Sizing chart | Insert & Flange


We know it can be overwhelming and difficult trying to figure out what pump it best for you, let alone what size breast shield or inserts you need!

We have created a visual representation and short guide to help you figure out your size. 

By using the correct size breast shield / insert you can avoid numerous complications such as pain, abrasions, and discomfort while you're pumping breast milk.

In the following guide, there is information on the nipple size measurement technique, breast pump flange size differences, and the uses of a pump flange and insert.


2 Mins : Know The Right Size For Your Breast Shield

What is the difference between a flange and insert?

A breast shield also known as 'Flange' is the cone shaped funnel that fits directly over your nipple, areola and breast. It forms a seal between your breast and the silicone that then creates a vacuum to extract milk from the breast, into your pump.


Breast shield / Flange

Breast Shield / Flange - onatural store



An insert has the same use by forming a seal between your breast and the silicone. Inserts are generally used when the required flange size isn't available. Our inserts fit all of our wearable breast pumps; meaning more of you Mamma's can use them.



While the average flange is between 24 and 27 millimeters (0.95 - 1.06 in) wide, nipple size can vary significantly. The correct flange size prevents nipple damage and allows the pump to extract the milk efficiently


How to measure your nipple size?

As mentioned before, a correct breast pump flange fit allows easy milk pumping and prevents damage to the nipple. To measure your nipple, take a ruler or a measuring tape and check your nipple diameter (without the areola).


Select your flange size based on your measurement. Common flange sizes relative to nipple measurements are listed below. If your exact size isn't available, look for the next closest size.

For example:

  • If you measure below 14mm, it's recommended using size 17mm.
  • If you measure up to a 15mm, use size 19mm.
  • If you measure up to 17mm, you’ll fit in a 21mm.
  • If you measure up to 20mm, use size 24mm.
  • If you measure up to 23mm, select size 27mm 

Note: All of our breast pumps contain 1 or more standard breast shield sizes. Check product information.

For an accurate measurement, we always recommend speaking to your lactation specialist or health visitor.


Keep in mind, breasts are not identical. Each breast may require a different size flange. In fact, your flange size may even change throughout your pumping journey. Before you replace your pump parts. It’s a good idea to remeasure to confirm you are still using the correct size.

How do i know if it's the correct fit?

You know you have the correct size flange / insert if the following apply:

  • The opening fits comfortably around your breast
  • Only your nipple and a small part or none of your areola enters the tunnel when pumping
  • Your nipple moves freely when pumping without rubbing against the side of the tunnel
  • After pumping, your breasts feel soft and emptied
  • Your milk supply remains the same or improves
  • Overall, your pumping sessions are comfortable and free of pain

Flange Size Calculator