Bra Sizes Explained

Most of us met bra sizes as a teenager and found them baffling, a number stuck to a letter with no obvious logic. There is logic, though, and it is simple once someone spells it out. Every bra size is made of two parts. The band size is the number, and it comes from measuring your ribcage. The cup size is the letter, and it comes from the difference between your bust and that ribcage measurement. Get those two and you have your size. This guide walks through both, how to measure them at home, what sister sizes are, and why the fit in the mirror matters more than the tape in the end.

How bra sizes work: the band and the cup

A bra size has two parts, and they measure two different things. The band size is the number, for example 30, 34 or 38, and it refers to your ribcage. The cup size is the letter, for example B, DD or G, and it refers to how much bigger your bust is than your ribcage. Because the cup is a difference rather than a fixed volume, the same letter means different things on different bands, which is the part that trips people up and the part that sister sizes explain.

Band size

Your band size is the number in your bra size. It is the measurement of your ribcage, taken directly under your bust where the band of a bra naturally sits. Wrap a tape measure around your ribcage at that point, keeping it level all the way round and snug but not tight. The band does most of the work of holding you up, so this is the measurement that matters most.

Cup size

Your cup size is the letter in your bra size. It reflects the size of your bust relative to your ribcage, not an absolute volume. To find it, measure around the fullest part of your bust, level with your nipple, then take your band measurement away from it. Each inch of difference is one cup, so a one-inch difference is an A, two inches a B, and so on up the ladder.

How to measure your bra size at home

You need one thing, a soft tape measure, ideally over a non-padded bra or nothing at all. It takes two measurements.

• Band. Measure directly under your bust, around your ribcage, keeping the tape level and snug but not tight. Round to the nearest whole inch. That number is your band size.
• Bust. Measure around the fullest part of your bust, level with your nipple, keeping the tape level and not pulling it tight. Round to the nearest whole inch.
• Cup. Take the band measurement away from the bust measurement. The difference in inches gives your cup letter, using the chart below.

Every UK cup size, in order

UK cup sizes do not run straight through the alphabet. After D comes DD, then E, and doubled letters appear again higher up, so the full ladder from smallest to largest is:

Band sizes run in even numbers, most commonly from 28 up to 48. Put the two together and you have the full range of standard UK sizes. The grid below shows how band and cup combine, so you can see, for instance, that a 34C and a 30F are different sizes entirely even though they might feel similar in the hand.

What is a sister size?

A sister size is another bra size that holds the same cup volume as yours, on a different band. It works because the cup letter is tied to the band, not fixed on its own. If you go down one band size, you go up one cup size to keep the same volume, and if you go up one band, you go down one cup. So the sisters of a 34C are a 32D and a 36B, all three holding the same amount in the cup on a tighter or looser band.

Sister sizes are useful when the cup feels right but the band does not, or when your usual size is out of stock. Rather than guessing, you step one band up or down and move the cup the opposite way.

Fit matters more than the tape

Whatever the measurements say, the fit is what counts, so treat your calculated size as a starting point and try sizes around it. When you put a bra on, lean forward and do a swoop and scoop, settling all of the breast tissue into the cup, then fasten it and check the band. The band should sit level around your body, firm enough that it does the supporting, not riding up your back. The cups should contain everything with no spilling or gaping, and the centre should sit flat against your breastbone.

Your size will also shift a little between brands and styles, which is normal, so the label is a guide rather than a rule. If the band digs or rides up, or the cup cuts in or gapes, adjust the band or cup size and try again. Our fitting guide walks through the full fit check step by step.

Common questions