Bra sister sizes: the full chart and a friendly finder
If your usual size is sold out, or the band feels too loose or too tight, a sister size keeps the same cups on a different band. Type the size you wear, in any country's labels, and we'll show you the family.
Sister sizes of 34D (UK)
- 34DThe size you typed. (US 34D)
- 32DDBand 2 in tighter, 1 cup up. For a band that rides up. (US 32DD)
- 36CBand 2 in looser, 1 cup down. For a band that digs in. (US 36C)
- 30EBand 4 in tighter, 2 cups up. Our advice: try the nearest sister first. (US 30DDD)
- 38BBand 4 in looser, 2 cups down. Our advice: try the nearest sister first. (US 38B)
32DD
tighter band
34D
your size
36C
looser band
Same cups, three bands: 32DD, 34D, 36C hold the same cup volume.
The rule is simple: one band down, one cup letter up. 34D, 32DD and 36C sit on the same line. A table in one country's letters can hide the split after DD: UK 32E is US 32DDD. Always check which system a label uses.
The sister size chart
Every cell on a highlighted diagonal holds the same cup volume. Select any size to redraw its line. Bands run across, cups down.
| Cup | 28 | 30 | 32 | 34 | 36 | 38 | 40 | 42 | 44 | 46 |
|---|---|---|---|---|---|---|---|---|---|---|
| AA | ||||||||||
| A | ||||||||||
| B | ||||||||||
| C | ||||||||||
| D | ||||||||||
| DD | ||||||||||
| E | ||||||||||
| F | ||||||||||
| FF | ||||||||||
| G | ||||||||||
| GG | ||||||||||
| H | ||||||||||
| HH | ||||||||||
| J | ||||||||||
| JJ | ||||||||||
| K |
- Same cup volume as the chosen size
- The chosen size
- Not in our table (our AU ladder follows Berlei through H and A Bra That Fits at HH; our approximate Japanese ladder stops at I). Brand charts may go further.
Fit problem? The size to try next
Worked out for 34D. Change the size above and these update.
| What you notice | Likely cause | Try |
|---|---|---|
| Band rides up your back, straps take the weight | Band too loose | 32DD |
| Straps dig in and carry the weight | Band too loose | 32DD |
| Band digs in or leaves deep marks on the loosest hook | Band too tight | 36C |
| Spilling over the top or sides of the cup, wire sits on breast tissue | Cup too small | 34DD |
| Empty space or wrinkles in the cup, with the band fitting well | Cup too big, or the wrong cup shape | 34C |
| Center front lifts off the breastbone | Cup too small, or a different style needed | 34DD |
| Wires poke or sit on breast tissue | Cup size or style mismatch | 34DD |
These are the checks used in bra fit research (band, cups, wire, center front, straps; see White and Scurr, 2012 and McGhee and Steele, 2010). A tape measurement can't make them for you.
Sister sizes for 14 selected sizes
Fourteen sizes with their nearest sister each way, in US labels. The sizes with their own page link to it.
| Size | Tighter band | Looser band |
|---|---|---|
| 32DD | 30DDD | 34D |
| 34C | 32D | 36B |
| 34D | 32DD | 36C |
| 36DD | 34DDD | 38D |
| 38D | 36DD | 40C |
| 34B | 32C | 36A |
| 34DD | 32DDD | 36D |
| 36C | 34D | 38B |
| 32C | 30D | 34B |
| 30D | 28DD | 32C |
| 32B | 30C | 34A |
| 36B | 34C | 38A |
| 38C | 36D | 40B |
| 40D | 38DD | 42C |
Every DD on the list has a tighter sister where UK and US labels part: 32DD to 30E (US 30DDD), 34DD to 32E (US 32DDD), and 36DD to 34E (US 34DDD).
How sister sizes work
A cup letter is not a fixed volume. It is the difference between bust and band, so the same letter on a bigger band is a bigger cup: a 36D cup holds more than a 32D cup. Each band step is 2 inches, and each cup letter is 1 inch of difference, so one band step is worth one letter. Move the band down one and the cup up one, and the volume stays the same. The G cup and H cup pages list each letter's sister sizes by band, in both the UK and US readings.
That is why the chart has diagonals. Bra Atlas computes the line as band ÷ 2 + cup position, which is constant along it, and uses the same rule on every page (methodology).
Sister sizes across countries
Our converter applies the sister-size rule on one grid and relabels it for each system. Brand conventions can differ. What changes is the letters after DD: the UK line through 34DD runs 32E, 30F, 28FF; the US line runs 32DDD, 30G, 28H; in EU labels it is 75E, 70F, 65G, 60H. Pick your system in the finder and the chart relabels.
Where sister sizing stops working
ThirdLove describes trying a smaller band and larger cup for a firmer band and deeper cup. Equal cup-volume categories do not guarantee the same fit in another size or style. Our advice is to try the nearest sister first, then check that the band stays level and the cups enclose the breast tissue comfortably.
What is a sister size?
A size with the same cup volume on a different band. Going down one band (2 inches) and up one cup letter keeps the volume: 34D, 32DD and 36C hold the same cup volume.
Do sister sizes fit the same?
Sister sizes share a cup-volume category, but their bands and proportions differ. ThirdLove describes a smaller band and larger cup as giving a firmer band and deeper cup. Our advice is to try the nearest sister first and check the fit rather than assume it will feel the same.
What is the sister size of 32DD?
Tighter: 30E (US 30DDD). Looser: 34D, then 36C. All four hold the same cup volume.
Sources
Each source was opened and checked on the date shown, or on 25 September 2026. Where sources disagree, the page says so instead of picking one silently.
- Wikipedia, "Bra size": UK and US cup ladders, the +4 method, EN 13402 and JIS cup steps
- A Bra That Fits calculator (community calculator of r/ABraThatFits): we submitted 18 measurement pairs in 6 systems on 25 September 2026; raw synthetic fixture outputs are linked from the methodology
- White J, Scurr J. Evaluation of professional bra fitting criteria for bra selection and fitting in the UK. Ergonomics 2012;55(6):704-711: 45 women; the traditional method overestimated band and underestimated cup
- McGhee DE, Steele JR. Optimising breast support in female patients through correct bra fit. J Sci Med Sport 2010;13(6):568-572: 104 women in Australia; 85% in ill-fitting bras
- Berlei Australia, size charts: AU band = underbust row, cups A to H with no FF or GG
- OneMall, "Japanese Bra Size Conversion" (11 September 2026): JIS cups in 2.5 cm steps; JP letter about one above the US letter
- ThirdLove, "Bra Size Calculator & Chart": bands 32 to 44, cups AA to H with half cups; 6 synthetic pairs submitted to the calculator; 5 result pairs recorded (opened 25 September 2026)