The short answer
Measure your underbust (snug, directly beneath the bust) for your band size, then your bust (relaxed, at the fullest point) for your bust size. The difference between the two, in inches, gives your cup size.
Underbust
Wrap the tape snugly beneath the bust. Round to the nearest whole number — that's your band.
Bust
Wrap the tape loosely around the fullest point. Round to the nearest whole number.
Subtract
Bust minus band gives your cup difference — plug both numbers into the bra size calculator for the letter.
What to avoid
Measure over an unpadded bra, keep the tape level front and back, and don't pull it tight — snug is enough for the band, and the bust measurement should barely touch, not compress.
Questions
Quick FAQs
A basic soft fabric tape measure works fine — the kind used for sewing, not a rigid metal one.
Both measurements can be taken solo in front of a mirror, though a second pair of hands can help keep the tape level.
Double-check that the tape stayed level and wasn't pulled too tight — a half-inch of slack is a common cause of an inconsistent result.
Want more detail?