We've outlined the first 2 steps to determining your flange size below, but, of course the ultimate size-decision is up to you!
STEP 1: To determine your flange size, most experts recommend that you measure the diameter of your nipple (not including your areola).
Make sure to perform this measurement on both sides in case you need two different flanges.
STEP 2: Select a flange that is 3-4mm larger than your nipple diameter. For example, if your nipple measures at 21mm, you would want to try the 24mm flange.
Note that your nipple swells while pumping, and since the rate of swelling varies between women, it’s important to take this into account when considering your size.
So, why does this even matter?
If your flange is too small your nipple may rub on the sides of the nipple tunnel. If your flange is too large, however, your areola may be pulled into the tunnel along with your nipple. Additionally, if your flange is the wrong size your breast may not completely empty (you'll still feel "full" after pumping).
How do you know if you have the right size?
When your flange size is correct your nipple moves freely in and out of the tunnel. It should mimic baby more like a gentle suck/tug and should not be painful!
Finally, note that over the course of breastfeeding, your flange size may change. So, measure and then measure again!