Finding your breast pump flange size starts with measuring your nipples. But the number you measure is a guide to what to try—not a guarantee that one size will fit perfectly.

Think of it like buying shoes. Knowing your size gets you close, but you still have to try them on to see what feels right for your body.

Below, you can download a free flange ruler, learn how to measure, and use those measurements to choose a few sizes to test. If you already have a size and want to check it in action, these photos show what a comfortable pump flange fit can look like.

What Size Pump Flange Do You Need?

Your flange size is the diameter of the tunnel your nipple moves through, measured in millimeters. The size included with your pump may be a useful option to try, but it may not be the size your body needs.

The goal is a flange that feels comfortable, removes milk effectively, and keeps your tissue healthy. Measuring helps narrow down where to start. Let’s do that first.

How to Measure Your Nipples for Pump Flange Size

You can use a printable flange ruler, a metric ruler, or digital calipers. Measure each nipple separately—you may need a different size on each side.

First: Print Your Flange Ruler at 100%

Print the guide at 100% or “actual size,” rather than “fit to page.” Before measuring, check that the 1-inch box at the bottom measures exactly one inch. You can also use a metric ruler to check the diameter of a labeled circle.

Second: Measure the Base of Your Nipple

Measure across the base of your nipple, right where it connects to your areola. You’re measuring the nipple itself, without including the surrounding areola.

With my printable guide, cut out a few circles close to your nipple size. The blue inside each circle helps you see whether you’ve cut out the whole opening. Find the smallest hole that comfortably slides down to the base without squeezing or forcing your nipple through. That number is your base measurement.

With a metric ruler, measure straight across your nipple at the base. With calipers, lightly bring the measuring edges to the sides of your nipple without compressing the tissue. Write down the measurement for each side.

What About Measuring the Nipple Tip?

The Flange FITS™ pilot study compared starting with the nipple tip measurement and trying nearby flange sizes against measuring the base and adding the extra millimeters recommended by the pump manufacturer. The individualized approach starting at the tip produced better comfort and milk output.

In my practice, my clients have found success measuring the base of the nipple and using that exact measurement as their starting point, without automatically adding extra millimeters.

You can measure both your nipple tip and base, then use those measurements to guide the sizes you try. To measure the tip, use a metric ruler or calipers across the tip itself without squeezing it. The two measurements may be similar, or they may give you different starting sizes.

In both my experience and the FITS™ study, the actual measurement, without additional mm being added to the result, resulted in a better fit. 

Either way, the measurement is a starting point. Just like shoes, knowing your size gets you close—but you still have to try them on to find what works comfortably for your body.

Third: Choose a Few Flange Sizes to Try

Start with a flange close to your measurement, then try nearby sizes if needed. For example, if your base measures 17mm, 16mm, 17mm, and 18mm could give you a small fit kit to compare. If your tip measurement is different, a size close to that number is another option to test.

Your nipple can change shape under suction, which is why the fit while pumping matters more than how it looks before you turn the pump on. If your tissue stretches a lot, this guide to fitting elastic nipples walks through what to watch for.

Breast Pump Flange Sizing Chart: A Starting Guide

Use this chart to help choose sizes to test, then check how your body responds. A chart can’t account for every nipple shape, flange design, or response to suction.

Getting Your Size: Replacement Flanges and Inserts

If the flange that came with your pump isn’t the size you need, you have two main options: replace the flange or use a compatible insert to reduce its tunnel size.

Replace Your Pump Flange

Replacement flanges come in different sizes, shapes, and materials. Some are made by your pump manufacturer, and others are made by companies that offer compatible parts. Check compatibility with your exact pump model before buying—including any connectors or adapters you need.

Size is only part of fit. Two flanges with the same millimeter label can feel different because of their shape, tunnel length, or material.

Use Silicone Inserts

Silicone inserts fit inside a larger flange to make the tunnel smaller. They can be helpful when you want to try several sizes or when your wearable pump doesn’t offer interchangeable flanges in the size you need.

Look for inserts designed for your flange’s size and shape. Thin, clear inserts can make it easier to see what your nipple is doing during pumping. An assortment of nearby sizes can help you compare without buying several complete flanges.

How to Check Your Flange Fit While Pumping

Center your nipple in the tunnel and begin at a low, comfortable suction. Watch what happens once suction begins and again as milk starts flowing. Your nipple may look like it fits before the pump turns on, then move quite differently under suction.

If you’re not sure which tissue you’re seeing, my nipple-versus-areola photos and flange-fit examples can help you compare.

Signs the Fit Is Working

• Pumping feels comfortable. You may feel a rhythmic tug, but you shouldn’t have to push through sharp, pinching, or burning pain.
• Your nipple can move in the tunnel. Gentle contact with the sides may happen, without scraping, sticking, or constriction.
• Your areola isn’t being repeatedly stretched deep into the tunnel. A little movement at the base can happen; watch for a large amount of tissue being pulled in or folding.
• Milk is being removed effectively. Consider milk flow, breast softening, and your usual output together.
• Your tissue looks and feels healthy afterward. Check for new swelling, damage, or lasting color changes.

Signs You Need to Recheck the Fit

Pain, rubbing, swelling, or a fold of tissue getting caught are reasons to stop and reassess. They don’t all point to the same size change—position, suction, and flange shape can matter too.

Pinching at the base can happen when a flange is slightly too big and pulls in a small fold of areola. These close-up photos show how that pinching happens and what you can try.

If a size feels comfortable, compare it across a few similar sessions before deciding whether it works better. One pump session can vary for plenty of reasons. Keep the timing and suction as consistent as you reasonably can, and look at comfort and tissue response along with output.

Don’t Forget About Friction

Even with a suitable size, friction can make pumping uncomfortable, especially with grippier silicone inserts. A small amount of a compatible lubricant may help your nipple glide more comfortably. Check your flange and product instructions first.

I explain what to use for pump flange lubrication and how to apply it in this guide. Lubrication can help with friction, but it won’t fix tissue being squeezed or pulled into the wrong place.

What to Do Next

• Measure both nipples. Write down the base measurement, and the tip measurement if you want to compare starting points.
• Choose a few compatible sizes. Use your measurements and the chart as guides.
• Try the fit while pumping. Watch your tissue, notice how it feels, and compare milk removal over similar sessions.

If you measured during pregnancy or your current setup starts feeling different, you can measure again. Your fit may change over time.

If you’re having trouble finding a comfortable fit, a lactation consultant who works with pumping can help you compare sizes and watch what happens under suction. You can also find more setup help in 10 things pumping parents should know.

Finding What Works for Your Body

You don’t have to chase the smallest flange or make your body match a chart. You’re looking for a setup that feels good, removes milk well, and keeps your nipples healthy.

And if your current setup already does that? You don’t need to change it just because a measurement suggests another size. The number helps you get started. Your experience helps you decide what works.