Wear a non-padded bra or no bra
Stand straight with your arms relaxed
Wrap a soft measuring tape around the fullest part of your bust
Keep the tape level across your back and front
Make sure the tape is snug but not tight
Read the measurement in inches or centimeters
Measure around your ribcage directly under your bust
Use the underbust measurement and bust measurement to determine bra size
Recheck both measurements for accuracy