Bra Sister-Size Guide: When the Cups Fit but the Band Doesn’t
Cups feel right, but the band feels too tight? A sister size can be a useful starting point: try one band size larger and one cup letter smaller to keep approximately the same cup volume. If the band is too loose, try one band size smaller and one cup letter larger.
Sister sizes are nearby sizes to try, not a guarantee of the same fit. Fabric, padding, cup shape and each style’s size chart still matter.
Read your Secret Land size label
On styles labelled 34/75 B, 34/75 identifies the band and B identifies the cup. The two band numbers are paired size labels, not two separate measurements to add together. Other styles use labels such as 75B or put the band numbers in the opposite order, such as 75/34 B. Always follow the size chart and labels on the product you are buying.
A cup letter is relative to its band. Moving from 34/75 B to 36/80 B changes both the band fit and the cup volume. It is not the same adjustment as moving to 36/80 A.
If the cups fit but the band is tight
First, check that the band is level, the straps are not overtightened and your breast tissue is sitting inside the cups. Cups that are too small or too shallow can also make the whole bra feel tight.
If the cups contain you comfortably and the band alone needs more room, try one band step up and one cup letter down. For example: 34/75 B → 36/80 A, or 34/75 C → 36/80 B. Check that your chosen style actually offers the suggested size.
If the cups fit but the band is loose
If the band rides up or shifts around, try one band step down and one cup letter up. For example: 36/80 B → 34/75 C. Do not keep tightening the straps to compensate for a loose band.
Sister-size examples
Read each row from left to right for a larger band and a smaller cup letter. Read from right to left for a smaller band and a larger cup letter. Cup volume is approximate within a consistent sizing system.
| Smaller band | Middle size | Larger band |
|---|---|---|
| 32/70 C | 34/75 B | 36/80 A |
| 34/75 C | 36/80 B | 38/85 A |
| 36/80 C | 38/85 B | 40/90 A |
For styles using labels like 75B, a comparable example is 75C → 80B → 85A, but only where those sizes are offered and the style’s chart follows that progression. These examples explain the adjustment; they do not promise availability or identical fit across products.
When not to use the sister-size shortcut
Band fits, cups overflow: keep the band and try a larger cup. Band fits, cups gape: check strap adjustment and cup shape first; a smaller cup or another cut may suit you better. Both band and cups feel wrong: start again with your measurements and the product’s chart rather than making repeated sister-size jumps.
Grouped cup labels such as 34/75 BC, 34/75 DE or 34/75 BCD do not represent individual cup steps. Do not apply the one-letter rule to these groups. Styles labelled M, L, XL, combined ABC cups or weight ranges also need their own size chart; body weight alone does not tell you band and cup fit. Avoid guessing conversions between brands or cup-letter systems, especially beyond D.
Take two simple measurements
- Use a soft tape and, if helpful, a lightly lined, non-padded bra. Measure around your ribcage directly under your bust, keeping the tape level and comfortably snug. Record the measurement without adding inches.
- Measure around the fullest part of your bust with the tape level and without compressing it. Record both measurements in the units used by the product’s chart.
- Compare both numbers with that specific style’s chart. Do not treat the 75 in a 34/75 label as an instruction to choose that size solely because your underbust measures 75 cm.
Check the whole fit, not just the label
The band should sit level and feel secure without painful digging. Cups should contain you without spilling or persistent empty folds. Straps should stay in place without needing to be pulled very tight. On a conventional wired bra, the wires should surround rather than press onto breast tissue. Move your arms, sit down and check whether the bra stays comfortable.
Shape matters too. If a cup cuts in at the top but has empty space elsewhere, changing size alone may not solve it. A more open neckline, a different cup depth or a more flexible cup can be worth trying. Padding, plunge cuts and strapless designs can fit differently even when their labels match. If one side is fuller, start by fitting the fuller side and adjust the straps separately where possible.
Start with just one sister-size step and reassess. Do not settle for discomfort simply because two sizes appear in the same row.
Ready to choose?
Shop the Bra collection, open the size chart for your chosen style and check the exact options before ordering. If you would like help comparing sizes, share the product name, your underbust and bust measurements, and which part of your current bra feels too tight or too loose.