What Are Sister Sizes? How to Find Yours and Use Them When Your Size Is Out of Stock

You have found the perfect bra — the right shape, the right colour, the right price — and then you discover that your size is completely out of stock. It happens all the time, and it is one of the most frustrating experiences in shopping. But there is a solution that most women never learn about: sister sizes.

Understanding sister sizes can open up a much wider range of options when your exact size is unavailable, and it also helps you understand why the same bra can feel different from one brand to the next.

What Are Sister Sizes?

Sister sizes are bra sizes that share the same cup volume but have different band sizes. When you go up a band size, you go down a cup size. When you go down a band size, you go up a cup size. The amount of space inside the cup stays the same.

This works because cup sizes are not absolute measurements — they are relative to the band. A D cup on a 32 band is a much smaller physical cup than a D cup on a 40 band. The letter only tells you the difference between the band size and the bust measurement, not the actual size of the cup in centimetres.

An Example to Make It Concrete

If your correct size is 34C, your sister sizes are:

• 32D — smaller band, larger cup letter, same cup volume
• 36B — larger band, smaller cup letter, same cup volume

All three of these sizes — 32D, 34C, and 36B — contain the same amount of space for your breast tissue. The difference is how the bra fits around your body.

How to Find Your Sister Sizes

Finding your sister sizes is simple. From your current size:

• To go up a band size, go down one cup letter (e.g., 34C → 36B)
• To go down a band size, go up one cup letter (e.g., 34C → 32D)

You can extend this in either direction:

• 34C → 36B → 38A
• 34C → 32D → 30DD → 28E

In practice, going more than one or two steps from your true size tends to result in a bra that fits noticeably differently. Sticking to one sister size in either direction gives the best results.

If you have not yet found your true starting size, use our bra size calculator to get your recommended size first, then identify your sister sizes from there.

When Should You Use a Sister Size?

When Your Size Is Out of Stock

This is the most common reason to reach for a sister size. If you are a 34D and that size is sold out, a 32DD or 36C will give you the same cup volume. Try both if possible — the difference in how the band feels may make one clearly preferable to the other.

When You Are Between Band Sizes

If your underbust measurement falls right between two band sizes and you are not sure which to choose, sister sizing can help you experiment. Try both your calculated size and the sister size one step in either direction and see which band feels more secure and comfortable.

When a Brand Runs Differently

Bra sizing varies significantly from brand to brand, even within the UK. A 34C from one brand may feel more like a 34B or 34D from another. If a bra from a new brand feels slightly off even though you measured correctly, try a sister size rather than dismissing the brand entirely.

When Should You NOT Use a Sister Size?

As a Permanent Fix for a Band That Is Too Loose

If you are wearing a 36C but the band rides up constantly, the answer is not to keep trying 38B — the answer is to go to 34D. Using a larger band sister size to accommodate an ill-fitting band just perpetuates the problem and means you lose support. Read our article on signs your bra does not fit to diagnose band fit issues properly.

When You Are Not Sure of Your True Size

Sister sizes only work as a tool if you already know your correct size. If you are still not sure what size you are, read our guide on how to measure your bra size at home and find your starting point before experimenting with sister sizes.

UK Sister Size Chart

The following chart shows common UK sister size groups. Each row contains sizes that share the same cup volume:

• 28D — 30C — 32B — 34A
• 28DD — 30D — 32C — 34B — 36A
• 28E — 30DD — 32D — 34C — 36B — 38A
• 28F — 30E — 32DD — 34D — 36C — 38B — 40A
• 28FF — 30F — 32E — 34DD — 36D — 38C — 40B
• 28G — 30FF — 32F — 34E — 36DD — 38D — 40C
• 28GG — 30G — 32FF — 34F — 36E — 38DD — 40D
• 28H — 30GG — 32G — 34FF — 36F — 38E — 40DD
• 28HH — 30H — 32GG — 34G — 36FF — 38F — 40E

To use this chart, find your size in the list and read across the row to find your sister sizes on either side.

A Note on UK Cup Sizing vs International

The UK uses a cup progression that differs from US sizing, which can cause confusion — particularly if you are shopping from American or European brands. UK sizing includes double letters (DD, FF, GG, HH) between the single letters, whereas US sizing does not. This means a UK DD and a US DD are not the same size. If you are shopping internationally, check our full guide on UK vs US vs EU bra sizes before using a sister size from a foreign brand's size chart.

How to Try a Sister Size in a Shop

When you are in a shop trying sister sizes, keep these things in mind:

• A smaller band will feel tighter than you are used to — this is normal, and it should feel firm rather than painful
• A larger band will feel looser — check whether it rises up when you raise your arms, which would signal it is too large
• The cups should contain all of your breast tissue with no spillage or wrinkling regardless of which sister size you choose
• Always do the scoop and swoop (scooping all breast tissue into the cup) before assessing the fit

Finding Your Sister Sizes Automatically

Our bra size calculator automatically shows you your primary recommended size along with your nearest sister sizes, so you can quickly identify alternatives without doing the calculations by hand. It is the fastest way to find the right starting point and know your options at the same time.

Sister sizes are one of those small pieces of knowledge that make an enormous practical difference — especially when you have found a great bra and need it in the right fit. Once you know your size and its sisters, you will never be completely stuck when your exact size is unavailable.