Most patterns are now printed with several sizes on one sheet. Pick the size range that the majority of your measurements fit into. You will be able to blend the sizes together to fit your body. If you have to choose between a pattern that matches your hip or waist measurement, choose the one closest to your hip measurement. It is easier to alter the waist line.
There are only a few basic measurements that are needed to choose the correct pattern size. You will need additional measurements to make alterations, but the only measurements needed to select your pattern size are: bust or high bust if you wear a C-cup or larger bra, waist, and hips.
The sizing of most commercial patterns is based on a B-cup bra. If you are larger busted, using your measurements may give you a pattern that is too big in the shoulders and back. In this case, it is better to choose a pattern based on your high bust measurement and alter the pattern to accommodate a larger bust. The high bust measurement is taken under the arms, but just above the bust.
You must have accurate measurements to select the correct size pattern. It is almost impossible to take precise measurements for yourself. Measurements should be taken in a good fitting bra and underwear. Use a good tape measure that has been checked for accuracy. The tape measure should be snug, but not tight. Do not add inches for looser fit, this is added in the design ease of the pattern.
Patterns do not follow the same sizing standards as ready-to-wear clothing. The major U.S. commercial pattern companies have standardized sizing. European pattern companies and independent pattern companies all have their own sizing standards. It is important to read the measurement chart for each pattern you are purchasing and select the appropriate size based on their measurement chart.