Use a soft measuring tape
Wear an unpadded bra or no bra
Measure your band size around your ribcage, just under your bust
Keep the tape snug and level
Round to the nearest whole number
Measure your bust size around the fullest part of your breasts
Keep the tape level and not too tight
Subtract the band 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
Try on bras in your calculated size
Adjust up or down if the fit feels too tight or loose
Check that the band sits flat and the cups fully contain your breasts