The FEMBRA journal

Bra Sister Sizes Explained: How to Switch Sizes Without Guessing

Fit advice, fabric stories and honest comfort notes for every day.

Sister sizes are bra sizes that hold the same cup volume on a different band. The rule is simple: go up one band and down one cup, or go down one band and up one cup. For example, a 34B has the same cup volume as a 32C and a 36A. Knowing this helps when your usual size is out of stock or the band does not feel right.

A note on FEMBRA sizes

FEMBRA bras come in sizes 32B to 42B, and every FEMBRA bra is a B cup. So at FEMBRA, your size comes from your band, using the FEMBRA bra size chart and the bra size calculator. The sister size rule below is general fit knowledge that helps you understand how bands and cups relate.

How the sister size rule works

The band number and the cup letter work together. When the band gets bigger, the cup letter must get smaller to keep the same volume. When the band gets smaller, the cup letter must get bigger.

Band feels too tight: go up one band and down one cup (32C becomes 34B).
Band feels too loose: go down one band and up one cup (34B becomes 32C).

Sister sizes for common bands

Here is how it works for the sizes we are asked about most. Each guide explains fit in more detail: 32B, 34B, 36B, 38B, 40B and 42B.

32B matches 34A and 30C
34B matches 36A and 32C
36B matches 38A and 34C
38B matches 40A and 36C

When to use a sister size

Use a sister size if the cups fit but the band is wrong. If the cups themselves are too small or too big, a sister size will not help. In that case, re-measure with the bra size calculator or follow how to measure your bra size at home. For the full list, see the bra size chart for Pakistan.

Signs you should change size

If the band rides up, the cups gap or spill, or the straps dig in, your size probably needs adjusting. Our guide to 8 signs your bra does not fit shows how to fix each one.

Find your fit at FEMBRA

FEMBRA bras are non-padded and wire-free. Browse all bras, check current prices, and enjoy free delivery in 3 to 5 business days with cash on delivery and 7-day returns. Not sure about your size? Message us on WhatsApp at +92 319 7171905.

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Sister size questions

What is a sister size in bras?

A sister size is a bra size with the same cup volume on a different band, found by going up one band and down one cup, or the reverse.

What is the sister size of 34B?

The sister sizes of 34B are 32C and 36A.

Can I wear a sister size instead of my size?

Yes, if the cups fit but the band is too tight or too loose. If the cups do not fit, re-measure instead.

Do sister sizes work for every brand?

They are a good starting point, but fit still varies by fabric and style, so check the product page and ask us if unsure.