter and it grows back.

On a size chart

The same sister has a different letter in the UK and in the US. For an EU 75E (UK and US 34DD):

A sister helps when the band rides up or digs in while the cups fit, or when a size is sold out. It doesn't fix a cup that gapes or overflows on a band that fits: change the cup and keep the band.

In Ruban Size

On a bra chart, Find my size adds a line under the answer: "Sister sizes (same cup volume): 70D · 80B" for a 75C. It lists one sister each way, in the chart's own system and never below a 60 band: two steps away, the band is a different fit altogether. The bra size calculator gives the same pair from an underbust and a bust.