Here's a step by step guide for finding out your correct* size. Make sure you take your measurements while standing up straight, relaxed and wearing a bra, but no extra layers. Keep the tape measure firm against your skin, not too tight and not loose. Exhale before measuring! For the cup measurement, you can use a slightly looser measuring technique. If you are in between measurements, it's best to round up to the next size. All measurements are in cm.
It's important not to use the "size" in inches (in the USA/UK row) as the equivalent of the band measurement for inches. They don't match. Use the dark grey row - and if you measure in inches, either check the other side of the tape measure for a cm measurement, or you convert your inches measurement to cm online.
Finding your correct size is a step in the right direction, but it's only the beginning. Part 2 is going to be about troubleshooting, and finding the right style of bra to suit your specific needs. I'd love it if you could leave any comments with your specific issues, so that I can make sure I cover them in the next post.
*This method works well for regular sizing. Ladies with a more generous cup size than a 'D' might need to take some additional advice from this chart which lists major manufacturers, their sizes and cup size equivalents to measurements. After a D cup, different countries and brands use different cup descriptions, in Australia we use D, DD, E etc, but the USA and Europe are different. I've listed the Australian cup sizes so they may not align with the sizing in your country for the larger cup sizes.