Flange size - and your fit - is determined by the width of the flange opening. You can measure your nipple to estimate which size flange is best, then check your flange fit by watching your nipple during pumping. A good fit can help make breast pumping comfortable and help you pump more milk. These flanges and inserts replace the standard size flange that comes with all Ameda HygieniKit Milk Collection System to ensure you find your best fit. This item includes two (2) 30.5mm breast flanges and two (2) 28.5mm reducing inserts that fit inside the 30.5mm flanges. Color: Clear.