Breast Pump Flanges

Carolyn Bakker

Updated September 14, 2026 16:25

Article reviewed and updated 9/10/26

My pump came with a few flanges. Which size should I use?

A breast pump flange is the funnel part of a pump that is placed against the breast and connected to the bottle or other collection device.

Using the correct flange size is very important. A flange that is too big or too small will not be as effective at removing milk. A proper fit ensures the maximum amount of milk can be expressed and will not cause pain. Choosing a breast pump can be overwhelming, but not settling for just any flange is as important when it comes to pumping success.

Choosing the correct flange size:

1. Measure your nipple.

Use a millimeter (mm) ruler or measuring tape to measure the diameter (length across) of your nipple at its base.

2. Use a sizing guide (like this one) to determine the correct flange fit.

3. Try it out with pump suction

Pumps often come with a couple different sizes, such as 24 and 27 mm. If neither works, inserts or other sizes can be ordered online and are fairly affordable. Your local WIC office should have additional sizes, such as 21, 30 and 36 mm. Ask if you need a smaller or bigger size.
Studies show that many women are using a flange that is too large, which can affect milk supply. The standard 24 mm flange that comes with most pumps is too large for many women.
Center the nipple into the breast flange and hold it against your breast.
Using a strong, but comfortable suction setting, observe the nipple and areola when pumping.

Does it fit?

When the flange fits correctly, the nipple stays centered in the flange and moves freely without excessive rubbing on the sides of the “tunnel” of the flange.

Too Small:

The nipple rubs along the sides of the “tunnel”
The nipple looks misshapen, white, red, or irritated during or after pump use
Pumping is painful

Too Big:

The nipple and too much of the areola are pulled into the “tunnel”
The areola looks red or irritated during or after pump use
Pumping is painful

Flange Facts:

It’s common for moms to need a different size flange for each breast.
The size a mom needs may drastically change during her breastfeeding/pumping journey.
If you start to notice pain, signs the flange is too small or big, or you are no longer able to express as much milk as you once did, it’s time to try a new flange size.

Help is available! Call your local WIC office and schedule an appointment to be measured for a flange. Our breastfeeding specialist can help over Zoom or you can get assistance from a lactation consultant in your area.