How to measure your bra band and cup

Band measurement is taken snug, directly under the bust, without a bra on

Wrap a soft tape directly under your bust, where a bra band sits, keeping it level, snug, and parallel to the floor — measure without a bra on for the most direct reading, or over a thin, unpadded bra if that's genuinely easier for you to hold steady, noting which method you used.

Bust measurement is taken at the fullest point, over the bust

Wrap the tape around the fullest point of your bust, generally across the nipple line, with the tape level all the way around your back and your arms relaxed at your sides — this figure, combined with your band measurement, determines your cup size.

Cup size comes from the difference between bust and band, not either figure alone

Subtract your band measurement from your bust measurement; each roughly one-inch difference generally corresponds to one cup letter step (A, B, C, and so on) — this is why cup size isn't a fixed, absolute measurement on its own but a relationship between the two figures, meaning the same cup letter can describe genuinely different bust sizes depending on the band it's paired with.

Round the band measurement following your chosen brand's own convention

Different brands round the raw band figure to the nearest even or odd number differently, and some add a few centimetres or inches of stretch allowance into the sizing itself — checking a specific brand's own guidance on how they round the raw underbust figure into a labelled band size avoids assuming every brand rounds identically.

Standing posture affects both readings

Stand naturally with relaxed posture and normal breathing for both the band and bust measurements, rather than arching your back or taking a deep breath, both of which shift the bust forward and change the reading away from your natural resting shape.

Measure both figures together, in the same short session

Taking the band and bust measurements back to back, rather than on separate days, reduces the chance that a natural day-to-day fluctuation in one figure throws off the relationship between the two — since cup size depends on that relationship, consistency between the two readings matters more than either figure's absolute precision.

This measurement feeds the bras chart, with its cup-lettering caveats

As covered in more depth on the bras chart elsewhere on this site, band size converts across US/UK/EU as genuine unit arithmetic, while cup lettering is a coded system that runs out of room differently across the three systems past a D cup — this measurement guide gets you the two raw figures; the bras chart handles converting them into a specific country's sizing.

Sister sizing means a borderline reading has a legitimate second option

If your band-and-cup calculation lands right at the edge between two combinations, remember that moving one band size up and one cup letter down (or the reverse) describes roughly the same actual cup volume — trying both the calculated size and its adjacent sister size is standard, sensible practice for a borderline reading.

A helper improves accuracy for both measurements

As with several other circumference measurements on this site, having someone else hold the tape level around your back for both the band and bust readings produces a more reliable result than attempting the same measurement solo with a mirror, particularly for the band figure which sits at an awkward angle to check yourself.

Re-measure after weight change, and periodically regardless

Band and bust measurements can both shift with weight change, and many people's actual size drifts gradually even without a major life event — treating this as a figure worth re-checking every year or two, rather than a number fixed since a single measurement years ago, is standard advice from fitting specialists.

Pregnancy and nursing call for current measurements, not a stored pre-pregnancy figure

Maternity and nursing bras are the one category on this site where an old measurement is actively misleading rather than just slightly stale — band and cup can shift multiple times across a single pregnancy and again once nursing begins, so treat each trimester as needing its own fresh measurement rather than a single number that carries you through.

A too-loose band undermines support even if the cup size feels right

Because the band, not the straps, is meant to provide the majority of a bra's support, a band that's measured or sized too loosely forces the straps to compensate, leading to shoulder discomfort and a bra that rides up at the back regardless of how correct the cup size seems — checking the band fits snugly on the loosest hook (to allow room to tighten as the band stretches with wear) is standard fitting advice.

Different bra styles tolerate a borderline measurement differently

A soft, unstructured bralette tolerates an imprecise band-and-cup calculation far more comfortably than a structured underwire style does, similar to how a full swimwear panel tolerates more error than a fitted structured piece — knowing which style you're sizing for changes how much precision the measurement actually needs.

Recording both figures together, with the date, keeps a useful long-term reference

Since cup size is a relationship between band and bust rather than either figure alone, recording both together — with the date taken — makes it easy to check later whether your calculated size has genuinely changed or whether the same relationship still holds even if your recalled bra size feels slightly off.

A fabric tape is genuinely necessary here, more than for most measurements

Because both the band and bust readings need to follow the body's curve closely and precisely to produce an accurate cup calculation, a proper flexible fabric or vinyl tape isn't just a nice-to-have here — a rigid or improvised tool introduces enough error in either figure to shift the calculated cup letter by a full step in some cases.

Teenage and first-time measurements benefit from extra patience and repetition

For anyone measuring for a bra size for the first time, taking both readings two or three times and comparing results, rather than trusting a single attempt, builds confidence in an unfamiliar process and catches an early mistake before it leads to an uncomfortable first purchase.

A band that rides up your back during the day is a fit signal worth acting on

If a bra's band consistently creeps upward at the back within an hour or two of wearing it, that's usually a sign the band itself is too large even if the string measurement seemed to check out at the time, since the band — not the straps — is meant to anchor level around the ribcage all day; sizing down one band and up one cup, per the sister-sizing relationship covered above, is the first thing worth trying before assuming the whole style simply doesn't suit you.

A recently changed exercise routine can shift both band and bust figures gradually over months

Taking up a new strength-training routine, or stopping one, can gradually shift both ribcage muscle and bust tissue over several months without any single dramatic change being obvious day to day, so it's worth treating a significant, sustained change in exercise habits as its own trigger to remeasure, separate from the weight-change and pregnancy triggers already covered.