Sister Sizing

Moving up one band step and down one cup step is a common way to compare a similar cup volume with a looser band. Moving down one band step and up one cup step makes the opposite comparison. John Lewis illustrates this inverse band-and-cup relationship in its sister-size chart. [F7]

The worked comparison below illustrates that relationship within one conventional sequence. It is not a guarantee across manufacturers or every construction.

Suppose the band moves too easily but the cups otherwise seem suitable. A smaller band with the corresponding cup adjustment may be worth comparing. If the band feels comfortable but the cups overflow, that is a different question: compare cup size or shape without automatically changing the band.

Write the problem before choosing the next size. This stops “sister size” becoming a catch-all answer to every fitting issue.

A nearby size is the most useful starting experiment. Moving several band steps away changes more than the number around the ribs. The garment’s proportions and strap position may no longer resemble the fit you originally liked.

Do not accept a substantially loose or painfully tight band simply because a retailer has described another size as equivalent. Similar capacity does not mean equivalent comfort.

Cup labels do not advance identically in every system. A “step” means the next size in the maker’s own sequence, not necessarily the next single alphabet letter. Check the relevant chart before applying the rule, especially around double-letter or translated sizes.

This is why the Book explains the relationship rather than supplying one supposedly universal grid.

Put the comparison bra on as though you have never tried it. Recheck containment, band stability, wire position and strap adjustment. Do not assume those details are settled because you changed sizes according to a rule.

The successful result is a garment that works, not a technically neat calculation. When several nearby sizes all produce the same discomfort, compare another style before continuing to chase the label.

In a conventional band-and-cup sequence, 34C is commonly compared with 32D for a smaller band or 36B for a larger band at a similar nominal cup volume. John Lewis's published sister-size chart shows those relationships. The example explains a direction of change; it is not a recommendation that you personally wear any of those sizes. [F7]

Write the starting size, brand and style down before comparing. A letter without its band number is not enough context, and the same label is not a guarantee of identical construction across brands. Check the actual manufacturer's sequence rather than counting letters informally through sizes such as DD, E and F.

Sister sizing is most useful as a comparison when the cup volume seems reasonably close and you are investigating the band. If the cup is also cutting in, empty in a particular area or the wrong shape for the wearer, preserving approximate cup volume may not solve the issue.

Use a sentence with two parts: “I want to compare the band, while keeping roughly similar cup capacity.” That reminds you what the experiment is for. After trying the alternative, reassess every area instead of assuming the rule has settled the result.

A different band size changes more than the printed number. The complete pattern may place components differently. Even if a chart suggests a similar nominal volume, straps, cup dimensions and the stretch of the particular garment still need to cooperate with you.

Treat a nearby sister size as one comparison, not a route through an unlimited chain of available stock. Buying a distant alternative simply because it is on sale defeats the purpose if the band no longer creates a useful foundation. A correct calculation is not a substitute for a comfortable garment.

Do not apply a band-and-cup calculation to a one-size adhesive cup, a bralette sold only as small/medium/large or a one-piece that uses its own body measurements. Follow that product's sizing system. For a bra-sized bodysuit, cup fit also leaves a separate question about torso length.

The Fitting-Room Check↗ new tab is the final step, not an optional extra. It asks whether the band, cups, wires and straps work together in ordinary movement. When several close comparisons leave the same problem, the useful next step may be another construction or a fitter's assistance, rather than another calculation.