Flange Sizing 101

Maintaining and increasing milk supply is one of the biggest questions women have in their lactation journey. We asked our community of experienced surrogates what they wish they'd known about pumping before starting their lactation journey. More than half of the women surveyed said they were unaware of how crucial it is to use the right pump and properly sized flanges. According to lactation experts, these are two of the biggest mistakes women make when pumping.

Pumping Essentials

“Nipple Rulers 100%. I made the mistake of not realizing how important it is to be using the right size flange. Amazon has a million different flanges & inserts that fit pretty much any pump with different conversion kits.” — Jordan P

“Making sure you have the right flange. You can get a flange sizing kit on Amazon for super cheap.” — Sasha V

“Get sized for flanges either using a ruler or working with a professional experienced in sizing. Not all Lactation Consultants are experienced in sizing flanges.” — Lisa L

What Exactly is Flange Sizing?

Flange sizing is something that not all lactating women are properly educated on, and many wish they knew more about. We consulted with Nikki Bredeson, a lactation expert, to explain what flange sizing is and why it’s so important:

“The flange is the cup or the shield that you attach to the breast when you pump. It is essentially the piece that attaches to the body that pulls the milk and allows it to drop into the bottle. The standard size is much larger than what the majority of women need. I believe right now the average size is 17 millimeters, and yet most pumps come with a 24 or a 25 millimeter flange.”

Why is Flange Sizing Important?

“For all of us pumping women, it is important to have flange sizing checked by a specialist,” says Nikki. “A lot of women get a size at the hospital that actually is not correct for them and then they go home. They assume their supply is low, but it isn’t. It’s just the incorrect size that is not allowing them to remove milk efficiently.”

Using the wrong flange size may lead to:

• Painful pumping experiences

• Reduced milk output

• Frustration and stress

With the right sized flanges, you can optimize your milk supply and make pumping more comfortable.

Measuring Your Flange Size

Flange sizing can change over time, so regular measurement is key. Nurse and Pumping Expert Brianna Seibel shares her advice:

“Measure your flange size early and often. It changes throughout your pumping journey, and the wrong size can make pumping painful. If your pump doesn’t fit, don’t use it. I made that mistake and ended up with shredded skin and blisters — it was unbelievably painful, and healing took forever.”

“Your size right after birth will likely be different from your size a few months later, and that’s completely normal. Use a nipple ruler and consider sizing inserts. I recommend having a few different sizes on hand and re-measuring regularly to make sure you’re using the right flange.”

“In the beginning, your body isn’t used to pumping, and the wrong size can make everything harder and more painful. Pumping isn’t always the picture-perfect moment you see on TV — it can be beautiful, but it can also hurt, and that’s normal. Learn from me: don’t use a flange that’s too big. The damage and the healing time just aren’t worth it.”

If your measurements change or you’re unsure about your fit, talk to an IBCLC for personalized guidance.

Step-by-Step: How to Measure Nipple Size for Pumping

1. Place the nipple ruler against your nipple (not including the areola).

2. Measure the diameter of your nipple at the base.

3. Compare with your pump flange size and add 1–3 mm for comfort.

4. Try inserts if your pump allows, or consider alternate flange sizes.

5. Re-measure regularly, especially in the first 3–6 months postpartum or throughout a surrogate’s pumping journey.

Tools to Find Your Correct Flange Size

Amazon Silicone Nipple Ruler

Free Printable Ruler

Looking for more lactation resources? Check out our Lactation Resource Guide.