Bra Sister Size Calculator
Use this calculator to find equivalent bra sizes with the same approximate cup volume. Enter your current bra size, then generate tighter-band and looser-band sister sizes.
If you have ever tried on a bra that was almost right but not quite perfect, sister sizing can save a lot of frustration. A sister size keeps the cup volume close while changing the band fit. This is useful when a brand runs snug in the band, when a specific size is sold out, or when your measurements sit between two standard sizes.
What Is a Bra Sister Size?
A sister size is an alternative bra size with a similar cup volume. The basic rule is simple:
- Go up one band size and down one cup size.
- Or go down one band size and up one cup size.
For example, if your regular size is 34D, close sister sizes are 32DD (tighter band) and 36C (looser band). The cup volume is similar, but the way support feels on your ribcage will change because the band changes.
Quick Example
Starting from 34D:
- One step tighter: 32DD
- One step looser: 36C
- Two steps tighter: 30E
- Two steps looser: 38B
How to Use This Calculator
Step 1: Enter Your Current Size
Input your band and cup exactly as you normally wear them. If your band number is odd, the calculator rounds to the nearest even size because most bras are sold in even band increments.
Step 2: Choose Number of Steps
Select how many sister sizes to generate in each direction. One to three steps is usually enough for fitting decisions.
Step 3: Compare Fit in Real Life
The results are mathematically equivalent, but fit still depends on:
- Brand size grading
- Cup shape (projected vs. shallow)
- Fabric stretch and construction
- Your personal comfort preference
When Sister Sizing Helps Most
- Your exact size is sold out.
- A band feels too tight but cups fit well.
- A band rides up but cups fit well.
- You are between two sizes during body changes.
- You are comparing fit across brands with different sizing quirks.
Fit Checklist After You Try a Sister Size
Band
The band should sit level around your torso and do most of the support work. If it rides up in back, it is usually too loose.
Cups
You should not spill over the top or sides, and there should not be major wrinkling or empty space. Small shape mismatches can happen even in technically correct sizes.
Center Gore
In many bra styles, the center panel should sit close to the sternum. If it floats far away, cup volume or shape may be off.
Straps
Straps should feel supportive but not carry all the weight. If straps dig in badly, the band and/or cup fit may need adjustment.
Important Notes About Cup Labels
US and UK cup systems are not identical. Some brands use DD, DDD, F; others use DD, E, F, FF. Always check brand-specific charts, especially when ordering online. Sister sizing math still works, but the printed cup label may differ.
Bottom Line
A bra sister size calculator is a practical tool for finding nearby sizes that keep cup volume similar while changing band tension. Use it to build a short try-on list, then confirm with real-world fit checks. Comfort, support, and shape together are what make the “right” size.