The Secret to Better Bra Fit

In This Guide

• What Is Sister Sizing?
• The Sister Size Rule
• When to Use It — and When Not To
• Three Mistakes Worth Avoiding
• Find Your Fit at Conturve

Most of us were measured once, probably years ago...given a size, and left to get on with it.

And mostly, that's fine until it isn't.

The band digs in by mid-morning. Or it creeps up your back every time you move. Or you've found a style you actually like, but your size is sold out, and you have no idea whether anything nearby will work.

Sister sizing is one of those things nobody really explains. But once you understand it, shopping for bras gets a lot less frustrating. It won't solve every fit problem. But for band issues specifically, it's genuinely useful — and much simpler than it sounds.

What Is Sister Sizing?

Sister sizes are bra sizes that share the same cup volume but have different band sizes. Change the band — the cup letter shifts with it. The space inside the cup stays the same.

If you've always thought of a C cup as just a C cup, that's understandable. But cup size isn't a fixed measurement — it's a ratio. It's the difference between your underbust and your bust. When the band number goes up, the cup gets physically larger even if the letter goes down. When the band number goes down, the cup gets smaller even if the letter goes up.

So a 34C, a 32D, and a 36B all hold the same cup volume. Different letters. Same fit in the cups.

That's sister sizing.

The Sister Size Rule

The rule goes in both directions.

Band too tight?
Go up one band size and down one cup size. 34C becomes 36B. The cups hold the same, but the band sits a little looser.

Band too loose?
Go down one band size and up one cup size. 34C becomes 32D. Same cup volume, but snugger around the ribs.

One step. That's it.

Here's a quick reference in UK sizing:

When to Use It — and When Not To

Sister sizing solves one specific problem: your cups fit, but the band doesn't.

If that's you — cups feel right, no spillage or gaping, but the band is uncomfortable — sister sizing is the right tool. Try one step in the direction you need, and you should be close.

It's also worth knowing when your exact size is sold out. If you're a 34D and only 36C is available in a style you want, that's a reasonable alternative. The cup volume should feel the same. The band will be a fraction looser, but it won't be a wildly different bra.

Where it doesn't help:

If both the band and the cups feel wrong, sister sizing won't fix that. You're likely in the wrong size altogether. So, go back to your measurements before trying anything else.

And stick to one step at a time. Jumping two band sizes, say from a 34C to a 38A, shifts too much. Wire width, cup shape, strap placement: all of it changes with the band. The volume might match on paper, but the fit won't feel right.

One step. Then reassess.

Three Mistakes Worth Avoiding

1- Keeping the same cup letter when you change the band
If your 34C digs in and you swap to a 36C, you haven't sister sized, you've bought a bigger bra. The cups will hold more. Sister sizing is 36B: same cup volume, adjusted band. The letter always moves with the band.

2- Going more than one step
Two steps away from your base size and the proportions of the bra start to shift in ways the volume calculation doesn't capture — wire width, cup shape, how the straps sit. One step keeps the fit predictable. Two steps, and you're guessing.

3- Using it to fix a cup problem
Sister sizing isn't designed for that. If cups are gaping or spilling, that's a cup volume issue; stay in your band size and adjust the cup letter up or down instead. Different problem, different fix.

Find Your Fit at Conturve

Once you know your size, or your sister size, the next step is a bra that's actually comfortable in it. These three are worth knowing about.

Best for: all-day wireless comfort

Ultimate Comfort Wireless Support Bra

Wire-free with foam-moulded cups, convertible straps, and a hook-and-eye closure that gives you room to adjust. The free bra extender included makes it a practical choice if you're landing between sister sizes — it adds a little flexibility to the band without losing the structure.

Over 200,000 women wear it. "I forget I'm even wearing it" and "lift AND comfort — didn't think that was possible without a wire" are things real customers have said, not marketing copy.

Currently £26.99 (was £46.98). Backed by a 60-Day Money Back Guarantee and free unlimited exchanges.

Best for: exact cup-size fit, A to FF

Adjustable Wire Free Shaper Bra

If the EBRA's S-to-4XL sizing feels too approximate, the BRAHE is sized by cup, A through FF.

That makes it the more precise option if you know your exact cup and want a structured foam fit. The straps are genuinely adjustable, not just slightly. "A proper fit for the first time" is how one customer put it after years of wireless bras that didn't quite get the cup right.

Includes a free bra extender. 60-Day Money Back Guarantee.

Best for: off-shoulder, strapless, and one-shoulder outfits

Convertible Strapless Bandeau Bra

Sister sizing matters most for strapless styles — because there are no straps to compensate for a band that doesn't quite fit. The band is doing everything. The BANDEAUBRA uses a stay-put silicone grip and soft side boning to hold its position without straps. Wears three ways: fully strapless, one-shoulder, or with both straps for everyday coverage.

"Comfortable for a whole evening, didn't think about it once." That's the bar for a strapless bra, and this one clears it.

If you're between sizes, go for the snugger band. It'll stay up far better.

Browse the full range at Conturve Bras →

The Right Size Changes Everything

Sister sizing isn't a workaround. It's just how bra sizing works — and knowing it gives you more options, not fewer.

If your cups feel right and your band doesn't, you've got a simple fix. One step, a quick try-on, and you'll know.

Ready to find a bra that fits properly? Shop Conturve Bras → or start with the best sellers → if you're not sure where to begin.

Frequently Asked Questions

How often should I check my sister sizes?

Any time something shifts, weight changes, a new bra stretches with wear, or you're shopping a brand that runs differently than what you're used to. Worth knowing in advance so you can adjust quickly without starting from scratch.

My size is 34DD — what are my UK sister sizes?

Sister size down is 32E,  tighter band, same cup volume. Sister size up is 36D,  looser band, same cup volume. If a brand tends to run tight in the band, 36D is usually the first alternative worth trying.

Can I sister size more than one step?

You can, but the fit becomes harder to predict. Wire width, cup shape, and strap placement all shift with the band. One step keeps the volume and proportions consistent. Two steps, and you're better off remeasuring.

Is sister sizing the same as going up or down a cup size?

No, and this is the part that trips most people up. Going from 34C to 34D changes the cup volume. Sister sizing "34C to 32D" keeps the cup volume the same and adjusts the band instead. Different purpose, different result.