How to Know Your Correct Flange Size (Step-by-Step Guide) What is a flange size? The flange is the funnel part of your breast pump. The size refers to the diameter of the tunnel opening (in mm) where your nipple goes in. Correct flange size helps: Better milk output Less pain and nipple damage More comfortable pumping sessions STEP 1: Measure your nipple (BEFORE pumping or latching) This is the most important step. Measure only the nipple, not the areola (brown part). Best ti