Bra Size Calculator
Sister Sizes
Free bra fit tool

Sister Size Calculator

Enter your current bra size to find sister sizes with similar cup volume and a tighter or looser band. It is the fastest way to compare alternatives when a bra almost fits or your size is out of stock.

Start calculator Learn sister sizing

Instant result

Enter your current bra size
Use US-style sizes with an even band number. Spacing and lowercase letters are fine.
Start with the size that fits you closest right now.

Quick answer

To find a sister size, change the band and cup in opposite directions: down one band and up one cup, or up one band and down one cup.

Best use

Try sister sizing when cup volume feels close but the band is slightly too snug, too loose, or sold out.

Fit check

Compare sister sizes in the same bra style so the band and cup changes are easier to judge.

Formula

How the sister size calculator works

Bra cup letters are not fixed volumes by themselves. A 34C cup is not the same volume as a 38C cup because cup volume changes with band size. Sister sizing keeps the cup volume close by moving the band and cup in opposite directions.

Too loose?Band down, cup up34C becomes 32D
Too tight?Band up, cup down34C becomes 36B
  • Use the tighter-band sister size if your band rides up or shifts around.
  • Use the looser-band sister size if the band digs in but the cup volume feels close.
  • Re-measure if both sister sizes still gape, overflow, or feel unsupported.

Examples

Sister size examples

Starting sizeTighter bandLooser band
32C30D34B
34C32D36B
36D34DD38C
38DD36DDD40D

These are one-step sister sizes. Some people can wear a second step away, but fit becomes less predictable as the band changes more.

Fit guidance

Sister size calculator FAQ

What is a sister size in bras?

A bra sister size is a nearby size with a different band and cup letter but a similar cup volume. For example, one common move is down one band and up one cup.

Why does my bra size have two sister sizes?

You can move one step tighter in the band and one step larger in the cup, or one step looser in the band and one step smaller in the cup.

Should I replace my main size with a sister size?

Use sister sizes as fit alternatives when your usual band feels off, your size is out of stock, or a brand runs tighter or looser than expected.

Can sister sizing fix cup fit problems?

Not always. If cups gape or overflow, recalculate your base bra size first and then test sister sizes.

Want deeper fit guidance? When should you try a sister size?