A general rule of thumb for sizing inserts: 

• When you need an insert: Your pump's flange tunnel is wider than your nipple diameter, causing excess areola to pull in, an unstable seal, or the nipple sliding around off-center.

• How to pick a starting size: Measure your nipple diameter at the base in millimeters. Choose an insert that brings the tunnel close to that number with just enough room for the nipple to move freely without dragging.

• When an insert is not the fix: If you already feel pinching, friction, or see redness after pumping, the tunnel is too tight. You need a wider tunnel or different flange size, not a smaller opening.

What a flange insert does

A flange insert is a small piece that sits inside a larger flange or collection cup to reduce the diameter of the tunnel opening. It does not replace the flange itself. It fine-tunes the internal fit so the tunnel matches your nipple more closely.

Think of it like a shim. The outer flange stays the same because it needs to be compatible with your pump system, but the insert narrows the space where your nipple enters. The result is a snugger tunnel that can improve comfort and help maintain a stable seal.

When a flange insert may be the right choice

You may need a flange insert when your measured nipple diameter is smaller than the smallest flange tunnel your pump includes. Many pump systems ship with a limited range of flange sizes, and not every nipple fits neatly into that range.

Inserts are also useful when you fall between two available sizes and the larger option leaves too much room. A flange insert for small nipples can bridge the gap between what the pump ships with and what your body actually needs.

If your current flange pulls excess areola into the tunnel, feels loose, or does not hold consistent suction, an insert that reduces the tunnel diameter may solve the problem without requiring a completely different flange.

Flange insert vs. flange size

Flange size and insert size describe different layers of the same fit decision.

Flange size is your starting point. Insert size is the adjustment layer. You choose a flange insert size based on your nipple diameter, not the flange size alone.

Start with measurement, not guessing

If you have not measured your nipple diameter at the base in millimeters, do that before choosing any insert. Nipple diameter is measured across the widest point at the base of the nipple (not including areola) while the tissue is at rest.

Different brands suggest slightly different approaches to translating that measurement into a tunnel size. Some recommend adding 1 to 3 mm to your nipple diameter, though the exact number varies by system and flange design. There is no single universal formula that applies to every pump.

One thing worth noting: nipple size can shift over weeks or months of breastfeeding. Hormonal changes, frequency of nursing or pumping, and even time of day can affect tissue. If your fit starts feeling off after weeks of working fine, re-measuring before swapping inserts can save you a round of unnecessary trial and error.

How to choose flange insert size after measuring

Once you have your nipple diameter, compare it to the tunnel size of the flange you are currently using (or plan to use). The gap between those two numbers tells you whether an insert could help.

If the flange feels too large

Signs of a too-large tunnel include excess areola being pulled into the opening, the nipple drifting off-center inside the tunnel, and an inconsistent seal. You may also notice that suction feels weaker or that comfort decreases over a session.

Example: Say your nipple measures 15 mm at the base, and your pump shipped with a 24 mm flange. That is 9 mm of extra space, which is a lot. Your areola gets pulled in with each suction cycle, and the seal keeps breaking when you shift position. A 17 mm insert placed inside that 24 mm flange would bring the tunnel much closer to your actual nipple diameter, reducing the extra tissue being drawn in and stabilizing the seal.

Choose an insert size that corresponds to your measurement plus a small buffer for comfortable movement, then confirm the fit while pumping.

If the flange feels too small

Pinching, friction against the tunnel walls, redness or discoloration after pumping, and soreness at the base of the nipple are signs the tunnel may be too tight. In this case, you need a wider tunnel opening or a different flange size, not a smaller insert.

Example: Your nipple measures 19 mm, and you are using a 19 mm insert because the number matched exactly. But during pumping, the sides of your nipple drag against the tunnel walls, and you notice a red ring at the base afterward. That zero-clearance fit is too tight. Removing the insert and trying a 21 mm option (or the base flange without an insert, if it is close to your size) would give the tissue room to move naturally without compression.

Adding an insert to an already tight fit will make the problem worse. If you are experiencing these symptoms, reassess your measurement or try the next size up.

If you are between sizes

Being between two insert sizes is normal. When that happens, try the size closer to your nipple measurement and check comfort and milk removal during a full session. A fit that is slightly snug but allows the nipple to move freely without rubbing is generally preferable to one with noticeable extra space.

Avoid defaulting to the wider insert opening just because it feels easier to put on. A tunnel that is too loose can reduce stimulation and pull in more areola tissue than needed.

Swelling during a session can also change how a size feels from start to finish. If the fit seems right in the first few minutes but becomes tight or uncomfortable toward the end, mild nipple swelling may be a factor. The next size up could be the better daily choice even if the smaller one matches your resting measurement on paper.

 

Signs you may need a flange insert

Symptoms often point to a sizing issue before you consciously identify one.

 

These symptoms can overlap, and more than one factor may be at play. If adjusting insert size does not resolve discomfort or output concerns, a lactation consultant can help evaluate fit in person.

How to check if the insert size is working

Put the insert in your flange, start pumping, and observe. A well-fitted insert should allow your nipple to move freely in and out of the tunnel without dragging against the sides. The nipple should stay roughly centered, with minimal areola being drawn in.

You should not feel pinching, squeezing, or significant friction. Some sensation is normal, but sharp or persistent discomfort usually signals a size mismatch.

Milk removal is the other half of the equation. If comfort improves after switching to an insert but output drops noticeably, the tunnel may now be too tight. If output stays consistent or improves alongside better comfort, the size is likely right. For more detailed fit evaluation, the fit guide covers what correct breast pump fit should look and feel like.

Flange insert sizing for wearable breast pumps

Wearable pumps present a specific sizing challenge. Because the pump sits inside a bra and relies on a sealed fit against the breast, the tunnel needs to be both snug and stable. Any extra space can cause the cup to shift, break seal, or allow tissue to move in ways that reduce comfort and output.

Many wearable systems, including Willow Go, ship with a set of flange sizes, but users whose nipple diameter falls between or below those sizes often benefit from an insert. A wearable pump flange insert reduces the tunnel to match a smaller nipple diameter while keeping the outer cup compatible with the pump body.

Stability matters more in a wearable context than with a stationary pump. A tunnel that is slightly too large on a tabletop pump may feel significantly too large in a wearable that shifts with your body throughout the day.

Common mistakes when choosing insert size

Skipping measurement entirely. Choosing an insert based on how your current flange "feels" without knowing your nipple diameter in millimeters often leads to cycles of trial and error that a two-minute measurement could prevent.

Copying someone else's size. Nipple diameter varies widely. Another person's insert size has no predictive value for yours, even if you use the same pump.

Chasing symptoms without reassessing fit. If you experience soreness or low output, the instinct may be to swap inserts repeatedly. A better approach is to re-measure (nipple size can change over weeks and months), check alignment, and then evaluate whether the tunnel diameter actually matches.

Assuming one size fits both sides. Your left and right nipples may differ in diameter by a millimeter or more. Measure each side independently and use different insert sizes if needed. Using a single size for both sides is one of the most frequent causes of lopsided comfort, and it is more common than most people expect.

Willow sizing support options

If you are choosing between insert sizes or starting the sizing process for the first time, the Breast Pump Sizing Kit includes measurement tools that help you identify your nipple diameter and match it to the right tunnel size. It is the most direct way to take the guesswork out of the process.

Once you know your size, Sizing Inserts are available to reduce the tunnel diameter in compatible Willow flanges. These inserts let you fine-tune fit without replacing the entire flange or collection cup.

FAQ

What size flange insert do I need?

Measure your nipple diameter at the base in millimeters, then choose an insert that brings the flange tunnel close to that measurement with enough room for comfortable nipple movement. The right size is a function of your measurement, your flange system, and how the fit holds up during pumping.

When do I need a flange insert?

You need a flange insert when your pump's flange or collection cup is compatible with your system but the tunnel opening is too large for your nipple. Signs include excess areola being drawn in, an unstable seal, or the nipple not staying centered in the tunnel.

Is flange insert size the same as flange size?

No. Flange size refers to the main tunnel diameter of the breast shield or cup. Insert size refers to the reduced tunnel diameter created when an insert is placed inside that flange. A 17 mm insert inside a 24 mm flange gives you a 17 mm tunnel opening, not a 24 mm one.

Can a flange insert help with pain or low output?

It can, if the pain or low output is caused by a tunnel that is too large. A closer tunnel fit can improve comfort and support more effective milk removal. If the tunnel is already too tight, adding an insert will make symptoms worse, and you need a wider opening instead.

Can I use flange inserts with a wearable pump?

Yes, if the wearable pump system supports inserts. Wearable pumps often benefit from inserts because a snug tunnel fit helps maintain seal and stability while the pump is worn inside a bra.

What if I have already measured but pumping still hurts?

Measurement is a starting point, not a complete answer. Pain during pumping can result from tunnel misalignment, incorrect insert size, swelling during sessions, or other fit factors. Re-check your measurement, confirm the insert is seated correctly, and review the fit troubleshooting guide for additional evaluation steps. If discomfort persists, consulting a lactation professional is a reasonable next step.

Do I need different insert sizes for each side?

Possibly. Many people have a measurable difference in nipple diameter between left and right. If one side feels comfortable and the other does not, measure each independently. Using two different insert sizes is completely normal and often the simplest fix for one-sided discomfort.

Conclusion

If flange insert sizing feels uncertain, the goal is not to guess better. The goal is to size more accurately.

That's where the Breast Pump Sizing Kit comes in. Take the guesswork out of sizing and give yourself peace of mind that you've found a better fit to make pumping more comfortable.