What cup size and underbust measurement are based on

Short answer: Underband is the circumference just under the bust, and cup size is the difference between the bust circumference and the underband. The letter is therefore not an absolute size, but a relation between two measurements.

Two measurements, not one

First the circumference just under the bust is measured, then the circumference over the fullest point. The difference in centimetres is turned into a letter on a fixed scale. The underband gives the number, the difference gives the letter. One measurement alone can never tell you what fits.

The underband decides the band

The underband decides how large the band should be, and the band carries much of the support in a bra. A band that is too loose rides up the back, and one that is too tight presses. Measure around the ribs with a soft tape while standing with your arms down.

The letter moves with the number

A C cup on a small underband holds less than a C cup on a large one. That is why the same letter can feel very different on two people. The letter only tells how big the difference is between the two measurements. See bamboo bra without underwire.

Why sister sizes exist

Go up one size in underband and down one in cup, and the volume is roughly the same. One example is 70 C and 75 B, which hold about the same in the cup. This helps when the band presses or sits loose and the cup otherwise fits.

The scale is fixed, but not identical

The scale is fixed, but brands cut the cup differently, and size labels vary between makers. The same size can therefore feel different at two brands. Use your measurement as a starting point and expect to try two sizes with a new brand. See bestsellers for women.

How to measure yourself

Use a soft tape and measure without heavy padding. Keep the tape level all the way round and pull it no tighter than you can breathe comfortably. Write the measurements down in centimetres and compare them with the scale of the brand you want to buy from. Measure twice to be sure.

How often to measure again

The measurement changes when weight changes, and over time the body can change for other reasons too. Measure again when the band no longer sits level or the cups begin to sit crooked. A size that fitted a year ago does not necessarily fit today.

When the band should sit right

The band should sit level and firm, but you should get two fingers under it. The cups should hold the whole bust without spilling or gaping. Straps are adjusted last. Bottoms use a different measure, since briefs are chosen by hip and waist, including period underwear.

When two sizes almost fit

Try both if you can. Choose the one where the band sits firm without cutting in and the cup holds the whole bust. A size that fits in only one place is rarely the right one. Consider the sister size too if you are unsure which way to go.