Current bra size measurements should be used.

Band Measurement - Measure around your ribs, directly under your bust, where the bra band sits. Keep the tape level and snug as you wrap it around your back. However don't wrap it too tight as you want a comfortably fitting bra.
Note your measurement in inches and round up to the nearest whole number. If the number you get is odd, round up to the nearest even number - you have your bra band measurement

Bust Measurement - Measure over the fullest part of your bust. Don’t pull the tape too tight, it can be fairly loose. Take the measurement in inches and round it to the nearest whole number.

Now it’s time to work out your cup size. Doing this requires a bit of maths and the conversion table below. You will want to subtract your band inches measurement from the inches of your bust to get the total difference in inches. You’ll use this to work out your cup size.

There is also an online bra size calculator to help with this.