Wear a non-padded bra or no bra
Measure your underbust snugly around your ribcage, just under your breasts
Round the underbust measurement to the nearest whole number
Measure your bust around the fullest part of your breasts
Keep the tape level and not too tight
Subtract the underbust measurement from the bust measurement
Use the difference to find your cup size
1 inch = A cup
2 inches = B cup
3 inches = C cup
4 inches = D cup
5 inches = DD or E cup
6 inches = DDD or F cup
Combine the band size and cup size to get your bra size
Try on a few sizes close to your measurement
Check that the band sits level and feels snug
Check that the cups fully contain your breasts without gaps or spillage
Adjust straps so they stay in place without digging in