Sister Sizes: Which Bra Sizes Hold the Same Cup Volume
Drop the band by one size and go up one cup letter, or raise the band by one and drop one letter, and the cup holds the same volume. A 34D, a 32DD and a 36C are the same volume on three different rib cages. That relationship fixes a band problem without giving up a cup that already worked — and it does nothing at all for a cup that gapes or spills.
By the Think Bra Editorial Team
Sister sizes are the sizes that hold the same cup volume on a different band. Move down one band and up one cup letter: 34D becomes 32DD. Move up one band and down one letter: 34D becomes 36C. All three cups hold the same amount.
The arithmetic behind it is simple. Cup letters are not absolute volumes — they are the difference between the bust and the band. Shrink the band by two inches and the same bust measurement produces a difference one inch larger, so the letter climbs by one and the volume stays put. Grow the band by two inches and the reverse happens.
That gives you a lever. If the band rides up but the cups fit, you do not have to accept a smaller cup to get a tighter band — you take the sister size and keep the volume. If the band cuts into your ribs, you go the other way. The one thing sister sizing cannot do is change how much the cup holds, which is precisely the thing most people reach for it to fix.
The rule, stated once
Every bra size has a volume index. Take the band number, halve it, and add the position of the cup letter in the run — AA is 0, A is 1, B is 2, and so on up through the doubled UK letters. A 34D is 17 plus 4, so 21. A 32DD is 16 plus 5, also 21. A 36C is 18 plus 3, also 21.
Anything sharing that index is a sister size. Sizes are sisters in both directions and to any depth: 30E, 32DD, 34D, 36C, 38B and 40A all sit at 21, and all six cups hold the same volume. What changes across that list is the circumference the band has to grip and the width the wire has to span.
There is nothing proprietary about the relationship — it falls straight out of how cup letters are defined. What is worth having is the volume actually attached to each coordinate, because that is what tells you how far apart two sizes really are.
The volume grid
The figures below are modelled, not taken off a manufacturer's mould. Each cup is treated as a half-ellipsoid whose base scales with the band and whose projection scales with the cup letter, giving volume in millilitres as roughly 5.9 × (half the band + cup position − 11)², rounded to the nearest ten. The formula is published here so you can check it rather than trust it.
Use the numbers as relative, not absolute. What matters is that the model is internally consistent: read any anti-diagonal — down one row and left one column, or up one row and right one column — and the figure does not change. Those diagonals are the sister-size families.
Cup (UK) 28 30 32 34 36 38 40 42 44 46 AA 50 90 150 210 290 380 480 590 710 850 A 90 150 210 290 380 480 590 710 850 1000 B 150 210 290 380 480 590 710 850 1000 1160 C 210 290 380 480 590 710 850 1000 1160 1330 D 290 380 480 590 710 850 1000 1160 1330 1510 DD 380 480 590 710 850 1000 1160 1330 1510 1710 E 480 590 710 850 1000 1160 1330 1510 1710 1910 F 590 710 850 1000 1160 1330 1510 1710 1910 2130 FF 710 850 1000 1160 1330 1510 1710 1910 2130 2360 G 850 1000 1160 1330 1510 1710 1910 2130 2360 2600 GG 1000 1160 1330 1510 1710 1910 2130 2360 2600 2860 H 1160 1330 1510 1710 1910 2130 2360 2600 2860 3120 HH 1330 1510 1710 1910 2130 2360 2600 2860 3120 3400 J 1510 1710 1910 2130 2360 2600 2860 3120 3400 3690 Modelled cup volume in millilitres at every band-and-cup coordinate. Find your size, then trace the diagonal in either direction — every cell on that diagonal is a sister size holding the same volume. Two things fall out of the grid that a plain sister-size list hides. The first is how much volume a single cup letter represents, and how that grows: at a 34 band, B to C is about 100 ml, while G to GG is about 180. A cup letter is not a fixed quantity, which is why "just go up a cup size" means something very different at a C than it does at a GG.
The second is how far apart two sizes with similar-looking labels can be. A 34DD holds around 710 ml in this model. A 38DD holds around 1000. Same letters, forty percent more volume, because the band grew and the difference was measured against a bigger number.
Direct lookup
If you would rather read a row than trace a diagonal, here is the same relationship laid out for eighteen common coordinates. Cup letters are UK; the volume column is from the model above.
Your size One band down One band up Two bands down Two bands up Modelled volume (ml) 32B 30C 34A 28D 36AA 290 32C 30D 34B 28DD 36A 380 34C 32D 36B 30DD 38A 480 34D 32DD 36C 30E 38B 590 34DD 32E 36D 30F 38C 710 36D 34DD 38C 32E 40B 710 36DD 34E 38D 32F 40C 850 30F 28FF 32E 26G (rarely made) 34DD 710 34F 32FF 36E 30G 38DD 1000 36F 34FF 38E 32G 40DD 1160 32G 30GG 34FF 28H 36E 1160 34G 32GG 36FF 30H 38E 1330 36G 34GG 38FF 32H 40E 1510 38G 36GG 40FF 34H 42E 1710 36H 34HH 38GG 32J 40G 1910 38DD 36E 40D 34F 42C 1000 40DD 38E 42D 36F 44C 1160 42F 40FF 44E 38G 46DD 1710 Take the row for the size you own, then pick the column matching the direction your band needs to move. Order the sister size, not a plain size up or down. The two-band columns are worth treating with more caution than the one-band ones. Two bands is a four-inch change in circumference, and while the volume holds, the wire span, the gore height and the strap placement have all moved substantially. A 34G and a 30H hold the same amount and are built for visibly different bodies.
Which direction to move, by symptom
Sister sizing is a band tool. Reach for it when the band is wrong and the volume is right; reach for something else when the volume itself is the problem.
What the bra is doing What that tells you The move Worked example Band rides up at the back, cups fit Band circumference too large; volume correct One band down, one cup letter up 36D → 34DD Band digs in and leaves a deep groove, cups fit Band circumference too small; volume correct One band up, one cup letter down 34DD → 36D Straps carry the weight and mark the shoulders The band is not holding the load, which is a loose band nine times out of ten One to two bands down with the matching cup increase, then loosen the straps 36F → 34FF, or 36F → 32G Sits on the tightest hook from new Band too loose at the coordinate you bought One band down, one cup up — there is no adjustment left to give 34G → 32GG Cup gapes at the top edge, band is fine Too much volume for the shape Stay in the band and drop a cup letter — a sister size will not help 34E → 34DD Spillage over the top, band is fine Too little volume Stay in the band and add a cup letter 34DD → 34E Band rides up and the cups spill at the same time Band too loose and volume too small — two faults, not one Two bands down and three cup letters up 38D → 34F Band comfortable, wires sitting on tissue at the sides Wire too narrow for the breast root One band up and one letter down widens the wire while holding volume; reassess before changing anything else 32F → 34E Match the symptom, then apply the move exactly as written. Rows five and six are the two cases where sister sizing is the wrong tool and a plain cup change is the right one. Where the relationship quietly stops working
Equal volume is not equal shape, and this is the part that catches people out after a good first experience with sister sizing.
Wire width scales with the cup letter, not with the volume. Moving from 34D to 32DD holds the volume and gives you a slightly wider wire on a narrower band. Moving from 34D to 36C holds the volume and gives you a narrower wire on a wider band. If your breast root is wide, the second of those will put wire on tissue even though the arithmetic is perfect.
Gore height moves too. Larger cup letters generally get a taller centre gore and a taller side wing; larger bands generally get a wider gore. Sister-size across two bands and both have changed at once.
Cup depth versus spread is entirely a design decision. A 32DD from a range drafted with deep, projected cups and a 32DD from a range drafted shallow and wide hold roughly the same volume in completely different distributions. Sister sizing moves you along one axis of a grid that has at least three.
And the band itself changes character. A 28 band and a 40 band with the same volume of cup hanging off them are doing very different mechanical jobs. Narrow bands at small sizes are usually fine; at a 40 with a 1,500 ml cup, band width and hook count start to matter more than the letter does.
Using sister sizes across two different brands
Stack the two corrections in a fixed order and the result is predictable. Convert first, then sister-size.
Start from a size you know works, in a run you know. Apply the offset between that run and the reference standard to get to a body-first coordinate. Apply the offset between the reference standard and the run you are ordering from. Only then, if the band still needs to move, take the sister size within the destination run.
Doing it the other way — sister-sizing first, then converting — produces the same answer only when both runs use the same cup step. They frequently do not. A run drafted at 2 cm per letter and a run drafted at 2.54 cm per letter drift apart by roughly a letter every four or five steps, and a sister-size move applied before the conversion carries that drift with it.
One practical shortcut: if you have a bra that works and you want the same volume from a different brand, order your calculated coordinate plus both one-band sister sizes. Three garments, one of which will be right, and the two that are not tell you exactly which direction that brand's run sits from the standard.
Things sister sizing cannot fix
- A cup that gapes at the top edge only — that is a projection and shape mismatch, not a volume one.
- A centre gore that will not sit down when the volume is already correct — the wire is too narrow for the root and no coordinate on the grid changes that.
- Straps that slip when the band is already firm and level — that is a strap set too wide for your shoulders.
- A run that stops before your coordinate. If the deepest cup a brand makes is DDD, no sister size reaches a 22 cm difference.
- A band that has lost its elastic. A stretched band is out of conformance, not out of size, and a sister size in the same worn condition will fail the same way.
- Two faults at once. Diagnose the band first, correct it, wear the result for a day, then look at the cup again.