STEP 1: Measure for band
size:
Keeping the measuring tape parallel with the ground,
measure around your bra directly under your bust
after expelling all air from your lungs - you want
this measurement to be as small as possible. Round
all fractional measurements to the nearest whole
number.
Note: After a 38 band
size, the band size should be the closest higher even number to
your actual measurement. So 41.5" is a 42 band. A 41"
measurement is a 42 band.
Subtract your band measurement (step 1) from your cup
measurement (step 2). Generally, for each inch in
difference, the cup goes up by one size. See the cup
size conversion chart below.
Example:
Step 1: 34" under measurement = 36" band size
Step 2: 39" over measurement
Step 3: 39" - 36" = 3" or C Cup Size
Your Size is probably a 36CThe cup size calculation starts falling apart at 4
inches larger than your band size, and if you have
droopy or pendulous breasts, your measurement will not
be accurate.

Important Note: this measuring
system tends to become less accurate as the cup sizes go
above a D. Additionally, some manufacturers name larger
cup sizes differently.