Sister sizing is the most-used fit-correction tool in bra fitting. When your measured size feels close but not quite right, when your size is out of stock in a style you love, or when your body changes through life stages, sister sizes give you three to five equivalent options to try. This guide covers what sister sizes are and how the math works, the full sister size chart covering bands 28 through 44, how to find your sister sizes in four steps, when to sister-up versus sister-down, where the rule breaks down (and what to try instead), and worked examples for common fit problems.
Find Your Sister Sizes
If you have not measured recently, use the home calculator first — it returns your size plus all sister sizes in one step.
Use the Calculator → Shop All Bras →- The rule: Up one band, down one cup. Down one band, up one cup. Cup volume stays constant.
- For a 34C, the sister sizes are 32D (firmer band) and 36B (looser band).
- Most wearers have 3–5 sister sizes — two on either side of the measured size.
- Use sister-down when the band rides up or feels loose (firmer band, same cup volume).
- Use sister-up when the band cuts in or restricts breathing (looser band, same cup volume).
- Sister sizing fixes band-fit problems, not cup-shape problems.
- One step in either direction is reliable; two steps may shift proportions; three+ rarely fits.
- Sister sizing works within a brand, not reliably across brands.
What Are Sister Sizes (and How They Work)
Sister sizes work because cup labels are relative to band measurement, not fixed volumes. A "D cup" doesn't mean a specific size in cubic inches — it means "4 inches larger than the band, whatever that band is." A 30D, 32D, and 34D all use the letter D, but they hold progressively more volume because the band reference point grows.
The reverse logic is what gives you sister sizes. If you shrink the band by one size (34 → 32), the cup letter has to grow by one (C → D) to hold the same volume. If you grow the band by one size (34 → 36), the cup letter has to shrink by one (C → B). The cup volume stays constant; the label simply reflects the new band reference.
Up one band, down one cup. Down one band, up one cup. A 34C ↔ 32D ↔ 36B. The bands change; the cup volume stays constant. One step in either direction is reliable; two steps still hold cup volume but the bra's proportions start shifting (strap placement, gore width, wire shape). Three or more steps rarely fit.
The Sister Size Chart
Find your measured size in the highlighted center column. Read across the row to see your sister sizes — every cell in the same row holds equivalent cup volume. Move left for firmer bands; move right for looser bands. The chart uses US sizing and covers bands 28 through 44, cups A through K.
| −2 Bands | −1 Band | Your Size | +1 Band | +2 Bands |
|---|---|---|---|---|
| — | 28B | 30A | 32AA | — |
| 28C | 30B | 32A | 34AA | — |
| 28D | 30C | 32B | 34A | — |
| 28DD | 30D | 32C | 34B | 36A |
| 28DDD | 30DD | 32D | 34C | 36B |
| 30DD | 32D | 34C | 36B | 38A |
| 30DDD | 32DD | 34D | 36C | 38B |
| 32DD | 34D | 36C | 38B | 40A |
| 32DDD | 34DD | 36D | 38C | 40B |
| 34DDD | 36DD | 38D | 40C | 42B |
| 34G | 36DDD | 38DD | 40D | 42C |
| 36G | 38DDD | 40DD | 42D | 44C |
| 36H | 38G | 40DDD | 42DD | 44D |
| 38H | 40G | 42DDD | 44DD | — |
If your measured size isn't in the chart, apply the rule directly: subtract one from the band and add one cup letter for sister-down; add one to the band and subtract one cup letter for sister-up. The chart covers the most-fitted sizes; the math extends to any band-cup combination across the standard sizing range.
Find your measured size in the center column. The cell to the immediate left has a firmer band on the same cup volume (sister-down). The cell to the immediate right has a looser band on the same cup volume (sister-up). The outer columns (±2 bands) are extended sisters — equivalent cup volume, but with more proportion drift. Most wearers find their best fit within one step of the center.
How to Find Your Sister Size in 4 Steps
The math is simple enough to do in your head, but the four-step framework below makes it explicit. Use it as a checklist when shopping or troubleshooting fit.
Identify Your Measured Size
Start with the size your underbust and bust measurements give you. If you haven't measured recently — or if your body has changed (weight, pregnancy, hormonal shifts) — measure again before applying sister sizing. Sister sizing refines a measured starting point; it doesn't substitute for measurement.
If you don't have a current measurement, use the home bra size calculator — it returns your measured size plus your full sister-size chain in one step.
Calculate Your Sister-Down (Firmer Band)
Subtract one from your band number and add one to your cup letter. A 34C becomes a 32D. A 36DD becomes a 34DDD. A 30B becomes a 28C. The cup volume stays equivalent; only the band changes.
Sister-down gives you a firmer band on the same cup volume. Use this option when the band rides up your back, feels loose, or doesn't anchor — the smaller band size pulls firm against the rib cage while the larger cup letter keeps the same cup volume you measured into.
Calculate Your Sister-Up (Looser Band)
Add one to your band number and subtract one from your cup letter. A 34C becomes a 36B. A 36DD becomes a 38D. A 30B becomes a 32A. The cup volume stays equivalent; only the band changes.
Sister-up gives you a looser band on the same cup volume. Use this option when the band feels tight, cuts into your skin, or restricts breathing — the larger band size relaxes against the rib cage while the smaller cup letter keeps the same cup volume you measured into.
Try the Equivalent Options to Find Your Best Fit
Most wearers fit one of three or four equivalent sister sizes. Try the measured size first. If the band rides up, sister-down. If the band feels too tight, sister-up. If the band is right but the cup spills or gapes, sister sizing won't help — that's a cup-shape problem, not a band problem.
Bra fitting is largely an exercise in working through a sister-size chain. Most refinement happens within one step of your measured size; if you've tried sister-down, sister-up, and the measured size and nothing fits, the issue is usually brand-specific cup shape rather than sizing math.
When to Use Sister Sizing
Sister sizing is the home fitter's most-used adjustment tool. The pattern is consistent: you've measured into one size, you try it on, something feels half-right. The sister sizes give you three or four equivalent options to test before changing brands, styles, or your assumptions. Five scenarios cover most cases.
Sister-Down vs Sister-Up — Which to Choose
Both sister sizes hold equivalent cup volume; the only difference is band tightness. Choose the direction based on what's wrong with your band fit, not on your preference for one over the other.
The Band Doesn't Anchor
- The back of the band rides up between your shoulder blades when you raise your arms.
- You can pull the band more than an inch away from your skin at the center back.
- The straps dig into your shoulders — meaning they're carrying weight the band should be carrying.
- The bra "rotates" on your body during normal wear (the cup gore drifts off-center).
Sister-down gives you a smaller band number and a larger cup letter — equivalent cup volume on a firmer band. A 34C goes to 32D; a 36DD goes to 34DDD.
The Band Restricts
- You can't take a full breath while the bra is fastened on the loosest hook.
- The band leaves deep red marks that are still visible 15+ minutes after taking the bra off.
- The underwire pinches inward at the side or center because the band is forcing it too tight.
- The band rolls or folds upward at the back even when fastened on the loosest hook.
Sister-up gives you a larger band number and a smaller cup letter — equivalent cup volume on a looser band. A 34C goes to 36B; a 36DD goes to 38D.
The Band Is Right but Something Else Is Wrong
- The cup gapes open at the top — your tissue isn't filling the upper cup. Try a smaller cup at the same band.
- The cup spills over the top or sides — your tissue is overflowing. Try a larger cup at the same band.
- The center gore floats off your sternum — the cup is too small to let the gore lay flat. Try a larger cup at the same band.
- The underwire pokes you at the side — the cup is too narrow for your breast root. Try a different brand or cup cut.
Sister sizing fixes band fit. Cup-shape problems require a same-band cup adjustment first, then a brand change if no cup letter at that band fits.
Where Sister Sizing Breaks Down (and What to Try Instead)
Sister sizing is a precise tool, not a universal solution. It solves specific problems (band fit, same-volume availability) and fails at others. Three patterns mark the limits.
Sister sizes are equivalent in cup volume, not in everything else. Strap placement, gore width, wire shape, and overall proportion shift slightly with each band step. One sister-size step (up or down) usually fits well; two steps can introduce noticeable proportion changes; three or more steps typically don't fit. If you've sister-sized twice and still aren't comfortable, the issue isn't sizing — it's brand or cup pattern.
Brand Variance Affects Sister Sizing
A 34C in one brand isn't necessarily a 34C in another — and the same applies to sister sizes. If your measured size in Brand A fits as 34C and you sister-down to 32D, the equivalent in Brand B might be 32D, 32DD, or 34C depending on how that brand grades cup volume. Some brands run small in the cup, others large; some have wider gores, others narrower. Sister sizing is most useful as a fit-correction tool within a single brand and style — when you've found a brand that fits your shape, sister sizes give you the adjacent options in that same brand's cut.
Style and Cut Affect Sister Sizing
Different bra styles react differently to sister sizing. Standard t-shirt bras, full-coverage bras, and balconettes follow the rule cleanly. Plunge bras and demi bras can shift visibly with a sister-size step because the cup shape is more sensitive to band position. Push-up bras with heavy padding may produce spillage at a sister-up because the foam doesn't accommodate the smaller cup letter as smoothly. Sports bras with compression construction (S/M/L sizing) don't use sister sizing at all — encapsulation sports bras do.
When the Brand Pattern Just Doesn't Fit
The most common case where sister sizing fails entirely: you've tried your measured size, sister-down, and sister-up, and none of them fit cleanly. This isn't a sizing problem — it's a brand-pattern mismatch. Some breast shapes (east-west, full-on-bottom, narrow-rooted, wide-set) need specific brand cuts. The fix is a different brand with cup geometry that matches your shape, not more sister sizing within a brand that doesn't fit.
Sister Sizing Examples — Real Fit Problems Solved
Six worked examples of common fit problems that sister sizing solves (or doesn't). Match your situation to the closest example.
Cup fits, but the band creeps up the back through the day. Sister-down to 30D — firmer band, same cup volume. The 30 band anchors flat; the D cup holds the same tissue as the 32C did.
Cup fits, but the band squeezes after a few hours. Sister-up to 38C — looser band, same cup volume. The 38 band relaxes; the C cup holds the same tissue as the 36D did.
Favorite bra style sold out in 34DD. Sister sizes 32DDD and 36D both hold equivalent volume. Try 32DDD first if the original band feels just right; try 36D if the original band was slightly tight.
Body has changed; familiar 30DD doesn't fit. Re-measure first — pregnancy and breastfeeding can shift you naturally into a sister size. If the new measurement is 32D or 34C, those are equivalent volumes on different bands.
Band fits, but breast tissue overflows the cup top. Not a sister-size problem. Try 34D first (larger cup, same band). If 34D still spills, try 36D (sister-up of 34D) for more capacity on a looser band.
Band fits, but the upper cup wrinkles or gapes. Not a sister-size problem. Try 38DD (smaller cup, same band). If the gape persists, the brand's cup shape isn't matching your tissue distribution — try a different cup cut (balconette or demi) rather than re-sizing.
"Sister sizing is the closest thing bra fitting has to a universal trick. It works because of how cup labels are calculated, not because of any clever design. Once you understand the math — same volume, different band — you can solve 80% of fit problems without leaving a brand you already like."
— HauteFlair Fit Editorial Team
Frequently Asked Questions About Sister Sizes
What are sister sizes in bras?
How do I find my sister size?
How many sister sizes does each bra size have?
What is the difference between sister-up and sister-down?
When should I use sister sizes?
Can I use sister sizes across different brands?
Do sister sizes work for sports bras?
Do sister sizes work for full-bust (DDD+) sizes?
Are sister sizes always equivalent in fit?
What if no sister size fits well?
This article is for informational and educational purposes. HauteFlair is not responsible for individual fit outcomes — bra sizing varies between brands and styles, and sister sizing is a starting framework rather than a guarantee. For best results, refer to each brand's specific size chart and consider a professional fitting consultation. Last reviewed: May 13, 2026.