[edit] Steps

1. Wear your best supporting bra. The bra doesn't have to fit perfectly (or else you could just check the size of that one and be done with it, right?). The objective is to get your breasts to the position that the ideal bra would support, with the weight suspended comfortably. This step is optional, however, so if you can't find a bra with good support, continue with the measurements.
2. 

   
   Measure your bandsize. Run a tape measure all the way around your body just underneath your breasts and take a measurement in inches. Make sure the tape measure is horizontal and fairly snug. Your arms should be down. If the number is even, add 4 and if it is odd, add 5. This will give you your band size. For instance, if you measured 28 inches, your band size will be 32.
3. 

   
   Measure your bra size. Measure loosely around the fullest part of your bust in inches (not too tightly or the measurement will be inaccurate). If you don't have a bra, or if the one you have is very poorly fitting, bend over at 90 degrees and measure around your chest, over your nipples. Round this measurement up to the nearest whole number.
4. Calculate your cup size. Subtract the band size measurement from the bra size measurement and determine your cup size as follows:
   
   
   • less than 1 inch = AA cup
   • 1 inch = A cup
   • 2 inches = B cup
   • 3 inches = C
   • 4 inches = D
   • 5 inches = DD or E (see Tips)
   • 6 inches = DDD or EE or F
   • 7 inches = DDDD or EEE or FF or G
5. Try on a bra with the band size and cup size you've arrived at in these steps. Because there can be wide variations in how bras are manufactured as well as differences between different styles of bras, this size should be starting point.
6. Check the band size. The correct band size is the smallest you can comfortably wear. It needs to be tight enough that the bra is still fairly supportive without weighing down on the straps.
   
   
   • You should be able to run your fingers around the inside of the band, but not much more.
   • It should fit on the biggest adjustment, but will probably be too tight if you try to fasten it on the smallest size. Bras are designed to fit like this to leave room for the bra to stretch after you've worn it a few times.
   • If the band is painfully tight, even on the biggest adjustment, then go up a band size (and down a cup size to compensate). For example, if a 32D is too tight, try a 34C.
   • If the band is loose enough for you to be able to comfortably fasten it on the tightest adjustment, try a smaller band (e.g. 32D -> 30DD).
7. Check the cup size. The correct cup size is the biggest you can completely fill out. You should fill out the cups, but not bulge out anywhere.
   
   
   • Check both the top of the bra and the sides for bulging.
   • Make sure the underwire encloses your whole breast and is flush against your rib cage.
   • If the cup size is too big, go down a size.
   • If it is too small, or even if it seems to fit, try on a bigger cup size as well to double check.

[edit] Tips

• This measurement applies to the US and UK, but it can be translated into bra sizes for other countries using an international bra size calculator.
• Cup sizes above D tend to vary much less reliably between manufacturers. Some brands go to the next letter of the alphabet for every added inch, while other brands may simply add on the same letter for every inch.
• Don't pay too much attention to the tightness of the straps. A well-fitted bra should provide support from the band, not the straps. You should be able to take off the straps and still feel supported by the bra.
• Always try on a bra before you buy, and keep an open mind about your size.
• It is possible to find out your bra size without a tape measure if you already wear a bra which seems to fit well in the cup, but is roomy in the band size. You can go down a band size and up a cup size until you get to a size you can only just get on - this is the right size. (e.g. 36C -> 34D -> 32DD)
• If you have uneven cup sizes, go with the bigger side.

[edit] Warnings

• As mentioned above, this is only to give you an approximate idea of what size to try on first. Women come in all shapes and sizes, and bra size can be affected by back proportions, breast shape, pectoral muscle mass, and so on. All of these factors can interact to affect your bra size in ways that are impossible to predict.
• Ignore anyone who claims to be able to tell you your size with certainty from your measurements - especially if they tell you to add several inches to your underbust measurement.
• If you have previously been told your size, but now find that a different size fits better, just wear the size that fits! The fit is more important than the number on the tape measure.
• Don't be tempted to buy the wrong size because it's cheaper.