The diagram below shows the key points to measure when calculating size, then simply select the correct size from the chart above. It's best to actually take body measurements rather than measuring over clothes. You shouldn't need to 'size up' or 'down' unless you have a particular fit preference of course. Keep your bra on and measure in inches around your ribcage directly under your bust. This gives you the band or back size. When measuring, you should always round down!