Bra Fitting Encyclopedia

How Bra Sizing Works

A bra size is a band number and a cup letter. The band is your ribcage measurement rounded to the nearest even number; the cup is the difference between your bust and your band. The part most people miss: cup size is a ratio proportional to band size, so a 32D and a 36D are not the same cup. There is no fixed D cup.

The band: the number

The band size is the number in your size (30, 34, 38). Measure snugly around your ribcage just under your breasts and round to the nearest even number: a 29-inch ribcage becomes a 30 band, a 31.25 becomes a 32. The band provides most of the bra's support, so it should feel snug and secure, never painful. Sources commonly cite 80 to 90 percent of support coming from the band; the exact figure varies by style and body, but the principle holds: the band does the heavy lifting, not the straps.

The cup: the letter

The cup size is the letter (A, D, G). Measure the fullest part of your bust without pressing down, subtract your band size, and the difference maps to a letter. Bust 36, band 32: a 4-inch difference is a D, so 32D. See the full cup chart for every letter.

Sister sizes: same volume, different band

If your bra is close but not right, try a sister size. To keep the same cup volume, go up a band and down a cup letter (34C becomes 36B), or down a band and up one (34C becomes 32D). Reach for one when the band feels too tight but the cups fit, or when the cups fit but the band rides up. The sister-size table lists the full ladder for every band.

Re-measuring

Re-measure every 6 to 12 months. Your size can change with weight fluctuation, hormonal changes, aging, pregnancy, nursing, or surgery. A bra that fit last year may not fit this year. If you would rather never think about it again, the Radical scan measures you from two photos each time you order.

The +4 vs +0 method split

Older bra-fitting guides added 4 or 5 inches to the underbust measurement to get the band size: a 29-inch underbust became a 34 band. That is the +4 method. Modern methods, including the r/ABraThatFits community calculator, use +0: the band stays close to the raw underbust, rounded to the nearest even number, so a 29 becomes a 30.

This is why modern methods produce smaller bands and larger cup letters for the same body. If you were fitted as a 34B under the +4 method, the +0 method might suggest a 30D or 30DD. The cup volume is similar; the band is snugger and the letter is higher. Neither size is wrong; the methods differ. This encyclopedia uses the +0 approach, so expect your measurements to point at a different size than an old department-store fitting did.