What is a sister bra size?

A sister size changes the band and cup label together to keep approximately the same cup volume.

When should I use a sister size?

Use a sister size when cup volume is already close but the band itself needs to be firmer or looser.

Are UK, US and EU bra sizes exactly standardized?

No. Common conversion ladders are useful starting points, but brand-specific cup labels, grading and construction can differ.

Sister Size Calculator & Bra Size Converter (UK, US, EU)

What sister sizes actually change

A sister size keeps approximately the same cup volume while changing the band. One step toward a firmer band normally means one cup-volume step larger; one step toward a looser band normally means one cup-volume step smaller. That relationship is useful when the cups are already close and the band is the part you are trying to correct.

Do not use sister sizing to hide a cup problem. If the cup is spilling, empty through most of its volume, too shallow, too tall, or using the wrong wire geometry, changing the band and preserving cup volume can leave the real problem untouched.

How UK, US and EU labels line up

The converter maps the common band and cup ladders used by these three systems. UK and US band numbers commonly share the same number while EU bands use a metric-labelled sequence. Above D, cup lettering diverges: for example, common charts map UK E to US DDD and EU F.

UBBRA keeps the conversion deliberately conservative. A manufacturer’s own chart should win when it disagrees with a generic conversion because brands can use different cup lettering, grading and construction.

Worked example: UK 34E

With the common progression used here, UK 34E maps to US 34DDD and EU 75F. Its immediate sister sizes in UK labels are 32F for a firmer band and 36DD for a looser band. The cup volume is the comparison being preserved; the frame, wire width, strap position and band tension can still feel different.

When to use this tool instead of Fit Rescue

Use this calculator when you already know the cups are close and you are intentionally changing the band. Use Bra Fit Rescue when you are not sure whether the band, cup volume, cup shape, wire, center or straps are creating the problem.

Reference points

The sister-size relationship is consistent with public guidance from Bravissimo. The UK-to-US cup ladder and common international conversions are cross-checked against Wacoal’s public size charts. These sources do not endorse UBBRA; brand-specific charts remain the authority for their own garments.

Sister-size FAQs

Is a sister size the same bra size?

No. It preserves approximate cup volume while changing the band. The overall frame and how it sits on your body can change.

Should I move more than one sister-size step?

The calculator intentionally emphasizes the immediate step. Moving farther changes the band and frame more dramatically and is less useful as a controlled fit comparison.

Why does UBBRA warn about US cup labels?

US manufacturers do not all use the same letters above D. If your brand uses F where another uses DDD, follow that brand’s progression rather than forcing a generic label.