Why sister sizes exist
A cup letter is relative to the band. A C cup on a 36 band is larger than a C cup on a 32 band, because the cup is sized against a bigger frame. That relationship has a useful side effect. If you make the band one size smaller and the cup one letter larger, the cup volume stays about the same. The reverse works too.
Those linked sizes are called sister sizes. The tool above takes any size and shows the sister one band down and the sister one band up.
The rule in one line
- Band down one size (2 inches), cup up one letter.
- Band up one size, cup down one letter.
So the chain around 34D in UK sizing runs: 30E, 32DD, 34D, 36C, 38B. Each holds about the same cup volume. In US sizing the same line reads 30DDD, 32DD, 34D, 36C, 38B. In EU sizing it is 65F, 70E, 75D, 80C, 85B.
Every double letter counts as one full step. The sister below a UK 32DD is a 30E, not a 30F.
When to use a sister size
Sister sizes are a fitting tool, not a sizing shortcut. Use them when the cups feel right but the band does not.
- The band rides up at the back, or feels loose on the tightest hook. Go down one band and up one cup. A 36C becomes a 34D.
- The band digs in and feels tight even on the loosest hook. Go up one band and down one cup. A 32DD becomes a 34D.
- A brand runs tight or loose in the band. Some labels are known for firm elastic. Moving one sister size can match the fit you get elsewhere.
- Your size is out of stock. A sister size is a reasonable second choice, as long as you check the fit when it arrives.
When the problem is the cup itself, sister sizes do not help. If the cups spill, go up a cup on the same band. If they gape, try a smaller cup or a different cup shape. The how a bra should fit explains how to tell which part is wrong.
What changes when you switch
The cup volume stays close. Other things do not.
The band length changes, so the bra sits differently around the ribcage. The way the wires, straps and center panel sit can change with it, and brands handle this in their own ways. That is why one step along the line usually feels like a near match, while two or three steps away can start to feel like a different bra.
A band also does most of the supporting work. Going up a band to relieve tightness gives up a little of that support. If the tighter band is only uncomfortable for the first few wears, it may relax. A band that hurts on the loosest hook from day one will not improve much.
Sister sizes when you buy a gift
If you are buying for someone and you know her size from a bra that fits her well, stick to that exact size. Sister sizes are useful when she tries things on. They are less useful when you are guessing, because you cannot check the band.
The exception is a size from the calculator after an odd underbust measurement. There the band below and the band above are both reasonable, and the calculator shows both. For a surprise gift, a wireless bralette in S, M or L avoids the band question entirely. The gift guide has more options when you are unsure.
Finding sister sizes for any size
Enter a size in the tool above, in any system, and it returns the size on each side plus the full conversion to US, UK, AU, EU and FR. The calculation comes from the same sizing logic as the rest of the site, so the results match the bra size chart and the calculator.
If you do not have a starting size yet, begin with how to measure bra size. Measure first, then use sister sizes to fine-tune.
Sister sizes are especially handy for strapless bras, where a firmer band does all the work: choosing a strapless bra size.
Sources & method: sizing rules follow the standard band-and-cup system used by UK, US, AU and EU brands, checked against fitting guides from specialist retailers and fitting communities. How we work.