This refers to UK bra
sizes, other countries' sizes may be different.
The sizing system for bras consists of a number, which is the band size (underbust) in inches, and a letter, which is the cup size (based on bust measurement.) Band sizes go up
in incraments of 2 inches, cup sizes go up in incraments of 1 inch.
To calculate bra size:
1 - measure underbust in inches
2 - round this measurement up to the nearest even number. This is your band size
3 - measure bustline in inches (wearing a well-supporting bra)
4 - round this measurement up to the nearest whole number
5 - work out the difference between the 2 numbers
6 - for every inch difference, go up a cup size, as follows:
less than 1 inch - AA
1 inch - A
2 inches - B
3 inches - C
4 inches - D
5 inches - DD
6 inches - E
7 inches - F
8 inches - FF
9 inches - G
If the size you get from this method is too tight, go up a band size and down a cup size.
eg underbust measures 31 inches, bust measures 37 inches.
Bra size should be 32DD, but if this is too tight, a 34D may fit better.
Cup size is relative - a 32DD is much smaller than, say, a 36DD. 36DD is equivalent to 34E, which is the same as 32F.
A new bra should fit on the BIGGEST adjustment to allow for stretch over time. If it already fits comfortably on the smallest adjustment, go down a band size (and up a cup size to compensate)
If you have uneven cup sizes, go with the bigger side.
It's better to buy a new bra when on your period, you're often bigger then.
The band of the bra should be horizontal and not ride up at the back: If you pull the shoulder straps upwards, the band should gently grip your back. If it slides up easily, the band size is too big.
If the top edge of the cups make a double bulge, or you feel squashed inside them, the cup size is too small.
If the shoulder straps dig in and leave red marks, they are either adjusted too short or they're taking too much weight. The weight should be taken by the band, so you may be wearing a band size which is too big. Similarly, if you find you keep having to shorten the shoulder straps, try going down a band size instead.
You should be able to run a couple of fingers around inside the band. If you can stretch the bra out from your chest or fit your whole hand under the band, it is too loose.