Every bra size has two direct sister sizes with exactly the same cup volume. The rule: go one band size (5 centimetres) down and you go one cup letter up — and vice versa. So an 80C has the same cup volume as a 75D and an 85B. Once you know this conversion, you never have to settle for a sold-out size or a band that digs in.
How sister sizes work
A cup letter means nothing on its own. The letter only describes the difference between your bust measurement and your underbust measurement — roughly 2 centimetres per letter. The actual volume of a cup grows along with the band size: the D-cup of a 70 band is considerably smaller than the D-cup of an 85 band.
That's why you can shift between sizes. If the band of your 80C digs in while the cups fit well, an 85B offers the same cup space with 5 centimetres of extra circumference. If the band rides up your back the moment you move, it's too loose, and a 75D will hold you more securely — without making the cup smaller.
Brands notice this too: the same size fits differently depending on the cut and the amount of stretch. Sister sizes therefore don't give you a new label, but a direction to search in when your standard size doesn't quite fit.
Sister size table: look up your size
The table shows the two closest sister sizes for each size. Read it as a decision tree: if the cups fit well but the band doesn't, move one column over.
| Your current size | Band digs in → looser band | Band too loose → tighter band |
|---|---|---|
| 70B | 75A | 65C |
| 70C | 75B | 65D |
| 75B | 80A | 70C |
| 75C | 80B | 70D |
| 75D | 80C | 70E |
| 80B | 85A | 75C |
| 80C | 85B | 75D |
| 80D | 85C | 75E |
| 85C | 90B | 80D |
| 85D | 90C | 80E |
Preferably try your new size in a style with a wide size range. The VitaSculpt Push-Up Bra runs from band size 75 to 110 and from cup B to H, meaning almost every sister size next to your current size is available to compare.
When a sister size is the right choice — and when it isn't
A sister size solves one specific problem: the cups fit, the band doesn't. Three situations where the chart helps. The band leaves red marks: choose the looser sister size. The band rides up your back the moment you raise your arms: choose the tighter one. Your size is sold out and you want a bra that fits today: both directions are worth trying. If your band has become too loose or too tight because your weight changed, also read new bra size after weight change: it shows you when a sister size is enough and when you're better off measuring again.
If the cup overflows or gapes, a sister size is the wrong tool. That's a cup problem: keep the same band size and only go up or down a letter.
Tip: always fasten a new bra on the outermost hook. Band elastic stretches noticeably in the first few months, so this keeps two hooks in reserve to tighten the band before you need to move to a sister size.
Measure correctly first, then convert
Sister sizes solve a band problem, not a measuring problem. Fit research has reached the same conclusion for years: a wide majority of women wear a bra size that's wrong, usually a band that's too loose combined with a cup that's too small. If you convert from the wrong base size, you're only passing the problem along.
So first measure your band size snugly under your bust and your bust size at the widest point. The difference between the two determines the cup letter, in steps of about 2 centimeters. How to do that step by step, including tape measure instructions, is in our guide measuring your bra size; if you're still unsure between two sizes afterward, our size advice can help.
Translating sister sizes to letter sizes
Seamless bras often use letter sizes such as S, M, and L instead of band-cup sizes. That's not a simplification but a different construction: the circular-knit stretch fabric accommodates several band-cup combinations within each letter size. This makes the system naturally more forgiving for anyone between two traditional sizes, exactly the group that most often looks for sister sizes.
The Luma Contour Bra (S to XXL, 12 colours) works according to this principle and moves with small fluctuations in band size. The Luma Forma Bra 2.0 adds pre-shaped cups for those who want more structure at the bust, with 13 sizes from XS to 3XL+ including half sizes. Check the size chart for each model: the transition points between letter sizes differ per cut.
Doesn't the sister size fit either? Look at the model
If, after measuring and converting, you still get pressure marks or poorly fitting cups, you're usually running into the limits of an underwired model. A wire-free, seamless bra distributes the weight across a wide band instead of two metal arcs, which significantly widens the fit window. We've previously explained the differences in support, shape and wearing comfort in the comparison seamless bra versus underwired bra.
Frequently asked questions about sister sizes
What exactly are sister sizes?
Sister sizes are bra sizes with the same cup volume but a different band size. With each step up in band size, the cup letter goes down one, and with each step down in band size, it goes up one. This keeps the cup volume the same while the circumference changes.
Is a sister size bigger or smaller than my size?
Neither. The cup volume is identical; only the band circumference differs by 5 centimetres per step. However, the feel does change: a tighter band gives more support, a looser band more freedom of movement.
Which sizes are the same as 80C?
The direct sister sizes of 80C are 75D (tighter band) and 85B (looser band). Two steps further you'll find 70E and 90A, although in practice the fit often deviates too much at that point. It's best to stay within one step of your measured size.
When should I choose a sister size?
When the cups fit well but the band pinches or rides up, and when your own size is sold out. It's a conversion for band problems — cup problems are solved with a different letter within the same band size.
How much support do I lose with a looser band?
Noticeably a lot: the rule of thumb among bra fitters is that around 80 percent of the support comes from the band, not the shoulder straps. Therefore, only choose the looser sister size if the current band demonstrably pinches, not just to be safe.
Do sister sizes also work for letter sizes?
Indirectly. With seamless bras, one letter size covers multiple band-cup combinations, so small shifts are absorbed by the material itself. If you're on the border between two letter sizes, choose based on your band size: that determines the support.
What if no sister size fits?
Then the problem is almost certainly with the style or the measurement, not the size. Measure again and consider a wireless seamless style with a wide size range; these are more forgiving for in-between sizes than pre-formed underwire bras.
Written by Team Novyre — specialists in seamless underwear. Last updated: 5 August 2026.
