h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Being large in that area, I sympathise. Two suggestions:

1. Measure yourself properly. This is done as follows: (Note: Do the measuring in inches, 'cos I've never bothered to work it out in cm!)
Measure just under your breasts. If the measurement is odd (e.g., 31 inches) add 5. If the measurement is even, add 4. This gives you your bra size. Call this measurement A.
Measure over your breasts at the fullest point. Call this measurement B.
Subtract A from B to find your cup size. If the measurements are the same, you are an A cup. 1 inch difference is a B cup, 2 inches is C, and so on.

2. Once you 'know' your bra size (or before, if you don't have a tape measure), go to a shop that does bra fitting. Marks and Spencer offer this service (if you're in the UK), and most good department stores will do it, too. They will measure you if you haven't measured yourself (and probably even if you have!).

Once you have found a manufacturer who makes a bra that fits you, stick with them. Although the size on the label may be the same, the fit won't be.

HI All;I'm new to this site, but of course drawn to it by the same problem everyone else is having -- getting a bra that fits. As someone who has sewn my own clothes all my life, I found a wonderful solution to the problem and became a professional custom bra-maker. Please visit my website at www.avaloncustombra.com -- I have been successful at fitting anyone I've worked with and am in the process of developing a system to take orders via the internet. There are some photos of bras I've created and lts of info there.

The best place to go to get fitted is Bravissimo. They make a point of throwing the tape measure away, so you end up with a bra that is actually your size, not "should be" your size.
See http://www.bigbrasizemystery.co.uk/index.php for more info.
The majority of people who wear the wrong size bra are wearing a band size which is too big. (eg, people wear a 36C instead of a 34D or even 32DD)
Taking your underbust measurement and adding 4 or 5 inches, or whatever people say, will usually give you a band size which is so big it will ride up your back and offer no support whatsoever!
Not only that, but anyone smaller than a D would be an A or less according to that method, since cup size is calculated after band size.

If you must use a tape measure, the most accurate way is to simply round up your underbust measurement to the nearest even number to get the band size, then the cup size is whatever fits! (usually 1 inch per cup size as mentioned before)

Actually, I re-measured a couple of months ago and found I was 'in-between' band sizes. I only had a metric tape measure so I had to convert to inches. Depending on the conversion factor (2.54 cm per inch or 2.5cm per inch) I got either 38.5 or 39.2. That's either a 38 bra or a 40. Factoring in the over-bust measurement gives either 38E or 40DD. I decided the smaller bra size was likely to cut in, so I went for a 40DD. That fits fine, although I have to fasten them bras on the middle set of eyes.

Using a tape measure may not work for everybody but it has worked fine for me. A lot of women can't be bothered with being fitted, so I reckon the calculation is a good enough method to get a reasonable fit.