Bra Sister Sizes: The Secret Every Woman Should Know

Quick Summary

Bra sister sizes share the same cup volume with a different band size, so a 34C equals a 32D or a 36B. Sister size up when your band is tight on the loosest hook, and sister size down when it's loose on the tightest hook. If your band fits but your cups don't, change only your cup size instead of switching to a sister size. Stay within one step of your usual size, since moving further shifts wire width, strap spacing, and center gore placement. Sister sizing works with Thirdlove's half cup sizes too, and the Fit Quiz can find your starting size.

Bra sister sizes share the same cup volume but have different band sizes, so a 34C equals a 36B or a 32D. If your band feels too tight, go up one band size and down one cup size. If it feels too loose, go down one band size and up one cup size.

Sister sizing works best when you move just one step. It's handy when your size is out of stock or doesn't feel quite right. It can also help you fix gaping cups, spillage, or a band that rides up.

Shopping for most clothes is pretty straightforward: sizes get bigger as they go up, and that's that. Bra sizes aren't that simple. You're balancing your unique breast shape, your band size, and your cup size, and they rarely line up neatly with one number and letter.

That's where bra sister sizes come in, and knowing yours can help you find your perfect bra. Below, you'll learn how sister sizes work, how to read a bra sister size chart, and the sister sizes for common bra sizes. You'll also learn when to size up or down and how to fix common fit problems.

What Are Bra Sister Sizes?

Bra sister sizes are groups of bra size equivalents that share the same cup volume. For example, a 36F (DDD) belongs to the same family as a 38E, a 40D, and a 42C. Stick with us, and we'll explain.

Chances are, you learned you had one bra size, and if it sort of fit, that was the end of the story. You may also have thought a B cup could never be a D cup unless your breasts changed size. Not quite.

Bra sizes work more like ratios. A cup's volume, or how much breast tissue it holds, changes with the band size. Here's how it breaks down.

Why Does Knowing Your Sister Size Matter?

Knowing your sister size gives you more ways to find a bra that fits when your usual size doesn't. If you've ever struggled to measure your bra size and find a bra that fits your unique shape, you're not alone. Remember: it's not you, it's your bra.

Your sister size is especially helpful when neither your cups nor your band fit properly.

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How Do Bra Sister Sizes Work?

Bra sister sizes work on an inverse rule: when your band size goes up, your cup letter goes down, and vice versa. To find your two closest sister sizes, go up a band and down a cup, or down a band and up a cup. It only sounds complicated, we promise.

On a bra size chart, like the one below, find your current size. Your sister sizes are the others in the same row, so if you're a 34C, they're 30E, 32D, 36B, and 38A. Let that sink in: an A cup can also be an E cup.

Sister sizes can feel counterintuitive, but they're key to a great-fitting bra. Say you wear a 34C and the cups fit, but the band is tight, so you think, "I'll just go up to a 36C." That usually leaves the cups too big, because a 36C is a sister size of a 34D.

Try a 36B instead, since it's the next sister size up from a 34C. The cups hold the same volume, but the band is roomier.

What Does a Bra Sister Size Chart Look Like?

In a bra sister size chart, each row is one sister size family, and every size in that row has the same cup volume. The chart uses Thirdlove cup letters, where DD equals E and DDD equals F.

What Are the Sister Sizes for Common Bra Sizes?

A 34C is the same as a 36B in cup volume, just with a bigger band, and every common size has sisters one step away. Here's a quick lookup, with the sister size down (smaller band, bigger cup) first and the sister size up second.

• 34C: 32D (down) or 36B (up)
• 34DD (E): 32DDD (F) (down) or 36D (up)
• 36DD (E): 34DDD (F) (down) or 38D (up)
• 32DDD (F): 30G (down) or 34DD (E) (up)
• 36DDD (F): 34G (down) or 38DD (E) (up)
• 38D: 36DD (E) (down) or 40C (up)

Should You Sister Size Up or Down?

You should sister size up if your band feels too tight and sister size down if it feels too loose. Sister size up means one band size larger and one cup size smaller. Sister size down is the opposite: one band size smaller and one cup size larger.

If your bra is on the loosest hook and still feels constricting, try your sister size up. For example, if you wear a 34D and the band feels snug, move to a 36C.

If your bra is already on the tightest hook and still feels loose, try your sister size down. For example, if you wear a 36C and the band feels loose, try a 34D for a snugger band with the same cup volume.

Here's how the two moves compare.

Stick to the sister sizes right next to yours. Moving further can leave the band too slack or too tight.

How Do You Find Your Sister Size?

You find your sister size by starting with your best-fit size, then moving your band and cup one step in opposite directions. Follow these steps, and use the bra sister size chart above to see each move.

1. Start with your current best-fit bra size.
2. If your band feels too tight, go up one band size and down one cup size (your sister size up).
3. If your band feels too loose, go down one band size and up one cup size (your sister size down).
4. Find your size's row on the sister size chart, since every size in that row shares the same cup volume.

Your next move depends on which part of your bra isn't fitting.

What If Your Cups Fit but Your Band Doesn't?

If your cups fit but your band doesn't, adjust your band and cup size in opposite directions. If your band is too small, go up a band size and down a cup size. If it's too large, go down a band size and up a cup size.

Ex: A 34C cup is the same as a 36B. A 40E (DD) cup is the same as a 42D.

What If Your Band Fits but Your Cups Don't?

If your band fits but your cups don't, change only your cup size. Go up a cup if they feel small, or down a cup if they feel big.

Ex: If a 32F feels small in the cups, go up to a 32G. If it feels big in the cups, go down to a 32E.

What If Neither Your Cups nor Band Fit?

If your band and cups are both too big or both too small, adjust only your band size. Don't worry about changing your cup letter, since cup volume shifts naturally with the band. If your band is too small and your cups are too big, go up in band size while reducing cup size, and vice versa.

This fix is a bit trickier, so plan to try on a few more bras to find your fit.

Ex: If a 32D is too small in both band and cup, try a 34D.

Sister sizes work with our signature half-cup sizes, too. If you're a 34B½, you're also a 36A½ and a 32C½. Since those cups are equivalent, choose the band size that feels best on you.

When Should You Wear Your Sister Size?

You should wear your sister size when your usual size isn't available or doesn't feel right in the band. One moment you're squeezing into a too-tight 34C, and the next you're comfortable in a 36B like it was made for you.

Reach for your sister size in these situations.

• Your size is out of stock: a quick look at the sister size chart gives you another size to try.
• Your band doesn't fit: move one band size while keeping the same cup volume.
• Your body changes: a sister size lets you adjust your band without losing your cup fit.
• You're between styles: a band that feels snug in one style may fit better in its sister size in another.

Sister sizes do have a limit. They work best when you move just one size up or down, since going further changes the bra's overall proportions.

Do Bra Sister Sizes Actually Work?

Yes, bra sister sizes actually work, as long as you stay within one step of your usual size. They rely on the fact that cup volume is relative to band size, so the same cup letter holds more on a bigger band. For example, a 34B and a 36B have different cup volumes, even though both are B cups.

Beyond one step, the fit starts to break down because the bra's frame shifts along with the size. Here's what changes.

• Wire width: the underwire's width changes with the size, so it may not sit where your breast tissue begins.
• Strap spacing: straps sit farther apart or closer together, so they may slip or dig in.
• Center gore placement: the gore shifts position, so it may not lie flat against your chest.

Sister sizes also don't change the shape or position of your breasts, since they only rebalance band and cup proportions. Still, they're a helpful tool on your way to a better fit. Get professionally fitted, or try our Fitting Room Quiz to get your best bra recommendation.

What Does a Well-Fitted Bra Feel Like?

A well-fitted bra supports your breasts without feeling tight, so you barely notice it's there. It keeps your breasts lifted and separated all day. Here's what to check.

• Band: it sits snug and level, and it provides most of the support.
• Cups: they fully encapsulate your breasts, with no spillage or wrinkling.
• Straps: they stay put without digging into your shoulders or slipping off.
• Center gore: it lies flat against your chest between your breasts.

How Do You Fix Common Bra Fit Problems?

You fix most common bra fit problems by adjusting your cup size, band size, or straps. Here are the most frequent issues and what to try.

• Gaping cups: go down a cup size, since the cups are likely too big, or try a different style.
• Spilling over the cups: go up a cup size or try full coverage bras.
• Band riding up: try your sister size down, since a loose band needs a smaller size.
• Straps digging in: let the band carry most of the weight and loosen the straps, or try wider straps.

Ready to Find Your Sister Size?

You're ready to find your sister size once you know your best-fit size and which way your band needs to move. If you'd like help, our quiz finds your bra size in just a few minutes, with bras in AA–H cups, including half cups.

Take the Fit Quiz

Our goal at Thirdlove is to help you find your perfect fit, so don't keep bra sister sizes a secret. Tell a friend, coworker, or, of course, your sister.

With additional reporting by Cathy Bishop and Nicole Yi.