Just when you start to understand sizes in MM, Pumpin’ Pal offers XS, S, M, L, and XL and confuses me all over again:

Our flange design is completely revolutionary.  Nothing else on the market takes into account tissue elasticity which is such a big topic these days.

The millimeter (mm) sizing that corresponds with the standard generic flange is simply the width across the ring at the bottom of the funnel (where the nipple goes in) and takes into account only the size of the nipple.  Pumpin’ Pals are tapered and oval at the area where the nipple goes in and so there is no place on our flange to take an actual measurement. Eliminating the ring to increase comfort and flow meant eliminating a place to take the measurement.

Also, because the flange tunnel is oval at the top and has an angled tunnel it allows the user to rotate the flange to whatever direction needed to accommodate the (rarely ever perfectly round) nipple and the placement of the bottle so the parent can sit with as good as posture as can be while still having the milk go into the bottle.  Rotating a standard flange yields no change to how it fits on the breast.