If a size fits everywhere except one spot, the fix is often a “sister size” same volume, different band.
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What a Sister Size Actually Is
Going up one band size loosens the ribcage grip, so the cup needs to be one letter bigger to hold the same volume. The reverse going down. A 34D and a 32DD can hold the same amount of breast tissue even though the labels look completely different.
When to Use a Sister Size
- The cup fits but the band feels loose (or tight). Try the sister size with the band adjusted.
- Your usual size is sold out. A sister size is a reasonable substitute while shopping.
- You’re between two sizes at a specific brand and need a nearby option to try.
Enter your measurements below to see your size along with its closest sister sizes in both directions.
The Math, One Level Deeper
Cup volume is fixed by your actual measurements. Band size just determines how that volume gets distributed around your ribcage.
Move the band up 2 inches and it grips more loosely, so the cup has to grow one letter to hold the same volume. Move it down and the reverse happens. That relationship is exact, not approximate.
Three Real Situations Where This Helps
Out of stock: your usual size is sold out in a style you want. The sister size holds the same volume and is worth trying before giving up.
In-between sizing: your measurements land between two standard sizes. A sister size can bridge the gap better than rounding blindly.
Switching brands: a new brand’s cut runs differently. Trying the sister size alongside your usual size covers more ground on a first order.
How Many Steps Are Reasonable
Stick to one step up or down (2 inches of band, one cup letter). Going further than that stops being a “sister size” and starts being a genuinely different fit, at that point, re-measure rather than guessing further out.
Sister Sizing Isn’t a Universal Fix
It solves band-vs-cup tension, not every fit problem. If straps dig in or the shape feels wrong regardless of band, that’s a different issue. Check Fit Problems instead.
A Worked Example From Start to Finish
Say your calculated size is 34D. Going up one band size (36) and down one cup letter (C) gives you 36C.
Same volume, looser band. Going down one band (32) and up one cup letter (DD) gives you 32DD.
Same volume, snugger band. Both are legitimate options for the exact same body; which one to actually buy depends on how the band feels, not which number looks more familiar.
When Sister Sizing Doesn’t Solve the Problem
Sister sizing fixes band-comfort issues while preserving volume, but if the actual problem is a shape mismatch (not enough projection, wrong strap placement) rather than band comfort, no sister size will fix it. That’s a signal to look at style or brand, not size.
The Math Behind Sister Sizing, Step by Step
Start with your true band and cup. Add 2 inches to the band, and the cup volume needs to stay identical.
But a looser band naturally supports less, so the cup letter has to increase by exactly one step to hold the same amount of tissue. Subtract 2 inches from the band and the reverse happens: a tighter band supports more, so the cup can shrink by one letter while still holding the same volume. This one-step relationship holds at every point on the chart, which is why the same math works whether you’re starting from a 30 band or a 44 band.
When Sister Sizing Isn’t the Right Fix
If both your band and cup feel wrong at your calculated size (rather than only one or the other) sister sizing won’t solve it, since it only adjusts the balance between an already-correct total volume. That situation usually means the original measurement needs a redo, not a sister-size swap.