Wear an unpadded bra or no bra
Use a soft measuring tape
Measure snugly around the ribcage directly under the bust
Round the underbust measurement to the nearest whole number
If the number is even, add 4 inches
If the number is odd, add 5 inches
This gives the band size
Measure around the fullest part of the bust
Keep the tape level and not too tight
Round the bust measurement to the nearest whole number
Subtract the band size from the bust measurement
Use the difference to find the 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
Try on bras in the calculated size
Adjust for fit, comfort, and brand differences