Firmer band One band down
30E
One band down, one cup up
Worth trying if the cups fit but the band rides up or feels loose even on the tightest hook.
Same cup idea. Different band fit.
Bra Sister Size Calculator
Cup size is graded with the band, so the same cup letter is larger on a larger band. A sister size keeps cup capacity approximately comparable while changing the band, which can help when a band feels too firm or too loose but the cups fit.
Not sure of your starting size? Work out your starting size
The sister-size principle Approximately comparable cups
Smaller band 30E Firmer band
Start Starting size 32DD Reference
Larger band 34D Looser band
Down one band size = up one cup. Up one band size = down one cup.
Your sister sizes
Sister sizes for US 32DD
Both options keep cup capacity approximately comparable. Choose depending on whether the band needs to be firmer or looser.
Starting size
32DD
Your reference size
If the band and cups both fit well in this size, there is no need to change it.
Looser band One band up
34D
One band up, one cup down
Worth trying if the cups fit but the band feels uncomfortably tight, after checking the cups are not the cause.
Ready to try bras on?
Before you change size
How does your current bra feel?
Sister sizing is mainly a band adjustment. Use these checks to see whether a sister size, a different cup, or a different style is more likely to help.
Check the band and straps together
If the band rides up at the back and the bra is not old or worn, a smaller band may help. Moving down one band and up one cup (for example 32DD to 30E) keeps the cups approximately comparable while firming up the band. Check that straps pulled short are not lifting the band first.
Quick check
Raise both arms. If the band slides up, it is likely looser than it needs to be.
Check band riding upCheck the cups before the band
Cups that are too small can make a band feel tighter. Try fastening the bra with the cups turned to your back: if the band alone feels comfortable, try a larger cup in the same band. If the band itself is too tight, a sister size up (for example 32DD to 34D) loosens the band while keeping the cups approximately comparable.
Remember
New bands are usually worn on the loosest hook and can feel firm at first.
Check a tight bandA sister size is unlikely to fix this
Change the cup or the style, not the band
Because sister sizes keep cup capacity approximately the same, they usually repeat a cup problem. If the cups spill over, try a larger cup on the same band. If they gape, the cup may be too large or the shape may not suit you, so compare another style before changing size.
Shape or size?
Different styles shape the cups differently, even in the same size.
Review bra styles How it works
How do you find your sister size?
A cup letter is not a fixed size. It describes a cup in relation to its band.
A
Smaller band → next cup up
For example 32DD to 30EA smaller band has a smaller cup for the same letter, so the letter moves up one step to keep cup capacity approximately comparable.
- A firmer band
- Wires and straps usually sit a little closer together
Rule of thumb: subtract 2 from the band and move one cup letter up the UK sequence.
B
Larger band → next cup down
For example 32DD to 34DA larger band has a larger cup for the same letter, so the letter moves down one step to keep cup capacity approximately comparable.
- A looser band
- Wires and straps usually sit a little further apart
Rule of thumb: add 2 to the band and move one cup letter down the UK sequence.
UK cup sequence
Each step below is one cup size when moving between sister sizes.
- D
- DD
- E
- F
- FF
- G
- GG
In UK sizing, DD is followed by E, then F, then FF. US brands use different letters above DD, so convert a US size before using this tool.
Reference chart
One step either side Bra sister size chart (US and UK)
| Smaller band (firmer) | Starting size | Larger band (looser) |
|---|---|---|
| 30D | 32C | 34B |
| 30DD | 32D | 34C |
| 30DDD | 32DD | 34D |
| 32G | 34DDD | 36DD |
| 34H | 36G | 38DDD |
| 36I | 38H | 40G |
| Smaller band (firmer) | Starting size | Larger band (looser) |
|---|---|---|
| 28DD | 30D | 32C |
| 30E | 32DD | 34D |
| 32F | 34E | 36DD |
| 34FF | 36F | 38E |
| 36G | 38FF | 40F |
| 30GG | 32G | 34FF |
US labels follow the brand conversion charts we use (for example DDD after DD). US brands label large cups differently, so check the brand’s chart. Moving more than one step changes the band, wires and straps more.
When to try one
When should you try a sister size?
The band feels different between brands
Band firmness varies with the materials and construction a brand uses. If the cups fit but the band is too firm or too loose in a particular bra, a sister size may help.
You are trying a different style
Different styles can place the band, wires and straps differently. A sister size lets you change the band while keeping the cups approximately comparable.
Your size is not available
A sister size can be an option to try when your usual size is out of stock. A size with a looser band may not give the same support, so check the fit.
Why sister sizes don’t always fit the same
A larger band usually has wider-set wires and straps. If your breasts are narrow at the base, a sister size with a larger band may place the wires further back than you need. Always check the wires and straps as well as the band.
Keep in mind
A sister size is an option to try, not an exact twin
Changing the band changes how the bra sits. Keep these points in mind:
- One step at a time: Try one band size either way before moving further.
- Hooks matter: A new band should fit on the loosest hook.
- Strap position: Larger bands usually set the straps further apart.
- Centre and wires: In underwired styles, check where the wires and centre sit.
Related tools
Explore related fitting tools
Bra Size Calculator
Work out a starting size from your underbust and bust measurements.
International Size Converter
Convert UK sizes to US, EU, French and Australian labels.
UK Bra Size Chart
See UK bands and cups in order, and how they relate.
Clear answers
Frequently asked questions
What is a sister size?
A sister size is a size with approximately comparable cup capacity on a different band. Going down one band size and up one cup (32DD to 30E) gives a firmer band; going up one band and down one cup (32DD to 34D) gives a looser band.
Is a 34DD the same as a 36D?
They are sister sizes, so the cups are approximately comparable in capacity, but the 36D has a looser band. They are not identical: wire width, strap position and overall shape can change with the band size.
Are 34D and 36C sister sizes?
Yes. Moving from 34D to 36C goes up one band size and down one cup, so the cups are approximately comparable while the band is looser.
What is the sister size of 32G?
In UK sizing, the one-step sister sizes of 32G are 30GG (smaller band) and 34FF (larger band). Note the double letters: UK cups go F, FF, G, GG.
Will the cups fit the same if I choose a sister size?
Not exactly. Cup capacity is approximately comparable, but brands grade cups differently as bands change, and a larger band often has wider-set wires and straps. Treat a sister size as an option to try and check the fit.
Can I step more than one sister size away (for example 32DD to 36C)?
You can, but the further you move, the more the band, wire width and strap position change. Many people find a band two sizes away no longer fits well, so check the band carefully.
Which hook should a new bra fasten on?
New bras are usually designed to fit on the loosest hook, so you can move to tighter hooks as the band relaxes with wear and washing.
Sources and review status
Sources last checked: . This page has not yet been reviewed by an independent professional bra fitter.
- Sister sizes explained (opens in a new tab) · Brastop
- How to measure bra size (opens in a new tab) · Boux Avenue Same rounding rules; worked example subtracts the rounded band.
- Bra fitting guide (opens in a new tab) · Bravissimo
Spotted something that looks wrong? Report a correction or read our sizing method.