How to find your bra size

First up, you have to know that it is just a starting point. Bras and breasts can vary a lot, so nothing beats trying it on. However, it is very good to have a starting point, as many of us have to order online, and it can also help to figure if a fitter is way off. So here goes:

Measure under your bust, all the way up in your breast crease, and make sure to keep it parallel all the way around. Measure this snugly.

Lean forward, so all your breast tissue is gathered. Measure across the fullest part of your bust in a straight line all the way around your body (so this is higher on your back than the under bust measurement)

Now you have 2 measurements. They need to be in inches. If you measured in centimeters, divide by 2.54.

 Subtract the two measurements as so:

Bust – under bust = cup size

If there is 1” difference = A, 2”=B, 3”=C, 4”=D etc.

For UK sizes, it looks as so:

It is just a starting point, so it is a bit of a moot point to discuss what to do with uneven numbers. Most who have a bit of extra padding on their ribs, and a heavy bust, will get better support by rounding down, and most skinny girls, with more exposed ribs, prefer to size up in the band. Generally, if you round down for the band (say, you measured 33, but went for a 32) you probably need to round up in the cup (so if you measured  37.5, you should go for a DD), or visa versa:

If you try a bra, and the band is too loose, you need to pick it’s sister size: that means going diagonal in the chart, as shown with colors.. If the loose-band bra was 34D, you need 32DD to get the same cup volume on a smaller band.

This is explained here and here, too.

^That means that it doesn't make much sense when people say things like "DD's are huge!" because a 28DD is the same size as a 36A for example.

Now, go out and get some bras in your estimated size, and follow these tips by Brastop!
Remember, you must always "scoop and swoop", or determining the fit is utterly meaningless. I cannot stress the importance of this enough.
Check here, here and here. 

“But wait! I am not in the UK!”

Convert cups:

Convert bands:

^You may notice that EU centimeter mesurements do not match up with UK inch-measurements (for example, 75cm is less than 30", yet a 75 band is 34", like the 34band) This is why it is so important to get a band that matches your measurement in inches, rather than cm!

I hope this can be of help :) If you have any questions or need advice, feel free to make a comment, and I'll answer you! :D