Flange sizing:

Flanges and flange sizing can be really confusing, especially if you’re new to pumping. Even if you’re not new to pumping, you may not really know how important a good-fitting flange is.

What is a flange?

It is the pump part with the funnel/shield that goes onto the breast and around the nipple. The number/size of the flange refers to the diameter of the tunnel. Most kits come with a 24mm flange, but you can buy on amazon from 15-36mm, depending on your size.

Why is flange fit important?

The funnel of the flange creates a vacuum and seal around the areola where the nipple should move freely within the funnel and extract milk.

If a flange doesn’t fit properly, you may not pump as much milk as you could and if this is happening over an extended period of time, this could lead to decreased milk supply because your breasts are not being completely emptied and therefore not signaling your brain to make more milk. (That is the basis of milk production: demand and supply).

Also it can lead to pain and nipple damage.

Signs of a poor-fitting flange:

Flange is too big:

  • Your areola may pull into the tunnel.
  • Painful nipple pulling, pinching, squeezing.
  • Decreased milk production.
  • Nipple/areola may become discolored/white.


  • Nipple is rubbing on sides of tunnel
  • Nipple pinching/squeezing
  • Nipple damage/cracks, bleeding
  • Painful when pumping
  • Decreased milk production


  • Nipple moves freely in the center of the tunnel
  • Areola is not being pulled inside the tunnel
  • No nipple pain
  • No discoloration of nipple/areola after pumping
  • Breasts are empty after pumping

How to measure for flange size:

  • You can use a nipple ruler like this one from Legendairy milk (it sometimes comes free with a purchase).
  • You can also print a ruler like the one above for free. (I prefer this way).
  • There are a few ways to measure:
    • BEFORE PUMPING: hand stimulate your nipple at the base (where the nipple and areola meet).
      • Measure the base of the nipple
      • Add 3-4 mm to your measurement to get the proper sized
      • Example: you measure 17 mm before pumping, you’d likely fit into a 21mm flange
      • Add 1-2 mm to your measurement to get the proper sized flange
      • Example: you measure 22 mm after pumping, you’d likely fit into the 24mm

Choices of flange sizes:

Flange sizes range from 15-36 mm, although the standard pump kits only bring 24mm usually. You can purchase many of these sizes on Amazon. If you are between sizes, you can use inserts that fit into the flanges and decrease the diameter by 1-2 mm.

Mostly all of the pump brands sell pump parts on their websites but they may not sell all of the sizes of flanges. There are many choices on Amazon that are compatible with wide neck and narrow neck bottles. I will link some of the companies here that sell those parts.

  • Nenesupply sells 17mm-28mm flanges
  • Maymom sells 15mm-36mm flanges
  • If you search on Amazon you can find many different sized flanges and likely the size you need.
  • Inserts fit into the flanges. These are great if you’re in between sizes and the flange sizes that are available aren’t working for you.
    • For example, if you are a size 19mm flange and the 17mm flange feels too small and is pinching/painful and the 21mm flange is pulling too much of your areola in, an insert could work really well. Check out these inserts here. There are many different sizes that are compatible with different flanges sizes and pump brands.

My next flange/pumping post is going to go over elastic tissue/nipples (my favorite topic!) and we will go over those options, too!


  • Nipple size can change throughout a pumping journey, so you may need to remeasure and resize your flanges
  • Nipple sizes between your 2 breasts can be different sizes, so measure them both.
  • You may have to try out a few different sizes and/or inserts to see what works the best for you because it’s not an exact science due to tissue elasticity.
  • This can make a HUGE difference in your pumping experience/journey so please get a good fit! 

You can also book a flange fitting consult with me:

If you are still totally confused and overwhelmed, don’t know where to start, or can’t get a good fit, I am happy to help and we can do a flange fitting consultation. Just go here to book a consult.