1 Measure your band size. This is the easiest part of the process — your band size should be fairly stable and straightforward.
Run a tape measure all the way around your body just underneath your breasts and take a measurement in inches. Make sure the tape measure is horizontal and fairly snug. Your arms should be down. Write down this number.
If this measurement is an odd number, then you should try out bras in both the size below your measurement and the size above. For example, if you measured 31 inches (78.7cm), your band size could be 30 or 32.
If your measurement is already an even number, this is almost always your band size, but you may need a smaller or larger size depending on your body type. For example, if your band size is smaller than 28, you may want to go up a band size since these sizes run quite tight. If you do this, you should go down a cup size so your cups will have the same volume (known as a ‘sister size’). So if you are a 26C, you may find a 28B is more comfortable.
2 Determine your cup size. Remember, your cup size isn't an absolute measure — it's in proportion to your band size.
Bend over so that your chest is parallel to the ground. This is so that you'll be measuring all of your breast tissue — not just what protrudes outward when you're standing up.
Measure around your torso, so that the tape is over the fullest part of your breasts. Don't pull the tape too tightly — it should be tight enough that it doesn't evenly move, but not so tight that's pressing into your breast tissue. Write down the number.
Make sure your tape measure is level to the ground. It shouldn't be a few inches down your back, or you'll end up with an uneven measurement. To combat this, try to measure yourself in front of a mirror, or ask your partner or close friend to help you.
Calculate your cup size. To do this, you'll subtract your band measurement from the cup measurement you just took. The difference between the two numbers determines your cup size:
Less than 1 inch (2.5 cm) = AA
1 inch (2.5 cm) = A
2 inches (5 cm) = B
3 inches (7.5 cm) = C
4 inches (10 cm) = D
5 inches (12.5 cm) = DD
6 inches (15 cm) = DDD (E in UK sizing)
7 inches (18 cm) = DDDD/F (F in UK sizing)
8 inches (20.5 cm) = G/H (FF in UK sizing)
9 inches (23 cm) = I/J (G in UK sizing)
10 inches (25.5 cm) = J (GG in UK sizing)
The majority of leading brands use UK cup sizing: AA, A, B, C, D, DD, E, F, FF, G, GG, H, HH, J, JJ, K, KK, L, LL. If you're shopping in the US you might see cup sizes such as DDD or DDDD. These are equivalent to E and F. If you're in any doubt, particularly with larger cup sizes, you can refer to an international bra sizing chart.