It happens that your favorite bra model is not available in your size, or that in the measured size the band is comfortable but the cup is not. That's when sister sizes come in handy. It's a simple rule that allows you to choose a bra with the same cup volume but a different band size. We explain how it works, when to use it, and when it's better not to.
What are sister sizes?
Sister sizes are bra sizes that have a similar cup volume, although they differ in their designation. This is because the cup letter does not describe a fixed size, but rather the difference between the bust and underbust measurements. The larger the band, the larger the cup with the same letter. A C cup in size 85C therefore holds more than a C cup in size 70C.
The rule is simple:
- band down one size, cup up one letter, e.g., 75C → 70D,
- band up one size, cup down one letter, e.g., 75C → 80B.
Cups 70D, 75C, and 80B have similar volume. They differ in band length and tightness.
Sister size chart
In each row, you will find sizes with similar cup volume. Your size is in the middle, the tighter band version is on the left, and the looser one is on the right.
| Tighter band | Your size | Looser band |
|---|---|---|
| 65C | 70B | 75A |
| 65D | 70C | 75B |
| 70D | 75C | 80B |
| 70E | 75D | 80C |
| 75E | 80D | 85C |
| 75F | 80E | 85D |
| 80F | 85E | 90D |
| 85G | 90F | 95E |
| 90G | 95F | 100E |
Sister sizes for any measurement can be calculated by the calculator on the bra size chart page.
When should you consider a sister size?
The band is too loose, the cup fits
The bra rides up at the back, and the straps take all the weight. This is the most common fitting mistake. Choose a size with a smaller band and a cup one letter larger, e.g., instead of 80B, try 75C.
The band is too tight, the cup fits
The band digs in, leaves red marks, and is difficult to fasten even on the loosest hook. Choose a larger band and a cup one letter smaller, e.g., instead of 70D, try 75C.
Your size is not available
Not every model is sewn in a full range of sizes. If you like a bra that is not available in your size, a sister size can be a good alternative. It's best to choose one with a tighter band, because the bra stretches over time, and an extender will help if the band is too tight at first.
Differences between brands
Manufacturers' sizing varies. If you know that a certain brand has tighter bands, a sister size with a larger band will allow you to maintain the cup volume. Before purchasing, check the manufacturer's chart on the product page.
When won't a sister size help?
Sister sizes change the band, not the cup shape. So they won't solve the problem when:
- both the band and cup are ill-fitting, e.g., the band is too loose and the cup is too small. In that case, you need to reduce the band and increase the cup by more than one letter, or remeasure yourself,
- the style doesn't suit your breast shape, e.g., for full breasts at the top, a balconette might dig in regardless of size,
- you are moving two or more band sizes. Then the underwire spacing, gore width, and strap length also change, so the bra might fit completely differently.
A sister size is a solution for one step up or down. For larger differences, it's better to remeasure yourself according to the guide how to choose your bra size.
Sister sizes for large busts
For cups E and above, the rule works the same, but the volume difference between adjacent sizes is greater, and the band is responsible for almost all of the support. Therefore, for large busts, it's not worth giving up a well-fitted, snug band for a looser sister size. You'll find more tips in the guide bra for a large bust and in the collection bras for large busts.
How to try on a sister size
- Fasten the bra on the loosest hook.
- Lean forward and scoop your breast into the cup with your hand, from the side to the center.
- Stand up straight and adjust the straps so they don't dig in or slip off.
- Check that the band lies horizontally, the gore lies flat against your body, and the cup doesn't gape or dig in.
- Raise your arms and bend over. A well-fitted bra does not shift with movement.
Bra extender vs. sister size
When the band is only slightly too tight, you don't always have to change the size immediately. A bra extender is a short strip with hooks that attaches to the bra and lengthens the band by a few centimeters. It's useful during pregnancy, after holidays, or with a new, stiff bra. However, if you use an extender constantly, it's a sign that you need a sister size with a larger band.
Sister sizes in UK and FR notation
The rule works the same in other sizing systems. In UK sizes, 34C has sister sizes 32D and 36B, and in French sizes, 90C has sister sizes 85D and 95B. Just remember that in the UK system, D is followed by DD, E, F, FF, so "one letter up" from D is DD, not E. You can find the conversion in the table on the bra size chart page.
Frequently asked questions
Is 75C the same as 80B?
The cups have a similar volume, but 80B has a longer band. For an underbust measurement of about 75 cm, 75C will fit better, as an 80 band will be too loose.
Do sister sizes work for every style?
The principle is the same, but in underwired and padded bras, the differences are more noticeable than in soft and bralette models.
I ordered a sister size and it doesn't fit. What can I do?
You have 14 days to return it. If you want a different size, send the product back and place a new order. Details can be found in the frequently asked questions.
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