Sister Sizes: Why Your Bra Hurts and How to Fix It

Has your bra ever felt a little off? Maybe the underwire psresses where it shouldn’t. The side boning digs in. Or nothing’s quite wrong — it just doesn’t sit right.

More often than not, the bra isn’t the problem. The balance of the size is. And that’s where sister sizes come in.

At aimerfeel, we size in Japanese measurements — from A70 (a 32A in US sizing) up to an H cup — and the sister-size idea works just as well here. Let’s walk through it together.

What Is a Sister Size?

A sister size is a different size with almost the same cup volume. The numbers on the label change, but the space inside the cup stays about the same.

In US sizing, it looks like this:
30D = 32C = 34B = 36A

Same cup, different label — the amount of bust each one holds barely changes.

Japanese sizes work the same way. In aimerfeel’s sizing:
A70 ≈ B65 ≈ C60
B70 ≈ A75 ≈ C65
C70 ≈ B75 ≈ D65

So Japanese bras have sister sizes too — and once you know yours, you have far more room to find a fit that feels right.

How Sister Sizes Work

The basic rule is simple:

• Go up a band size → go down a cup
• Go down a band size → go up a cup

That’s because cup volume isn’t a fixed number. It depends on the band it sits on.

Why a bra starts to hurt
When the balance is off, a few things tend to happen:

• A cup that’s too small pushes the bust out, so the underwire presses in.
• A band that’s too tight bends the boning, so the sides dig in.
• A band that’s too loose lets the bra shift, so the underwire slides out of place.

In other words, the discomfort usually isn’t the bra. It’s the fit balance — and that’s something you can adjust.

How aimerfeel Designs for Fit

An aimerfeel bra is built to do more than match your measurements. It’s built to lift you comfortably — to ease the pressure while still shaping a beautiful line.

• The Chomori Maximum Boost Bra lifts from inside the cup, with a built-in underwire that adds volume and holds its place all day.
• The Side Support Push-Up Bra uses an L-shaped underwire that keeps direct pressure off the center of the bust, so you get a natural lift line and stay comfortable.

That design thinking is our own, and it’s why a small change in size still feels stable, supported, and easy to wear. Because a sister size isn’t really about the numbers. It’s fit, structure, and design working together to keep you comfortable.

The Sister Size Chart

	BAND SIZE →
	65	70	75	80
← CUP VOLUME	A65	—	—	—
	B65	A70	—	—
	C65	B70	A75	—
	D65	C70	B75	A80
	E65	D70	C75	B80
	F65	E70	D75	C80
	G65	F70	E75	D80
	H65	G70	F75	E80
	—	H70	G75	F80
	—	—	H75	G80
	—	—	—	H80

Japan to US Size Chart

	BAND SIZE →
	30	32	34	36
← CUP VOLUME	30A	—	—	—
	30B	32A	—	—
	30C	32B	34A	—
	30D	32C	34B	36A
	30DD	32D	34C	36B
	30DDD	32DD(E)	34D	36C
	30G	32DDD(F)	34DD(E)	36D
	30H	32G	34DDD(F)	36DD(E)
	—	32H	34G	36DDD(F)
	—	—	34H	36G
	—	—	—	36H

Here's how the sizes shift:
- Across = adjusting the band
- Down = adjusting the cup

That's the basic idea behind sister sizes.
A quick note: this is a general guide. How a bra actually feels can vary with its design and with your own body.

Sister Sizes in Action

Case 1

Let’s say you’re wearing a C70.
The cup fits just right, but the band feels a little tight — and you’re already on the loosest hook.
So what’s the sister size here?
→ We’d suggest a B75.

A little tip from us: if you just want to ease the band, a bra extender works too.

Check our bra extender from here.

This one’s handy if:
• Swapping sizes feels like a hassle
• The size you want is out of stock

Case 2

Here’s a trickier one. You’re wearing an F80.
Your bust looks ready to spill from the cup, and the band feels a little loose — even on the tightest hook.
So what’s the sister size here?
→ The answer is H75.

Let’s break it down.
There are two things going on at once: the band is loose, and the cup is a little small. So we’ll take them one at a time.

1. The cup — your bust is overflowing, which means the cup is too small. Go up one cup: F → G. That gives us a G80.
2. The band — the band is loose, so we size it down: 80 → 75.
3. Apply the sister size — the sister size of a G80 is an H75.

The result → an H75.

When It’s a Cup Adjustment, Not a Sister Size

Not every fix is a sister size — and it’s worth knowing the difference.

Let’s say you’re wearing a C65. Even with the removable pad taken out, your bust feels like it might spill a little over the cup — but the band itself feels fine.

When the band is right and only the cup falls short, you don’t need a sister size at all. You simply go up a cup and keep the same band:
→ a D65.

A little tip from us: if one side is fuller than the other, choose the cup that fits the larger side, then even things out with the pad.
(F-G-H cup bras don’t include a removable pad.)

Check our bra pad from here.

When to Reach for a Sister Size

A sister size comes in handy when:

• The band feels tight — go up a size.
• The cup isn’t quite right — adjust it to rebalance.
• You’re caught between sizes — use a sister size as the in-between.
• Your size is sold out — reach for a sister size instead.

It’s an especially useful thing to know when you’re shopping online.

FAQ

Q1. Why does the same size sometimes still hurt?

Usually for one of two reasons:

• The band pressure isn’t right for you.
• The cup volume doesn’t match your shape.

Trying a sister size can often help.

Q2. Is there a difference between Japanese and international sizing?

The idea of a sister size is exactly the same. For how we measure and our full size chart, see our Size & Fit page.

Q3. Can sister sizes solve everything?

Not quite. Sister sizes are great for small adjustments and a bit of fine-tuning. But if the shape of the bra itself doesn’t suit you — or the design doesn’t match your body — that calls for a different kind of change.

Q4. Why does the same size vary from brand to brand?

A few reasons:

• The pattern, or how the bra is cut.
• The shape of the underwire.
• How much the fabric stretches.

Size is only ever part of the picture — a starting point, not the whole story.

Find Your Perfect Fit

When a bra doesn’t feel right, it doesn’t mean there’s anything wrong with your body. More often, it just means you haven’t found the right balance yet.

Sister sizes open up your options. So you don’t have to give up on the bra that’s almost-but-not-quite. A small change to the band or the cup can make a surprising difference in how it feels.

When in Doubt, Just Ask Us

And if you’re not sure which size to choose — or sister sizes still leave you uncertain — you don’t have to work it out on your own. Tell us about your shape and what’s been bothering you, and we’ll help you find the right fit. Just email us anytime.

Fitting a bra was never about landing on one perfect number. It’s about finding the fit that feels good to you, and a sister size is simply one way to get closer to it. More often than not, one small change turns discomfort into comfort — and “this doesn’t quite work” into “oh, so this is how it’s meant to feel.”

We hope this guide leaves you a little more confident about finding the fit that’s truly yours.