Wear a non-padded, well-fitting bra or no bra
Measure snugly around the ribcage directly under the bust
Round the underbust measurement to the nearest whole number
Measure around the fullest part of the bust
Keep the tape level and parallel to the floor
Round the bust measurement to the nearest whole number
Subtract the underbust measurement from the bust measurement
Use the difference to determine cup size
Match the underbust measurement to the band size
Try sister sizes if the band or cup feels off
Check that the band sits level and firm
Check that the cups fully contain the breasts without spillage
Check that the center gore lies flat against the chest
Check that straps stay in place without digging in
Re-measure after weight changes, pregnancy, or hormonal changes