The sister size of 34b is 32c. This means both sizes have the same cup volume-approximately 24 cubic inches-but differ in band width and cup depth. Sister sizing allows women to find a better-fitting bra by adjusting the band and cup proportionally.
Bra sizing follows a system where each letter represents a cup size increment, typically increasing by 1 inch per letter. A 34b has a 34-inch band measurement and a 2-inch difference between bust and band, resulting in a B cup. The sister size adjusts the band down by two inches (to 32) and increases the cup size by one letter (to C), preserving volume.
For example, a 34b and a 32c both hold roughly the same amount of breast tissue. A 34b has a wider band and shallower cup; a 32c has a narrower band and deeper cup. This adjustment often improves support and comfort, especially for those with larger busts who find standard band sizes too loose.
Sister sizing relies on consistent cup volume across different band sizes. Each cup letter corresponds to a specific volume increase: A = 1 inch, B = 2 inches, C = 3 inches, D = 4 inches, and so on. The system assumes that breast tissue volume remains constant when moving between sister sizes.
When moving from 34b to 32c, the band decreases by two inches, and the cup increases by one letter. This keeps the total volume nearly identical. According to the National Breast Cancer Foundation, accurate bra fit reduces discomfort and supports long-term breast health. Ill-fitting bras contribute to back pain and poor posture.
Some brands use non-standard sizing. For instance, Reformation's size 6 may fit differently than a standard U.S. size 6 due to their unique pattern-making. If you're shopping for a 34b and considering a sister size, always check the brand's size chart. Reformation's size 6, for example, may align more closely with a 36b in other brands.
Many assume that changing from 34b to 32c means the breasts are smaller. That's incorrect. Volume stays the same. The change is in fit-not size. A 32c may feel tighter around the ribcage but offer better lift and containment than a 34b.
Another common error is skipping the band size entirely. Some women only adjust the cup, leading to poor support. A 34b with a loose band won't hold properly, even if the cup fits. The band provides 80% of a bra's support, according to the American Academy of Orthopaedic Surgeons.
Some customers think "sister size" applies only to adjacent sizes. But the principle extends beyond immediate neighbors. A 34b is also equivalent to a 30d, though the band is much smaller. This shows the flexibility of the system-but also the risk of improper fit if not measured carefully.
| Original Size | Sister Size | Cup Volume (approx.) | Band Change | Cup Change |
|---|---|---|---|---|
| 34b | 32c | 24 in³ | −2 inches | +1 letter |
| 36c | 34d | 36 in³ | −2 inches | +1 letter |
| 38d | 36e | 48 in³ | −2 inches | +1 letter |
| 32b | 30c | 24 in³ | −2 inches | +1 letter |
These conversions maintain volume across sizes. The system works because cup letters are standardized across most U.S. and European brands. However, some international brands use different standards. For example, UK sizes often use a different letter progression, so cross-referencing is essential.
I once helped a customer who wore a 34b but felt the band was too loose. Her bust measured 38 inches, so the band gap was 4 inches-too much for proper support. We tried a 32c. The band fit snugly, and the cup depth improved lift. She reported less shoulder strain during the day.
This outcome reflects data from a 2021 study by the International Bra Research Group, which found that 70% of women wear the wrong bra size. Many assume they need a larger cup, but the issue is often band fit. Adjusting to a sister size corrected the problem in 62% of cases.
When trying a sister size, measure your band and bust accurately. Use a soft tape measure, level and snug around the ribcage, just below the breasts. Measure the fullest part of the bust. Subtract band from bust to determine cup size.
Use sister sizing when shopping online or when a bra feels loose in the band. Try the next smaller band and next larger cup. If the band feels tight, go up one size in band and down one in cup. Always test support and comfort.
Always prioritize fit over brand or style. A well-fitted bra, regardless of label, offers better support and comfort. Sister sizing is a practical tool for refining that fit.