The Hug Journal · Lace & Lift ·
Bra sister sizes explained, with a DD sister size chart
Bra sister sizes explained: the one rule, a DD sister size chart from 34 to 48 bands, when a sister size works and when it will not, with fit checks.
Bra sister sizes are band and cup combinations that hold the same cup volume. The rule is simple: if you go up one band size, go down one cup letter, and if you go down one band, go up one cup. So 38DD, 36DDD and 40D are sister sizes. Use them when the cups fit but the band does not, or when a brand does not carry your exact size.
Why do sister sizes work?
A cup letter is not a fixed size. It describes the difference between your bust and your band, so the actual cup gets bigger as the band gets bigger. Brastop, a UK bra retailer, puts it plainly: a DD on a 28 band is much smaller than a DD on a 38 band. That means when you change the band, you have to change the letter to keep the same cup.
HerRoom, a large US lingerie retailer, explains sister sizes as the sizes right around your base size. Going diagonally keeps the cup volume the same: from 36C, both 34D and 38B hold the same amount. Going straight across, from 36C to 38C, makes both the band and the cup bigger.
What are the sister sizes for DD cups?
Here is a DD sister size chart in US sizing. In each row, the three sizes hold roughly the same cup volume. The cup after DD is DDD in US sizing, which some brands sell as F.
- 34DD: 32DDD or 36D
- 36DD: 34DDD or 38D
- 38DD: 36DDD or 40D
- 40DD: 38DDD or 42D
- 42DD: 40DDD or 44D
- 44DD: 42DDD or 46D
- 46DD: 44DDD or 48D
Labels past DD change from brand to brand, and UK sizes use different letters (DD, E, F, FF). Always check the size chart of the bra you are buying before you convert.
When should I use a sister size?
- The cups fit but the band rides up or feels loose on the tightest hook: go down one band and up one cup. A 40DD becomes a 38DDD.
- The cups fit but the band digs in or feels tight on the loosest hook: go up one band and down one cup. A 40DD becomes a 42D.
- A brand does not make your exact size. A sister size one step away is a good starting point.
- A style runs small or large in the band. Some lace and front closure bands have less stretch than smooth fabric ones.
The University of Portsmouth breast health research group recommends the same approach: if a bra does not fit well, try a sister size, and search for your fit rather than your size.
When will a sister size not work?
- When the cup is the problem, not the band. If you spill over the top or the wire sits on breast tissue, you need a bigger cup on the same band, not a sister size.
- When you go more than one step. Two bands away changes how the bra sits on your ribs, how far apart the wires are and where the straps land.
- When the band is already at its limit. Brastop notes that a band that is too loose gives poor support and can leave your shoulders sore, while one that is too tight can cause pain and bruising.
- When you change the band but keep the letter. Going from 40DD to 42DD gives you a bigger band and a bigger cup at the same time. That is not a sister size.
Do sister sizes feel the same in every style?
Not quite. The cup volume stays about the same, but the shape changes. A bigger band spreads the wires a little wider and moves the straps further out on your shoulders, and a smaller band does the opposite. Lace with little stretch and front closure bands with one setting are less forgiving than smooth knit bands with three hook columns. So treat a sister size as a strong guess, not a sure thing, and try it on before you remove the tags.
How do I check the fit of a sister size?
Try the bra on the loosest hook and go through the Portsmouth five point check:
- Band level and firm, not rising in back when you lift your arms.
- Cups smooth, no spilling or gaping.
- Wire sitting in the crease under the breast, not on tissue.
- Center lying flat on your breastbone.
- Straps steady on your shoulders without digging in.
If you have not measured in a while, start there. Our step by step guide to measuring your bra size at home takes two minutes, and underwire vs wireless for a large bust helps you pick the style.
How do sister sizes work with our bras?
Our Lace Underwire Bra and Brief Set is one option made for full busts: every size is a DD cup, in bands 36 to 46. If your usual size is a DDD, a DD on the next band up holds the same volume, so a 38DDD could try the 40DD. If your usual size is a D, the DD on one band smaller is your sister size, so a 42D could try the 40DD. The Front Close Lace Lift Bra is sold by band, 34 to 50, with each band fitting C to E cups, so choose your usual band and size up if you are between two. Both ship from our partner warehouse in China and take longer to arrive. Bras are final sale, so check your numbers against the chart before you order; defects are always covered.
Quick recap
- Up a band, down a cup. Down a band, up a cup.
- Stay within one step.
- Use it to fix the band, not the cup.
- Confirm with the five point fit check.
Mentioned here
Questions
What is a sister size in bras?
A sister size is a different band and cup combination with the same cup volume. Go up one band and down one cup, or down one band and up one cup: 38DD and 40D hold about the same amount.
What is the sister size of 38DD?
Going down a band, 36DDD (often sold as 36F). Going up a band, 40D. All three have a similar cup volume on different bands.
Is a DD the same size on every band?
No. Cup letters are relative to the band. Brastop notes that a DD on a 28 band is much smaller than a DD on a 38 band.
Can I go two sister sizes away?
You can try, but the band will feel very different. Sister sizing works best one step away; beyond that you are usually guessing.
Should I wear a sister size all the time?
Only if it fits well. Brastop cautions against using a sister size as a habit to avoid your true size, because a loose band gives poor support and a tight one hurts.
Keep reading
Sources: HerRoom: Your sister bra sizes and your bra size neighborhood · Brastop: Bra sister sizes explained · University of Portsmouth Research Group in Breast Health · Journelle: How to measure your bra size at home