Once you decide which pattern company you will purchase your pattern from, the next question is generally, "What size do I need to make?" Like RTW and patterns for sewing other garments, not everything is sized the same. Bra pattern sizes may be slightly different from company to company, so to start with you'll need your pattern instructions and some accurate measurements.
How do I measure?
All you need to measure is a reasonably well fitting bra (I realize some of you may not have a perfectly fitting bra, and that's why you're making one!) a flexible tape measure, a pen and something to write on. Measuring is best done with a helper - this doesn't have to be anyone experienced in sewing, their main purpose is to help you make sure that the tape measure is level and horizontal across your back when measuring, to help hold the tape measure and to write down numbers. Lacking a helper, you can use two mirrors to check the position of the tape measure in the back and a binder clip to hold that tape measure together in the front while you adjust the back.
What do I measure?
You'll only need two measurements to find your size. Each pattern company has its own set of directions for measuring and its own size charts, but they all follow the same general pattern. That pattern is to find the underbust measurement, which will help determine your band size and to find the overbust measurement and possibly the high bust measurement which will help determine cup size.
How do I measure the underbust?
While wearing your best fitting bra, loop the tape measure around yourself and measure around the body, underneath the bust making sure that the tape measure is level and horizontal across the back. Be sure that the tape measure is snug and not loose for this measurement. Do not exaggerate your posture or puff your chest out. The bottom band of the bra is a major player in the support system of the bra and you don't want a band that is too loose. If your pattern uses American sizes, jot this number down, and then follow the instructions in the pattern to find the band size. Usually you will add 4" or 5" to this measurement to come up with an even number. For example, if your under bust measurement is 33", you would add 5" to equal 38". This is the band size. By contrast, most European patterns do not do the the add 4" thing, your band size is the size you measure in cm.
How do I measure the overbust?
While wearing your best fitting bra, loop the tape measure around yourself and measure around the bust, across the fullest part of the bust, making sure the the tape measure is level and horizontal across the back. Do not pull the tape measure tight for this measurement. Measure comfortably, but not droopy loose, either. Write this number down. You will need this number to help determine cup size.
How do I determine cup size?
This measurement really is pattern specific and there are two or three general ways of doing this via American sized patterns - use the method in your pattern! Here are few ways to do this. One way is to subtract your band size from the overbust measurement and then determine the cup size from the difference; a difference of 1" equals an A, 2" equals a B, etc. For example, if your band measurement is 38" and your overbust measurement is 41" then you have a difference of 3" and your cup size is a C. Some patterns (like Kwik-Sew) will have you measure what is sometimes referred as the high bust measurement. This measurment is taken above the bustline, right at armpit level, with the measuring tape level and horizontal across the back, with a comfortable tension - not pulled tight. This high bust measurement is then subtracted from the overbust measurement and cup size is determined the same way; a difference of 1" is an A cup, etc.
Measuring is not hard, but it is essential to put you at the right starting point for fitting. If you can get a reasonably close fit, then it will take fewer adjustments to get you to a great fit. Like all sewn clothing, you have to have a starting point and fit from there. The measurements are only a tool to help you get the right fit.