When you take the right measures of your bra you will get a fitted bra with more feeling of comfort, it will make your clothes fit on your body and will make your style looks better.
Here I'll tell you how you can figure out your own bra dimensions.
1 - Relax while you are stands straight. Be normal and avoid inhaling to expand your rib cage.
2 - Use a simple tape measure to measure all around your body by placing the tape measure just beneath your breasts. Let us say - for example the measure is: 29 1/2.