Here's the correct way to measure:
Step 1: Grab yourself a nice soft tape measure with inches on (preferably not a rusty steel rule from the garage)
Step 2. Underbust Measurement - Run a tape measure horizontally around your body, just underneath your breasts, and take a measurement in inches. Make sure the tape measure is snug and relax your arms as far as possible. If the measurement is an odd number, try the two sizes around it. For example, if you measured 31", start trying on bras in a 30" and 32" size. If it was an even number, use that to start and remember that every bra is different (NOTE: Some bra fitting guides and calculators will tell you to add four or five inches to your underbust measurement, but this is outdated advice. The "add four" method was popularised by Warners in the 1930s when bra design was in its infancy - it simply doesn't work well with modern bras.)
Step 3: Since everyone's breasts are different in terms of shape (even between your own breasts), the most accurate way to determine your cup size is by using your current bra size as a starting point. If you don't have an existing bra, take a look at the sized cups for your band and give the one that you feel will be the best a try. The cups are sized relative to the band, so if you were to try a smaller band size but keep the same cup size, the cups would be too small - if you go down a band size you need to go up a cup size. If you decide to go up a band size, then to maintain the cup size you will need to go down a cup size.
Step 4: Put the bra on around your body first doing it up on the LOOSEST SETTING. The elastic in the bra will loosen with age, so you'll want to be able to tighten it through it's life and maximise its lifespan. Wiggle your breasts into position, yes wiggle, it works! Tighten the shoulder straps so that they do not dig in and hurt, or slip off the shoulder easily. The band around the body should be taking the main weight of the breasts.
Does the bra feel tight around the body? If so, good! It may feel too tight, but this is because we are used to wearing bras that are too big for us. It should feel tight, but not dig in. You should be able to get 2 fingers side by side between the bra and your skin, and it should have a little bit of give. Check that the weight of the breasts is well supported so the shoulder straps are not digging in at the top. It should of course feel comfortable.