A well-fitted bra implies, first, a right size, second, a right form, and third, an ideal length of shoulder straps. 

A bra should be tight enough that it does not evenly move, but not so tight, that is pressing your breast tissue. 

Bra size consists of three major measurement: 1 – under bust circumference, 2 – cup size and 3 – length of shoulder straps. Length of the latter, generally, can be adjusted and fitted individually during wear. 

To calculate your bra size you should know two measurements:  the first one – bust circumference (when inhaling), the second one – under bust circumference (when exhaling). 

To calculate a bra cup size you need to know two measurements: bust and under bust measurements. 

Bust circumference is measured with the help of the tape measure running across the widest part of your bust strictly in a horizontal position. 

Under bust circumference is measured right away under the bust with the tape measure running strictly horizontally. 

One should bear in mind that while measuring, a person should stand up and the tape measure is to be parallel to the floor. If you stand up with your arms up, your rib cage size changes thus misleading you with a bra size result. That is why there should be a second person for measuring. 

To determine you cup size, refer to the given below chart using your bust and under bust measurements to find your perfect match.