When the cup of a bra fits well but the band feels wrong, you do not need a different cup volume: you need a sister size. Sister sizes are bras with the same cup volume on a different band. This calculator lists them for your size and estimates the cup volume, using the rule explained on Wikipedia’s bra measurement page.
How to use the sister size calculator
- Enter your band number, the even number in your size (34 in 34C).
- Choose your cup from the list (US letters).
- Read the four sister sizes: one and two bands down, one and two bands up.
- If the band is loose, try the smaller band; if it is tight, the larger band.
Sister size formula
underwire size = band + 2 × (cup number − 2), with A = 1, B = 2, C = 3
cup volume V = π × D³ ÷ 12, D = 9.7 cm + 0.847 cm × (underwire size − 30) ÷ 2
Each step of two inches on the band is matched by one step on the cup scale, because one cup letter is one inch of difference between bust and band. The same underwire is used for the cups of 36A, 34B, 32C and 30D, so they have the same volume (Wikipedia, retrieved 2026-10-15). The diameter and volume rule uses the hemisphere formula V = 2πr³/3 given there.
| Your size (US) | Two bands down | One band down | One band up | Two bands up | Cup volume (cm³) |
|---|---|---|---|---|---|
| 30D | — | 28DD/E | 32C | 34B | 391 |
| 32C | 28DD/E | 30D | 34B | 36A | 391 |
| 34B | 30D | 32C | 36A | — | 391 |
| 36A | 32C | 34B | — | — | 391 |
| 34C | 30DD/E | 32D | 36B | 38A | 484 |
| 36D | 32DDD/F | 34DD/E | 38C | 40B | 714 |
| 38DD/E | 34G | 36DDD/F | 40D | 42C | 1006 |
| 40C | 36DD/E | 38D | 42B | 44A | 852 |
Underwire sizes and cup volumes
| Underwire size | Cup diameter (cm) | Volume of one cup (cm³) | Sizes that share it (US) |
|---|---|---|---|
| 30 | 9.7 | 242 | 32A, 30B, 28C |
| 32 | 10.6 | 310 | 34A, 32B, 30C, 28D |
| 34 | 11.4 | 391 | 36A, 34B, 32C, 30D, 28DD/E |
| 36 | 12.3 | 484 | 38A, 36B, 34C, 32D, 30DD/E |
| 38 | 13.1 | 592 | 40A, 38B, 36C, 34D, 32DD/E |
| 40 | 14.0 | 714 | 42A, 40B, 38C, 36D, 34DD/E |
Worked example
Your size is 34C and the band feels loose.
- One band down, one cup up: 32D. Two bands down: 30DD/E.
- One band up, one cup down: 36B.
- All share underwire size 36 and a cup volume of about 484 cm³ (π × D³ ÷ 12 with D = 4.83 in = 12.28 cm).
Limits of sister sizing
- Shape matters. Same volume does not mean the same shape; wire width and cup depth vary between brands.
- Letters differ by country. This page uses US letters; in UK sizing 34DD is the sister of 32E and 30F, so use the Bra Size Converter (US/UK/EU/AU/FR/JP) for UK or EU letters.
- Chart edges. Sizes below band 28 or above 60 are not shown.
- Not a diagnosis. The volumes are approximations, and a fitter can judge a fit much better.
To find your size start with the Cup Size Calculator and Band Size Calculator, or the UK Bra Size Calculator. This page is general information, not a professional fitting.
Sources
Every figure above comes from these pages, read directly on the dates shown. Brand names are used only to identify published charts.
- Wikipedia — Brassiere measurement (sister sizes, underwire diameter and cup volume table) — checked 2026-10-15
- Brava Lingerie — bra size chart (sister size definition) — checked 2026-10-15
Frequently asked questions
What is a sister size?
A sister size is a bra size with the same cup volume on a different band. Going down one band size (two inches) and up one cup letter keeps the volume, so 34C, 32D and 36B are sisters. Wikipedia gives 30D, 32C, 34B and 36A as an example.
When should I use a sister size?
When the cups fit but the band is too loose or too tight. A band that rides up needs a smaller band and a larger cup; a band that digs needs a larger band and a smaller cup. Try on the sister size, since brands differ.
Why does the cup volume change with the band?
It does not in a sister size, which is the point. Between different band sizes with the same cup letter, the volume is larger on the larger band, because the cup letter measures a difference, not a fixed volume. A 34C holds more than a 32C.
How is the cup volume estimated?
The calculator treats a cup as a hemisphere of volume πD³/12, using the underwire diameter that Wikipedia gives: 9.7 cm at size 30, growing by a third of an inch for every two sizes. It is an estimate, not a medical measurement.
Do sister sizes work in UK sizing?
Yes, the rule is the same for UK sizes, but the cup letters differ after DD: this page uses US letters. Convert with the Bra Size Converter (US/UK/EU/AU/FR/JP).