Sizing is based on your rib cage circumference just under your bust.  If you don’t have access to a tailors tape measure, you can use your bra size (the number portion). When making a custom garment, there is no such thing as “too much information”, so the more measurements you can give me, the better and more accurate the fit will be. You can enter any additional information into the “Additional Notes” section at the bottom of each product page.