Sister Sizing Explained: Why Your True Bra Size Might Surprise You

You're a 34C, right? Maybe. But you might also be a 32D, or a 36B — and all three could fit you just as well.

This is sister sizing, and once you understand it, bra shopping changes completely.

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What Is Sister Sizing?

Sister sizes are groups of bras that have the same cup volume but different band sizes. When you go up a band size, the cup letter goes down. When you go down a band size, the cup letter goes up.

Band	Cup
32	DD
34	D
36	C
38	B

All four of these hold the same amount of breast tissue. The 34D, the 32DD, the 36C, and the 38B are sister sizes.

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Why Does This Matter?

Because cup letters don't mean the same thing across band sizes.

A 32D cup holds less tissue than a 36D cup. The letter "D" isn't a fixed measurement — it's a ratio between your band and bust. When the band goes up, the cup gets physically bigger even though the letter stays the same.

This is why someone might try on a 36B at one store, hate it, try a 34C at another, and feel like a different person. Same cup volume — completely different experience based on band fit.

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When Do You Use Sister Sizing?

When you can't find your exact size in stock. If you're a 32C and the store only has 34B and 34C, grab the 34B — it should fit your cups the same way, just with a slightly looser band.

When you're between sizes. If the 34C fits your cups but the band is just slightly too loose on the loosest hook, try a 32D. Same cup volume, a tick tighter on the band.

When you're in a new style or brand. Different brands cut their bands differently. A 34C in one brand might run like a 32D in another. Sister sizes give you a framework for troubleshooting.

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The Rule (In Plain English)

• Going down a band? Go up a cup. (34D → 32DD)
• Going up a band? Go down a cup. (34D → 36C)
• 34D → 36D is NOT a sister size — that's a genuinely bigger cup.

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What Sister Sizing Doesn't Fix

Sister sizing addresses cup volume — it doesn't change the physical fit of the band.

If a 36B fits your cups but the band is way too loose, a 34C will fit your cups AND give you a tighter band. But if a 32C fits perfectly everywhere except the cups are slightly too small, the sister size is 30D — a tighter band that might not work at all.

The goal is always to get the band right first, then adjust the cup. Sister sizing helps you do that without starting from scratch.

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A Note on UK vs US Sizing

If you've ever looked at UK bra sizes, you'll notice the letters diverge above DD:

US	UK
D	D
DD	DD
DDD / E	E
G	F
H	FF
I	G
J	GG

The UK system is more granular in larger cups. Many specialty brands (Panache, Freya, Elomi) use UK sizing. If you're shopping in a size above DD, it's worth knowing which system a brand uses before you order.

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The Easiest Way to Figure Out Your Size

The calculator-based methods give you a starting point, but they're not always accurate. Breast shape, density, and placement all affect fit in ways a tape measure can't capture.

The most reliable method is a physical fit check. That's exactly what our free Raddi fit check does: no tape measure, just a 5-minute walk-through that ends with a personalized size and fit summary.

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Raddi was built from encyclopaedic bra fitting knowledge by Ashley Wen, a certified bra fitter. Try the free fit check at wearingradical.com/pages/find-your-fit.