How to Measure at Home
Underbust (Band)
Breathe out normally, then wrap the tape snugly around your ribcage just beneath your bust. Keep it level all the way around. Use the nearest even number as your band size.
Overbust (Cup)
Wrap the tape around the fullest part of your bust, usually at nipple level. Keep it level and relaxed so it rests against you without pressing the tissue down. This is your bust measurement.
Work Out Your Cup Size
Subtract your band from your bust measurement. Each inch of difference usually moves you up one cup: 0″=AA, 1″=A, 2″=B, 3″=C, 4″=D, 5″=DD, 6″=DDD…
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Sister Size Calculator · World Edition 2026
Find Your Sister Sizes
Keep roughly the same cup volume while changing how the band feels. See your complete sister-size chain across six international sizing systems.
Sister Size Bra Calculator: Find a Better Band Fit Without Losing Cup Room
Use the calculator above when your cups feel right but the band does not. It shows nearby sister sizes that keep the cup capacity closely matched while giving you a firmer or roomier band. Below, you’ll also learn when sister sizing is genuinely useful—and when measuring again will save you more frustration.
A sister size is a nearby bra size with a different band and a closely matched cup capacity. Move down one band size and up one cup letter for a firmer fit. Move up one band size and down one cup letter for a roomier fit. For example, 34D, 32DD and 36C are sister sizes: the cups hold about the same amount, while the band becomes firmer or looser.
What Does “Sister Size” Mean?
A sister size is not just the next label on the rack. It is a deliberate adjustment that changes the band while keeping the cup capacity close to your current size. Cup letters never work on their own: a 30D cup is smaller than a 40D cup, so you always need to read the band number and cup letter together.
The easy rule is to move the band and cup in opposite directions. A smaller band needs a larger cup letter; a larger band needs a smaller cup letter.
| Fit Goal | Rule | Example | What Changes |
|---|---|---|---|
| Firmer band | Band −2, cup +1 | 36C → 34D | A firmer anchor with closely matched cup room |
| Softer band | Band +2, cup −1 | 34D → 36C | More room around the ribs with closely matched cup room |
| Primary size | No adjustment | 34D | Stay here when both the band and cups already feel right |
When Sister Sizing Is Worth Trying
Sister sizing can help when
- The cups feel right, but the band digs in or feels restrictive.
- The cups fit, but the band creeps upward at the back.
- Your usual size is unavailable in the bra you want.
- A particular brand or style runs tighter or stretchier than usual.
- You want a firmer anchor in a strapless or high-support bra.
Measure again or change cup size when
- Breast tissue spills over the top or sides of the cup.
- The center gore floats away from your chest.
- The underwire rests on breast tissue instead of surrounding it.
- Both the band and the cups feel noticeably wrong.
- Your body or measurements have changed since your last fitting.
Important: Sister sizing works best when your current size is already close. If the cup is clearly too small, too large or the wrong shape, shifting the band can disguise the real problem rather than solve it. Start with the Bra Size Calculator when you are not confident about your starting size.
Sister Size or Wrong Size? Here’s How to Tell
A useful sister size should make the band feel better without creating a new cup problem. When the cup itself spills, gaps badly or sits on breast tissue, the issue is not just band tension. That difference matters because band problems and cup problems need different fixes.
| Symptom | Likely Meaning | Best Fix |
|---|---|---|
| Band rides up, cups fit | Band too loose | Try the smaller-band sister size |
| Band digs in, cups fit | Band too tight | Try the larger-band sister size |
| Cup spillage | Cup too small | Keep the band and move up a cup first |
| Cup gaping | Cup too large, shape mismatch, or loose band | Check the band, then assess cup size and shape |
| Wire poking | Wire width or cup size mismatch | Remeasure and compare a different wire or cup shape |
Using Sister Sizes When Shopping Internationally
The sister-size rule works across regions, but the labels do not always match. US, UK, EU, AU, Japanese and French systems can use different letters for the same general cup step—especially around DD, E and F. The calculator keeps the size relationship consistent and shows the closest label in each system.
Treat the results as a practical fitting range, not a promise that every brand will feel identical. A UK band may feel firmer than a US one, while a European molded cup may be shallower than a UK full-cup bra. Use the conversion as your starting point, then let the actual fit decide.
How Sister Sizing Works in Sports Bras, Strapless Bras and Bralettes
Sports bras
When the cup coverage feels right but the band is uncomfortably firm, compare the roomier sister size. Keep in mind that compression sports bras often feel tighter than everyday bras by design.
Strapless bras
A smaller-band sister size can sometimes feel more secure because a strapless bra relies heavily on the band for support. It should still feel snug rather than painful.
Bralettes
A roomier sister size may feel better in a stretchy pull-on bralette, but going too loose can reduce support and cause the band to roll or ride up.
SHEFIT Ultimate Sports Bra
This high-support sports bra lets you adjust both the band and straps, which makes it easier to compare a firmer or roomier fit without guessing.
- Best for: high-impact training, strong bounce control and fuller-bust support.
- Fit note: when the cups feel secure but the band is too restrictive, compare the larger-band sister size.
Evelyn & Bobbie Beyond Bra
This wireless pull-on style gives smooth everyday coverage without the pressure of an underwire.
- Best for: work-from-home days, travel, errands and breaks from underwire.
- Fit note: when the band rolls or creeps upward, compare the smaller-band sister size.
Affiliate disclosure: As an Amazon Associate, we earn from qualifying purchases.
Three Sister-Size Myths That Cause Confusion
“Sister sizes are identical.”
They are close in cup capacity, not identical in fit. Band tension, wire width and strap position can all change.
“You should always wear a sister size.”
Your regular size is usually the best place to start when it fits well. Sister sizes are useful alternatives, not automatic upgrades.
“Sister sizing fixes every fit issue.”
Sister sizing mainly helps with band tension. It cannot rescue a cup that is clearly too small, too large or simply the wrong shape for your body.
When to Check Your Size Again
Measure again whenever several bras suddenly feel different, or after noticeable body changes such as weight fluctuation, pregnancy, postpartum changes, hormonal shifts or menopause. It is also worth checking the fit when you switch brands, because band stretch and cup depth can vary more than the label suggests.
What to Do Next
When your current bra feels close but not quite right, begin with the calculator above. If the result still does not solve the problem, check your starting size with the Bra Size Calculator, confirm international labels with the Global Bra Size Converter, and work through the exact fit symptom with the Bra Fit Problem Solver.
Frequently Asked Questions About Sister Sizes
What is a sister size bra?
A sister size changes the band while keeping the cup capacity close to your current size. For example, 34D, 32DD and 36C sit on the same sister-size line.
How do I calculate my sister size quickly?
For a firmer band, move down one band size and up one cup letter. For more room, move up one band size and down one cup letter.
If my band rides up, which sister size should I try?
Try the smaller-band sister size. If a 36C rides up while the cups fit, compare a 34D. The band will feel firmer while the cup capacity stays close.
If my band feels too tight, which sister size should I try?
Try the larger-band sister size. A 34D, for example, becomes a 36C. You gain room around the ribs while keeping the cup capacity close.
Do sister sizes work for US and UK sizing?
Yes. The rule works in both systems, but the cup letters begin to differ around DD, E and F. Use the region converter before ordering from an international brand.
When should I remeasure instead of using a sister size?
Measure again when the cups spill or gape badly, the center gore floats, the wires sit on breast tissue, or several bras in the same size all feel wrong.
