How sister sizing works
A cup letter describes the difference between bust and band. It does not describe a fixed volume. So when the band changes, the letter has to change in the opposite direction to keep the cup the same size:
- Band down one size, cup up one letter: 34C becomes 32D.
- Band up one size, cup down one letter: 34C becomes 36B.
The cups of a 32D, a 34C and a 36B are made to hold the same amount of breast tissue. What changes is the length of the band around your body. For a fuller explanation of why this works, read what sister sizes are.
The full UK sister size chart
Every size in a row shares a cup volume. Find your size and read left for a firmer band or right for a looser one.
| 28 band | 30 band | 32 band | 34 band | 36 band | 38 band | 40 band | 42 band |
|---|---|---|---|---|---|---|---|
| 28B | 30A | 32AA | – | – | – | – | – |
| 28C | 30B | 32A | 34AA | – | – | – | – |
| 28D | 30C | 32B | 34A | 36AA | – | – | – |
| 28DD | 30D | 32C | 34B | 36A | 38AA | – | – |
| 28E | 30DD | 32D | 34C | 36B | 38A | 40AA | – |
| 28F | 30E | 32DD | 34D | 36C | 38B | 40A | 42AA |
| 28FF | 30F | 32E | 34DD | 36D | 38C | 40B | 42A |
| 28G | 30FF | 32F | 34E | 36DD | 38D | 40C | 42B |
| 28GG | 30G | 32FF | 34F | 36E | 38DD | 40D | 42C |
| 28H | 30GG | 32G | 34FF | 36F | 38E | 40DD | 42D |
| 28HH | 30H | 32GG | 34G | 36FF | 38F | 40E | 42DD |
| 28J | 30HH | 32H | 34GG | 36G | 38FF | 40F | 42E |
| 28JJ | 30J | 32HH | 34H | 36GG | 38G | 40FF | 42F |
| 28K | 30JJ | 32J | 34HH | 36H | 38GG | 40G | 42FF |
| 28KK | 30K | 32JJ | 34J | 36HH | 38H | 40GG | 42G |
| – | 30KK | 32K | 34JJ | 36J | 38HH | 40H | 42GG |
| – | – | 32KK | 34K | 36JJ | 38J | 40HH | 42H |
| – | – | – | 34KK | 36K | 38JJ | 40J | 42HH |
| – | – | – | – | 36KK | 38K | 40JJ | 42J |
| – | – | – | – | – | 38KK | 40K | 42JJ |
This chart uses UK cup lettering, which runs D, DD, E, F, FF, G, GG, H, HH, J, JJ, K, KK. If you are reading an American label, convert it first with the UK to US converter.
Sister sizes for common sizes
Size-specific pages, with when to choose each neighbour: 32D, 34C, 34DD, 36DD, 38D, 30E and 32F. For FF cups and above, see sister sizes for a larger bust.
When a sister size is the right move
The cups fit but the band is loose
The band rides up your back or you are already on the tightest hook. Go down a band and up a cup. This is the most common fix of all, because many people wear a band that is too big. More on that in why your bra rides up at the back.
The cups fit but the band hurts
If the band leaves deep marks or feels restrictive even on the loosest hook, go up a band and down a cup. First check that the cups really are right. A cup that is too small pushes the whole bra away from your body and makes the band feel tighter than it is. See is my bra band too tight?
Your size is sold out
Either neighbour can stand in. The firmer band usually gives the better result, because bands stretch with wear and a bra that starts a little loose only gets looser.
A fuller decision guide is in when to sister size up or down.
When a sister size will not help
- The cups are wrong. Sister sizes keep the cup volume the same on purpose. If you are spilling out or the cups gape, you need a different cup on the same band.
- You are several steps away. Going from 34C to 38A keeps the volume on paper, but the wires, straps and proportions are built for a much broader frame. Stay within one step where you can. See do sister sizes really fit the same?
- You do not know your real size yet. Sister sizes are adjustments around a correct starting point. If you are unsure of yours, measure first.
Frequently asked questions
What is the sister size of a 34C?
One band down it is 32D, and one band up it is 36B. All three hold the same cup volume. Choose 32D if the 34 band feels loose, and 36B if it feels too tight.
Should I sister size up or down?
Go by the band. If the band rides up or you fasten it on the tightest hook, go down a band and up a cup. If the band is painfully tight but the cups fit, go up a band and down a cup.
Do sister sizes fit exactly the same?
The cup volume matches but the bra is not identical. Wires get wider, straps sit further apart and the cup shape changes slightly as the band grows. One step usually works. Two steps is usually a noticeably different fit.
Are UK sister sizes the same as US ones?
The principle is the same but the letters differ above DD. A UK 36DD goes to a UK 34E, where an American chart would say 34DDD. Check which system a brand uses before applying a sister size.
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