Bra sister sizes explained

A sister size is a bra size with the same cup volume on a different band. To find one, go one band down and one cup up, or one band up and one cup down. A 34C (EU 75C) has two sisters: 32D (EU 70D) and 36B (EU 80B). The cups hold about the same; the band is tighter or looser.

Sister sizes exist because a cup letter isn't a fixed size. It only means something next to its band number.

Why a cup letter depends on the band

The European standard EN 13402-3 builds a bra size from two measurements:

• The band is the underbust, rounded to 5 cm: an underbust of 76 cm is a 75 band.
• The cup is the bust minus the underbust: AA for a difference of 10 to 12 cm, then one letter for every 2 cm more (A 12 to 14, B 14 to 16, C 16 to 18, D 18 to 20, and so on).

So a C means "the bust is 16 to 18 cm bigger than the ribcage", whatever the ribcage. On a larger band, that same difference wraps around a larger body, so the cup is wider and holds more. An 80C cup is bigger than a 75C cup.

That is why the rule works. Drop a band and the cup gets a little smaller; go up a letter and it grows back. The volume stays about the same.

The rule, in every system

• Band down, cup up: 75C becomes 70D.
• Band up, cup down: 75C becomes 80B.

One band is 5 cm in EU, French and Japanese sizes, and 2 in UK, US and Australian band numbers. One cup is the next letter in that system's own sequence, which matters after D (see below).

A sister size chain

Each row below is the sister of the rows next to it:

Italy and Japan write the same 75C as 3C and C75.

The chain is useful one step at a time. Two or three steps away, the band is a different fit altogether and the cup shape changes with it: a 65E and an 85A share a cup volume on paper and very little else. Ruban Size lists one sister each way.

After D, letters get tricky

The systems agree on cup letters up to D, then go separate ways:

"One cup up" means one step in your own system's row. Take a 34DD, which is a UK and US size (EU 75E):

The same sister is a 32E in a UK chart and a 32DDD in a US one. Mixing the two rows is the usual way to land on the wrong sister.

When a sister size helps

1. The band rides up or feels loose, the cups fit: one band down, one cup up.
2. The band digs in, the cups fit: one band up, one cup down.
3. Your size is sold out: a sister may work, knowing the band will feel firmer or softer.

A sister size doesn't help when the cup itself is wrong. If the cups gape or overflow on a band that fits, change the cup and keep the band. How to measure your bra size covers both measurements.

Find your size and its sisters

Enter an underbust and a bust. The calculator gives the size those measurements make, with its sister sizes:

For example, an underbust of 76 cm and a bust of 93 cm: the band rounds to 75, the difference is 17 cm, which is a C. The size is 75C, or 34C in UK and US sizes, with sisters 70D and 80B (32D and 36B). The calculator also has its own page, the bra size calculator, and the bra size conversion chart lists every band and cup.

How Ruban Size shows sister sizes in a store

On a bra chart, Find my size asks for the underbust and the bust and reads the chart's band and cup grid, merged band cells included. The answer gives the chart's own size, then two lines:

• "The same size in other systems: FR 90C · IT 3C · UK 34C · US 34C · AU 12C · JP C75"
• "Sister sizes (same cup volume): 70D · 80B"

Sister sizes are written in the chart's own system, so a UK chart shows UK letters. When the chart's sizes aren't bra sizes, as on a sports bra chart from XS to XL, the answer adds "Your bra size from these measurements (EN 13402)" with the standard size in all seven systems: EU, FR, IT, UK, US, AU and JP.

If the suggested size is sold out, the answer says so and the sister sizes line is still there, which gives the shopper something to try. The ends of the scale are handled too: an AA has no smaller cup, so it gets one sister, and no band below 60 is suggested.

Three templates come ready for this: Bras, Sports bras, and Maternity and nursing bras. Rings, shoes, belts and the other products with their own sizing rules are on the fit by product type page.

Questions

What is the sister size of 34C?

32D (one band down, one cup up) and 36B (one band up, one cup down). In EU sizes, 75C has the sisters 70D and 80B.

What is the sister size of 36DD?

34E in UK sizes, written 34DDD in US sizes, and 38D. In EU sizes, 36DD is 80E, with the sisters 75F and 85D.

Do sister sizes fit the same?

The cups hold about the same volume; the band does not fit the same. One band down feels firmer around the ribcage, one band up softer.

Can I go two sister sizes away?

You can on paper, but the further you go, the more the band and the cup shape change. Stay one step away, and if that doesn't fit, measure again.