Use a soft measuring tape
Measure around the fullest part of the bust while wearing a non-padded bra or no bra
Keep the tape level across the back and parallel to the floor
Measure snugly without compressing the breasts
Record the bust measurement in inches or centimeters
Measure around the ribcage directly under the breasts
Keep the underbust tape level and snug
Record the underbust measurement
Subtract the underbust measurement from the bust measurement
Use the difference to estimate cup size
Use the underbust measurement to estimate band size
Check the measurements against a bra size chart
Recheck both measurements for accuracy