How to Measure Bra Cup Size? | Band And Bust Math

Bra cup size is the inch difference between your bust and underbust measurements, with each inch typically representing one cup letter.

Getting the right bra size comes down to two numbers and a quick subtraction step. Measure your band snugly beneath your breasts, measure your bust at its fullest point, then subtract the band number from the bust number. That difference maps directly to a cup letter in most US sizing guides. It takes about three minutes and requires nothing more than a soft measuring tape.

What You Need Before You Start

Grab a soft cloth measuring tape and stand in front of a mirror. Wear a nonpadded, non-push-up bra or a fitted T-shirt — Good Housekeeping’s fit guidance specifically recommends a nonpadded style so the tape sits against your natural shape. Some resources suggest measuring without a bra entirely; either works as long as the tape rests directly against skin or a single light layer.

Skip anything with padding or underwire that changes your silhouette. The point is to capture your true measurements, not the shape a bra creates.

The Two Measurements That Matter

Step 1: Measure your band. Wrap the tape snugly around your ribcage, directly under your breasts. The tape should sit level and parallel to the floor all the way around your back — no dipping, no riding up. Pull it snug but not tight enough to leave a mark; you want the tape to feel like a well-fitted band would.

Step 2: Measure your bust. Move the tape up to the fullest part of your bust, keeping it straight across your back and level at the front. The National Breast Cancer Foundation’s bra fit guide advises measuring loosely here, letting the tape sit against your body without compressing anything. Write both numbers down in inches.

Turning Inches Into A Cup Letter

Subtract your band measurement from your bust measurement. The difference in inches determines your cup size. In standard US sizing, the math runs like this:

So if your band measures 32 inches and your bust measures 36 inches, the 4-inch difference puts you at a 32D. A 5-inch gap would make you a 32DD, and so on as the difference grows.

If your numbers land between sizes, round up to the next full inch for a more forgiving fit. The same measurements that point to a D in one brand might suggest a DD in another — this roundup of A-cup bras is a good starting point if your results sit at the smaller end. Sizing varies enough between manufacturers that trying multiple styles in your calculated size plus one sister size up and down is worth the effort.

Why UK Sizing Confuses Everyone

If you shop British or European brands, the simple 1-inch-equals-one-cup rule stops working. The NHS bra measurement guide uses a different system: take your underbust measurement, add 4 inches if it’s an even number or 5 if it’s odd, and use that adjusted number as your band size. Then subtract that adjusted band from your overbust measurement and compare the result to a UK cup table.

British cup progressions also run longer than US ones — AA, A, B, C, D, DD, F, FF, G, GG, and beyond — so a UK 34F is not the same as a US 34F. Double-check which sizing standard a brand uses before ordering, and use that brand’s own size chart rather than relying on memory. Most retail sites include a conversion guide, and their charts trump any general rule.

FAQs

Should I measure with a bra on or off?

Either works. Good Housekeeping recommends a nonpadded, non-push-up bra or fitted T-shirt for the most reliable numbers, while other guides suggest measuring without a bra entirely. The key is that the tape rests against your natural shape — avoid padded bras that change your silhouette or push tissue upward, since either will skew your results by a size or more.

How often should I remeasure my bra size?

Remeasure at least every six months, or any time your weight changes by more than a few pounds, you start or stop hormonal birth control, or you become pregnant. Your size also changes with age and fitness level as muscle and fat distribution shift. If your current bras leave red marks, ride up in back, or let your breasts spill over the cups, remeasure instead of tolerating the discomfort.

Why does my cup size differ between brands?

There is no universal sizing standard. US, UK, and European brands each use different band-to-cup math, and even brands within one region cut their cups differently. A 34C at one store may fit like a 34D at another. Use each brand’s own size chart, and rely on the fit — the center gore should lie flat, wires should encircle your breast tissue, and the band should stay level.