You've seen pump companies and other lactation helpers recommend to measure your nipples and add
2-5 mm to get your flange size. We disagree.
Pumping (and nursing) should feel like nothing or a gentle tug: no pain, no pinching, and no damage. Getting the perfect flange fit for each side of the body is the key. Left side is often different than the right side (they are sisters, not twins!).
There is actually no evidence for the flange sizes that come as standard with pumps (!) and now there IS clinical evidence that this sizing method works better--because it helps you determine which flanges to try and then YOU decide what feels best, gets the most milk out and in the least amount of time. Try it today or recommend it to someone you care about.