Sister Sizing Explained: How to Size Up or Down Without Losing Cup Volume

The sister size of a 34B is a 36A. Go up a band, down a cup, and the cups hold the same amount. That is sister sizing in one line.

It is the trick that lets you rescue a band that digs in or rides up without losing the cup that actually fits. Below is the chart, the quick maths, and how to use it when you are buying a sports bra rather than an everyday one. Sports bras play by slightly different rules, and most size charts never tell you that. More on it further down.

What is a sister size?

A sister size is a different band-and-cup combination that holds the same cup volume. When the band number goes up, the cup letter comes down. When the band comes down, the cup letter goes up. So a 34B, a 36A and a 32C all have the same size cup. Only the band changes.

Here is the bit most people get wrong. The letter on its own does not mean a fixed amount of breast. A DD on a 32 band is a much smaller cup than a DD on a 38 band. The letter only means something next to its number.

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What is the sister size of a 34B? And a 36A?

A 34B sisters down to a 32C and up to a 36A. A 36A sisters down to a 34B and up to a 38AA. Same cup volume all the way along. Only the band gets tighter or looser.

Here are the most-asked sizes laid out. Read along each row: every size on that row holds the same cup, with a tighter band on the left and a looser one on the right.

Find a size you already own on the chart, then slide left for a snugger band or right for a looser one. The cup comes with you.

How do I work out my sister size?

Move one step at a time. Up two inches in the band, down one cup letter. Or down two inches in the band, up one cup letter. The cup volume stays put while the band changes.

Say you are a 34D and the band feels loose. Drop the band and go up a cup, and you land on a 32DD. If instead the band digs in and you want more room, go the other way to a 36C. Same cups, different band, sorted.

How do I measure in the first place?

You need two numbers: your underband and your bust. Measure the underband first. Put the tape flat, snug and level just under where your bra sits, and read it in inches. If that number is even, it is your band size. If it is odd, add one inch.

Now the bust. Same tape, but around the fullest part of your chest. Your cup is the difference between the two. One inch is an A, two inches a B, three a C, four a D, and on it goes. So a 32 inch underband with a 36 inch bust is a 32D.

One honest caveat. A tape measure only ever gives you a starting number. Sizes shift between shops the same way clothing sizes do, so two bras with the same label can fit completely differently. The real test is how the bra sits on you, which is where it helps to know the signs of an ill-fitting bra. Most brands also publish their own measuring guide, so check ours on the size guide before you order.

Does sister sizing work for sports bras?

The maths is the same, but there is one rule a sports bra adds that an everyday bra does not. About 80% of your support comes from the band, not the straps. So do not size the band up just because it feels comfier, because a looser band on a sports bra means less support exactly when you need it most.

From my years of fitting, eight out of ten people are already in a band that is too big for them. With a sports bra that matters more than ever. If your usual band feels tight at rest but you need the hold, that snugness is doing its job, not failing you.

This is where a fit you can fine-tune earns its place. The Solidarity high-impact sports bra has an adjustable Overband^®, the curved panel that sits over the top of the chest and controls the upward bounce most sports bras leave alone. You set the band where it holds you and tune the Overband^® tension under each arm, rather than sizing up and losing support. If compression versus structure is new to you, here is the difference between encapsulation and compression.

That is sister sizing, start to finish. Learn it once and you will never be stuck between a band that digs in and a cup that gapes again. If you would rather have a second pair of eyes, book a free fitting with one of our fitters. Around twenty minutes, in person or online, no purchase needed.

Sister sizing FAQs

What is the sister size of a 34B?

The sister size of a 34B is a 36A if you want a looser band, or a 32C if you want a snugger one. All three hold the same cup volume. Only the band changes.

What is a 36A sister size?

A 36A sisters down to a 34B for a tighter band and up to a 38AA for a looser one. The cup volume stays the same across all three.

How do I find my sister size?

Take your usual size, then move one step at a time. Up two inches in the band means down one cup letter. Down two inches in the band means up one cup letter. So a 34D becomes a 36C with a looser band, or a 32DD with a tighter one.

Does sister sizing change the cup volume?

No, and that is the whole point of it. A sister size holds the same amount as your usual size. You are only changing how tight the band sits, not how much room the cup gives you.

Is my sister size the same as my real size?

It is an equally valid size that fits the same cup volume, not a separate "true" size. Your best fit is the one where the band stays level around your body and the cup holds all of your breast with no gaping or spillage. Sister sizing just gives you more than one label that can do that.

Can I use sister sizing for a sports bra?

Yes, the maths works the same, with one caveat. The band carries about 80% of a sports bra's support, so do not loosen the band just for comfort or you will lose the hold. If you are between sizes for sport, a fitting or an adjustable band is the safer route.

Why do the same size bras fit differently in different shops?

Bra sizes are not standardised, so a 34C in one shop can feel like a 34D in another, exactly like clothing sizes. Always check the brand's own measuring guide, and judge the fit on how the bra sits rather than the number on the label.