Wear an unpadded bra or no bra
Measure snugly around your ribcage directly under your bust
Round that number to the nearest whole inch or centimeter
Measure around the fullest part of your bust
Keep the tape level and not too tight
Subtract the underbust measurement from the bust measurement
Use the difference to find your cup size
Use the underbust measurement to find your band size
Check a bra size chart for your region
Try on the calculated size and adjust for fit
If the band rides up, go down a band size
If the cups gape or spill, adjust the cup size up or down
Sister sizes can help if the band or cup feels off
Recheck measurements regularly because body size can change