Step 1, Measure for Band Size:
A bra size is made up of two measurement components: band size (i.e. 34, 36, 38) and cup size (i.e. A, B, C).
To find your correct bra size, begin by wearing your best fitting bra - unpadded, and not a minimizer or sports bra (underwire doesn't matter). If you are sagging in this bra, tighten your straps until the crest of your breasts is approximately half way between your shoulder and your elbow - this is the proper alignment for most breasts. If possible, have someone other than yourself take your measurements. Not only is it faster, it also tends to be more accurate.
Measuring Your Band Size
1. Holding the measuring tape gently but firmly, measure around your bra directly

2. When you take these measurements, round to the closest whole inch, either up or down. For example, if your measurement is 29 3/8", round to 29". If your measurement is 29 1/2" or more, round to 30".
3. Bras come in even band sizes. If the rounded measurement you come up with is an even number, add 4 inches. If the rounded measurement you come up with is an odd number, add 5 inches.
4. The result is your band size. Example: 33" + 5" = 38" band size
Option 2:
Another method for finding your band size is to measure around your body, straight across your back and chest directly above the bust at underarm level. Again, expel all your air, stand up straight, and keep your arms at your side. High-busted women and women with implants will have trouble keeping the tape measure parallel to the floor; do your best. If this measurement is an odd number, add one inch and the result will be your band size.
Measuring Your Cup Size
1. Standing straight, with your arms at your side, measure around your body at the

2. Wrap the tape measure around you so that it is just touching, but not binding. If one of your breasts is significantly larger, put some type of padding in the cup of the smaller breast to make it even with the larger breast before taking your measurement.
3. Round up or down to the nearest inch as you did to find your band size.
4. Now, subtract your band size measurement from this second measurement. The difference is your cup size.? Example: 38" - 34" (band size) = 4" = D cup size. The chart below shows you how to determine cup size.
Difference Standard Cup Size
0"-1/2" (1.3 cm) AA
1/2"- 1" (2.6 cm) A
2" (5.1 cm) B
3" (7.6 cm) C
4" (10.2 cm) D
5" (12.7 cm) DD or E
6" (15.2 cm) DDD or F
7" (17.8 cm) G
8" (20.3 cm) H
9" (22.9 cm) I
10" (25.4 cm) J
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.
If the measurement is even, add 4"
if the measurement is odd, add 5"
Step 2, Measure for Cup Size:

Standing straight, with your arms at your side, measure at the fullest part of your bust (while wearing a non-padded bra) making sure the measuring tape is parallel with the ground and not binding.
Round all fractional measurements to the nearest whole number.
Step 3, Calculate Your Bra Size:
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 conversion chart here.
Example:
Step 1: 34" under measurement +4" = 38" band
Step 2: 40" over measurement
Step 3: 40" - 38" = 2" or Cup "B"
Your size would be 38B
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. See "The Proper Measuring Techniques for Plus-Size Women". - COMING UP