Bra Sister Size Calculator & Chart
A similar cup volume on a different band - and when the swap actually helps.
A sister size holds roughly the same cup volume on a different band. Go down one band and up one cup, or up one band and down one cup. AU 12C, 10D and 14B are sisters. They are not interchangeable – the band changes how tight the bra is around you, which changes how it feels and how well it supports.
How the swap works
Cup letters are relative, not absolute. A C cup on an AU 10 band holds less than a C cup on an AU 16 band, because the cup is graded up as the band grows. So when you shrink the band by one step, you have to grow the letter by one step to keep the volume roughly the same.
- Band feels too loose? Go down a band, up a cup. AU 14C becomes 12D.
- Band feels too tight? Go up a band, down a cup. AU 14C becomes 16B.
“Roughly” is doing real work in that sentence. The volumes are close, not identical, and the shape of the cup changes too – a smaller band with a bigger cup is deeper and more projected.
When a sister size is the right move
- The cup fits well but the band rides up or you can pull it far from your back.
- The cup fits well but the band leaves deep marks or makes it hard to breathe comfortably.
- Your size is sold out and you need the nearest usable alternative.
- A brand runs consistently tight or loose in the band and you have decided to compensate.
When it is the wrong move
- The cup is the problem. If tissue is spilling out, you need a bigger cup at the same band – not a sister size, which keeps volume constant.
- You are chasing a smaller-sounding letter. Moving from 12E to 16C does not make anything smaller; it makes the band looser and the support worse.
- You have already gone two steps. Past one step in either direction the shape diverges enough that the fit rarely holds up.
Sister size chart
Find your size in the first column. The other columns are the two sizes worth trying.
Scroll the table sideways on a narrow screen.
| Your size | Band feels loose - try | Band feels tight - try |
|---|---|---|
| 8A | - | - |
| 8B | - | 10A |
| 8C | - | 10B |
| 8D | - | 10C |
| 8DD | - | 10D |
| 8E | - | 10DD |
| 8F | - | 10E |
| 8FF | - | 10F |
| 8G | - | 10FF |
| 8GG | - | 10G |
| 10A | 8B | - |
| 10B | 8C | 12A |
| 10C | 8D | 12B |
| 10D | 8DD | 12C |
| 10DD | 8E | 12D |
| 10E | 8F | 12DD |
| 10F | 8FF | 12E |
| 10FF | 8G | 12F |
| 10G | 8GG | 12FF |
| 10GG | 8H | 12G |
| 12A | 10B | - |
| 12B | 10C | 14A |
| 12C | 10D | 14B |
| 12D | 10DD | 14C |
| 12DD | 10E | 14D |
| 12E | 10F | 14DD |
| 12F | 10FF | 14E |
| 12FF | 10G | 14F |
| 12G | 10GG | 14FF |
| 12GG | 10H | 14G |
| 14A | 12B | - |
| 14B | 12C | 16A |
| 14C | 12D | 16B |
| 14D | 12DD | 16C |
| 14DD | 12E | 16D |
| 14E | 12F | 16DD |
| 14F | 12FF | 16E |
| 14FF | 12G | 16F |
| 14G | 12GG | 16FF |
| 14GG | 12H | 16G |
| 16A | 14B | - |
| 16B | 14C | 18A |
| 16C | 14D | 18B |
| 16D | 14DD | 18C |
| 16DD | 14E | 18D |
| 16E | 14F | 18DD |
| 16F | 14FF | 18E |
| 16FF | 14G | 18F |
| 16G | 14GG | 18FF |
| 16GG | 14H | 18G |
| 18A | 16B | - |
| 18B | 16C | 20A |
| 18C | 16D | 20B |
| 18D | 16DD | 20C |
| 18DD | 16E | 20D |
| 18E | 16F | 20DD |
| 18F | 16FF | 20E |
| 18FF | 16G | 20F |
| 18G | 16GG | 20FF |
| 18GG | 16H | 20G |
| 20A | 18B | - |
| 20B | 18C | 22A |
| 20C | 18D | 22B |
| 20D | 18DD | 22C |
| 20DD | 18E | 22D |
| 20E | 18F | 22DD |
| 20F | 18FF | 22E |
| 20FF | 18G | 22F |
| 20G | 18GG | 22FF |
| 20GG | 18H | 22G |
| 22A | 20B | - |
| 22B | 20C | 24A |
| 22C | 20D | 24B |
| 22D | 20DD | 24C |
| 22DD | 20E | 24D |
| 22E | 20F | 24DD |
| 22F | 20FF | 24E |
| 22FF | 20G | 24F |
| 22G | 20GG | 24FF |
| 22GG | 20H | 24G |
| 24A | 22B | - |
| 24B | 22C | 26A |
| 24C | 22D | 26B |
| 24D | 22DD | 26C |
| 24DD | 22E | 26D |
| 24E | 22F | 26DD |
| 24F | 22FF | 26E |
| 24FF | 22G | 26F |
| 24G | 22GG | 26FF |
| 24GG | 22H | 26G |
| 26A | 24B | - |
| 26B | 24C | 28A |
| 26C | 24D | 28B |
| 26D | 24DD | 28C |
| 26DD | 24E | 28D |
| 26E | 24F | 28DD |
| 26F | 24FF | 28E |
| 26FF | 24G | 28F |
| 26G | 24GG | 28FF |
| 26GG | 24H | 28G |
| 28A | 26B | - |
| 28B | 26C | 30A |
| 28C | 26D | 30B |
| 28D | 26DD | 30C |
| 28DD | 26E | 30D |
| 28E | 26F | 30DD |
| 28F | 26FF | 30E |
| 28FF | 26G | 30F |
| 28G | 26GG | 30FF |
| 28GG | 26H | 30G |
| 30A | 28B | - |
| 30B | 28C | 32A |
| 30C | 28D | 32B |
| 30D | 28DD | 32C |
| 30DD | 28E | 32D |
| 30E | 28F | 32DD |
| 30F | 28FF | 32E |
| 30FF | 28G | 32F |
| 30G | 28GG | 32FF |
| 30GG | 28H | 32G |
| 32A | 30B | - |
| 32B | 30C | - |
| 32C | 30D | - |
| 32D | 30DD | - |
| 32DD | 30E | - |
| 32E | 30F | - |
| 32F | 30FF | - |
| 32FF | 30G | - |
| 32G | 30GG | - |
| 32GG | 30H | - |
Worked example
Someone is wearing AU 14C. The cups are fine but the band rides up when she raises her arms and she can pull it 8 cm from her back.
- The cup is not the problem, so keep the volume.
- Down one band: 14 becomes 12.
- Up one cup: C becomes D.
- Try AU 12D. If the band is still loose, 10DD is the next step.
Confirm with the seven fit checks before buying more than one.
Get both sister sizes automatically
The calculator returns your starting size plus both sister sizes, and tells you which to reach for depending on how the band feels.