Most of you won’t be the size you think you are!!!
Now about these measurements, you need 3, and you must have someone else do it. The reason someone else needs to do it is when you bring your arms up to take the measurement it alters the measurement, but these need to be accurate for a good fit.
Also, use our handy MS/Excel spreadsheet: BraSizeFinder (right-click the link, and select “Save File As” to download it for your use). Enter your measurements into this spreadsheet, and your size will be calculated based on these measurements. If your computer will not process this spreadsheet correctly, the rest of this page tells you how to compute your size.
Put your best-fitting bra on and adjust so your boobs aren’t sitting down on your waist or under your chin!!!!
1. Keep tape parallel to floor around your back. Measure the under bust, snugly and no “bit extra” and do not pull it too tight .Write it down. Enter this number as “under” in the chart.
2. Full bust is measured same as above, around the fullest part of the bust. Enter this number as “around” in the chart.
3. Upper bust is where the breast starts to come out from the ribs in a gentle slope. Enter this number as “over” in the chart.
Got it all. This is how we work out our size. You may or may not be the bra size you are wearing. 80% of women wear the wrong size. Most of you’ll just prove this!!!!LOL
Band size – add 4″ to your underbust measurement to give what size band you’ll be wearing. e.g., if you measure 36″ add 4″= 40″. If you measure an odd number or a fraction go up to the nearest WHOLE, EVEN number. eg.: if you measure 33 1/2″, add 4″= 37 1/2″, then round UP to 38″. The 4″ is for ease, otherwise you’d think you were in a vise!!! I know most of you feel that way now.
Subtract your upper bust from the full bust to give cup size. Every 1″ of difference represents a cup size. If you are less than 1”, it becomes AA cup. eg FB 42”- UB 38”=4” difference. Makes it A,B,C,D- the D’s have it-Should be easy enough! Just keep going down the alphabet for cup size if you are more than 4″ difference.
1 inch difference A cup
2 inches difference B cup
3 inches difference C cup
4 inches difference D cup
5 inches difference E cup
Culled from: http://sewing.about.com