Use a specific bra that fits as your reference. Convert both labels into the same sizing system, then compare the exact models by band firmness, cup volume, cup depth, wire width, cup height and material. Brand charts alone cannot show whether two three-dimensional garments fit alike.

Start with a known bra

Record the brand, model, sewn-in size and sizing system. Add the hook used, band feel, cup depth, wire width, cup height, strap placement and approximate age of the bra.

A worn bra is a weaker reference because stretched elastic changes how the label behaves.

Step 1, normalise the sizing system

Confirm whether each product uses UK, US, EU, French, Australian or another sizing system. Check the original label rather than relying only on the retailer’s conversion.

US cup sequences need particular care after D because brands do not all use the same labels.

Put both labels into one system

Convert the complete band and cup before comparing the garments.

Convert the Sizes

Step 2, compare the bands

Look at the material, wing height, number of hooks and repeated model-specific fit notes. A high, reinforced band can feel firmer than a narrow stretch band in the same labelled size.

If the known band fits and the new model is consistently firmer, the closest looser sister size may be useful. Do not assume that every model from the brand follows the same rule.

Step 3, compare cup capacity

Start with the converted equivalent. If the new model consistently cuts in or places the wire on tissue while the band is stable, compare one cup larger. If loose fabric appears throughout the cup, compare one cup smaller.

Do not change the band and cup randomly. Use a sister size only for a band question.

Step 4, compare cup geometry

A similar nominal volume can be distributed differently.

  • A wide, shallow cup spreads the room across a broader base.
  • A narrow, projected cup places more room forward.
  • A tall cup may gape near the strap.
  • A closed neckline may cut into upper fullness.
  • A wide gore may sit on close-set breast tissue.

If the original size has both empty space and pressure, another shape is often more useful than another volume change.

Step 5, compare materials and construction

Rigid foam, spacer fabric, stretch lace and multi-part seamed cups react differently to the body. A flexible upper cup can accommodate asymmetry, while dense moulded foam keeps its own shape.

The product name does not provide enough information. Use photos and technical descriptions.

Use wearer feedback carefully

Prioritise reviews that name the exact model, sewn-in size and comparison bra. Repeated specific observations can support a test choice. A few vague comments should not become a rule such as “this brand runs one cup small”.

For published brand guidance, record the source, market, date checked and confidence level.

Choose a small test set

A useful order is the converted baseline, one adjacent cup in the same band and the closest sister size only when band firmness is uncertain. Add another construction when the geometry is clearly different.

The useful conclusion

Compare garments, not logos. Normalise the system, anchor the comparison to a known bra and test one controlled difference at a time. The result is a more useful starting size and a clearer understanding of the fit.