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. Last resort: the wires sit narrower. (US 30DDD)
- 38BBand 4 in looser, 2 cups down. Last resort: the wires sit wider. (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. What most charts miss is that the letters change from country to country after DD, so the same line reads 32E in the UK and 32DDD in the US.
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
- No label in that system (Australian brands stop at HH; the Japanese column is approximate and stops at I)
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 |
| Centre front lifts off the breastbone | Cup too small, or wires too narrow | 34DD |
| Wires poke or sit on breast tissue | Cup too small, or wires too tall | 34DD |
These are the checks used in bra fit research (band, cups, wire, centre front, straps; see White and Scurr, 2012 and McGhee and Steele, 2010). A tape measurement can't make them for you.
The sister sizes people look up most
Monthly Google searches for "[size] sister size", US and UK, from Google Ads data pulled 25 September 2026. Sisters in US labels.
| Size | Tighter band | Looser band | US searches | UK searches |
|---|---|---|---|---|
| 32DD | 30DDD | 34D | 2,900 | 320 |
| 34C | 32D | 36B | 2,400 | 320 |
| 34D | 32DD | 36C | 2,400 | 320 |
| 36DD | 34DDD | 38D | 1,900 | 110 |
| 38D | 36DD | 40C | 1,300 | 140 |
| 34B | 32C | 36A | 1,000 | 140 |
| 34DD | 32DDD | 36D | 1,000 | 90 |
| 36C | 34D | 38B | 1,000 | 110 |
| 32C | 30D | 34B | 880 | 170 |
| 30D | 28DD | 32C | 720 | 170 |
| 32B | 30C | 34A | 720 | 140 |
| 36B | 34C | 38A | 720 | 90 |
| 38C | 36D | 40B | 720 | 70 |
| 40D | 38DD | 42C | 320 | 30 |
Our read: the most searched sizes cluster at 32 to 36 bands with C to DD cups, and the tighter sister of every DD on the list is where UK and US labels part (UK E, US DDD).
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.
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
The rule holds in every system because every system is the same grid relabelled. 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
The cup volume matches, but the frame does not. A band 2 inches longer spreads the wires wider and moves the straps outward, so a sister two or three steps away can fit the volume and still gape at the sides or slip off the shoulders. Use the nearest sister first.
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?
The cup holds the same volume, but the band length changes by about 2 inches per step, and the underwire and strap positions move with it. One step usually works; two or more is a stretch.
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 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 output is in our repository
- 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