A Guide to Finding Your Willow Flange Size

Take care of your nips, mama!

All about Pump Flange Fit

Why does fit matter?

When it comes to wearable pumps, size really matters. Wearables aren’t one-size-fits-all, and if you’ve used a traditional pump, you may find that flange fits aren’t consistent between the two. Plus, having a flange that fits is critical for performance (that’s right, size impacts output!) and comfort. This is why we include a standard 24mm flange in your pump kit. Designed for use with our silicone inserts, which improve fit and comfort if an out-of-the-box flange isn’t working for you, they help make sure that you have what you need to pump with confidence.

What are the risks of pumping with a poor fit?

A poorly-fitting flange can impact stimulation, which can then affect the hormonal rush of oxytocin that triggers letdown and milk production, and decrease output in turn. If your flange is too large, you’ll have less pressure on the alveoli and less stimulation overall, which can result in insufficient emptying. (And when you leave milk behind, you may decrease your supply.)

If your flange is too small, your nipple may rub against the tunnel, causing discomfort, pain, and compressed milk ducts that restrict emptying.

Can I measure my nipples when I’m still pregnant? What about when my milk comes in?

Your nipples will undergo a series of changes in both shape and size as you shift from pregnancy to establishing breastfeeding to reaching a steady state. It’s normal to need a different flange size a few times before you finish your pumping journey, so be prepared to measure a few times—and possibly swap out your flanges or inserts each time.

If you’re still pregnant and want to get an idea of your flange size, the ideal time to do so is before your sixth month of pregnancy. After that, they may actually be larger than they’ll be during most of your pumping and breastfeeding journey.

If you’ve purchased your flanges during pregnancy, you’ll want to measure them again once you start pumping — ideally before you pump or at least an hour after you’ve finished. If you’re sized in the hospital, this is the stage when your nipples are likely at their largest, so you should resize again in a few weeks — ideally around six to eight weeks postpartum.

In summary? You can measure your nipples when you’re pregnant — just remember to do it before the six-month mark. If you’re sized at the hospital, take your measurements again after 6-8 weeks (you should have reached a “steady state” at this point), as it’s possible your nipples may be smaller than they were right after giving birth. After that, measure any time you notice pain, discomfort, or a change in supply.

When should I measure my nipples?

First and foremost, you should measure anytime you feel any discomfort when pumping, or experience a sudden change in output. Remember to always measure both nipples — no two breasts are the same (even if they’re both yours!), and they may change in size throughout your breastfeeding journey.

We also recommend you re-measure before you order replacement accessories to ensure your size is still correct.

Aside from nipple size, what else impacts fit?

Finding the right fit also depends on your skin elasticity and breast tissue sensitivity. Once you know what flange or insert might be right for you based on your measurements, don’t be afraid of experimenting. You may find that using a flange or insert a size above or below your measured size is more comfortable and effective.

What is a flange and how does it work?

A flange is a cone-shaped funnel, typically ending in a tunnel, that seals over your nipple and breast and creates a vacuum to transmit milk. Sizes are based on the diameter (or width) of your nipple (not your areola, which is a common misconception!).

The Willow Go comes with a 21mm and 24mm flange and is also available in 27mm. The Willow 3.0 comes with a 24mm flange and is available in 21mm and 27mm. For both pumps, the 24mm flange is what you’ll use with an insert.

What is an insert?

Breast pump sizing inserts, made of flexible silicone, optimize comfort and output for moms who measure smaller than 21mm. (It’s a way to get a more custom-feeling fit!) Willow inserts are designed to fit into a 24mm flange and are available in sizes 13mm, 15mm, 17mm, 19mm and 21mm.

Why do you need an insert?

Based on years of sizing data (more than 300k women have pumped with Willow!), we know that most moms find the best fit using a17mm or 19mm insert, and rarely need a size larger than 24mm. (P.S. If you’re using Willow Go and measure 21mm, use the 21mm flange included in your kit — no insert needed!)

Why do Willow pumps come with 24mm flanges if most moms measure smaller?

There’s no way around it: wearable pump sizing is an art! If you need a size that’s smaller than 24mm (and you might!), you’ll still need to use a 24mm flange, since that’s the size that our inserts are designed to fit into. So how does that work, exactly? If your nipples measure 17mm, you’ll pair a 24mm flange with a 19mm insert.

Because your size can change over time, we encourage you to size frequently, and experiment with inserts to maximize comfort and output.

Once I have my measurements, how do I know which size to choose?

When it comes to wearables, you’ll need a fit that’s more snug than what you might be used to with a traditional pump. Most moms find that adding 1-3mm to their measured nipple size (or 0-2mm if you’re pumping with the Willow 3.0) results in optimal output and comfort. After measuring the width of your nipple, use this handy sizing chart to find your size.

How do I know I have the right fit?

Center your nipple in the flange, and start pumping. You should feel pulling, not pinching. If your nipple rubs against the side, it’s likely too small. If you don’t feel suction, it’s likely too big.

What do I do if my flange or insert doesn’t fit?

  • If you’ve tried pumping and the fit seems off, try using an insert, or going up or down to the next flange size.
  • Willow’s customer care team can help you troubleshoot fit issues. Learn more here.
  • You may benefit from the Willow Perfect Fit Bundle if you’ve had trouble measuring your nipples on your own using an at-home measurement tool, and are unsure about whether your flange fits properly.
  • You can also schedule a virtual visit with SimpliFed to talk to a lactation consultant. They’re trained to help with sizing for Willow pumps as well as answer questions about supply, pumping schedules, and more.

The facts about nipples

The art of sizing is exactly that—an art! It’s not a science. However, it does take a little bit of patience to find the fit that will result in a whole lot of pumping payoff. There are several factors that impact fit, and make it unique to each mom.

  • Nipple fact #1: Nipples can change size from morning to night, before feeding and after feeding, and throughout your breastfeeding journey. They’re magical (and mercurial!) like that.
  • Nipple fact #2: Elasticity can impact sizing. Some nipples stretch wider while some expand longer. Elasticity can also change depending on breastfeeding duration. We recommend elastic moms to use as snug an insert size as possible to prevent their nipples from being pulled too far into the flange and mushrooming.
  • Nipple fact #3: All moms require different levels of stimulation to trigger let down and the letdown reflex is a trained response. This is why experimenting with multiple sizes is a good idea if you’re struggling to find a fit that’s comfortable and resulting in optimal output.
  • Nipple fact #4: This might be the most important one of all! If your current Willow flange size is resulting in great output and it’s comfortable, there’s no need to stress and change what you’re doing — even if the size chart suggests you might need a different size. As they say: If it’s not broken, don’t fix it.

The Perfect Fit Bundle

The Perfect Fit Bundle

Need extra sizing support? Get a set of four inserts (they help create a more snug flange fit), access to a Sizing Specialist for 1:1 guidance, and more.

How to Measure?

The good news is that it’s easy to measure yourself at home. We recommend sizing with Willow's cut-out nipple sizing tool, or printing it out if you don't have a copy handy. Alternatives include using a cloth measuring tape or ruler (physical or digital works!).

What part of my nipple do I measure?

Measure the diameter of your nipple from where it meets the areola on one side to the other. This ensures that the flange—a cone-shaped funnel—seals over your nipple, creating a vacuum to extract milk. And don’t forget to measure both nipples. No two breasts are the same (even if they’re both yours!).

How do I measure with the circle cutout sizing guide?

If you already have a Willow pump, the Nipple Sizing Guide (it has the circle cutouts!) that came in your pump kit is easy to use and accurate, too. You can also print the sizing tool and cut out the circles yourself.

Here’s how to find your size:

  1. Pick a time before pumping/breastfeeding or at least one hour after
  2. Slide each circle over your nipple. Select the circle that can slide all the way to the base without touching any part of the nipple. This is your nipple size.
  3. Do the same for your other nipple. Remember: no two breasts are the same (even if they’re both yours!).
  4. Once you know your nipple size, find your flange using our sizing chart.

Note: your flange size should closely match your nipple measurement. Most moms have the greatest success in a size 1-3 mm larger than their exact measurement, or 0-2mm if you’re pumping with Willow 3.0.

How to measure your nipples with a smartphone app

If you don’t have easy access to a printer, download an app to your phone and follow these instructions:

  1. Download a measurement app for iOS or Android on your smartphone
  2. Set the measurement. If using the iOS app, change the measurement settings to millimeters. If using Android, select the default measurement settings
  3. Pick a time before pumping/breastfeeding or at least one hour after
  4. Stimulate the nipple (you’ll need to do this to ensure it’s perked up enough for correct sizing)
  5. Place the phone screen perpendicular to your nipple — the edge of your nipple should line up with the zero points in the corner of the app screen
  6. Slide the measurement bar to the opposite side of your nipple (what you see is your nipple measurement!)
  7. Do the same for your other nipple
  8. Once you know your nipple size, find your flange using our sizing chart.

Remember, we recommend using a flange that’s 1-3mm larger than your nipple measurement, or 0-2mm if you’re pumping with Willow 3.0.

How to measure your nipples with a ruler or tape measure

You can also measure your nipples with a ruler or soft measuring tape.

  1. Stimulate the nipple (you’ll need to do this to ensure it’s perked up enough for correct sizing)
  2. Line up the end of the ruler or measuring tape where your nipple begins, and measure only the nipple (include minimal areola). Secure excess measuring tape under your arm so it’s not hanging down (or ask a loved one to hold it!)
  3. Use a mirror to guide you when placing the ruler or cloth measuring tape against the nipple, so that you can more easily distinguish between the nipple and areola.
  4. Repeat with the other side
  5. Once you know your nipple size, find your flange using our sizing chart.


Can I switch back and forth between sizes?

We recommend allowing up to a week for your body to adjust to a new size. Because your letdown reflex is a trained response, your body will need time to adjust!. Switching back and forth between different sizes can do more harm than good(and lead to less output), so we recommend consistently using one size for at least a few days before trying something new.

I measured my nipples, but the size I chose based on my measurements isn’t comfortable.

Sometimes, the size you choose based on your measurements just won’t feel comfortable — and while that’s frustrating, it’s okay! Remember that in addition to your nipple size, flange fit is also dependent on your skin elasticity and breast tissue sensitivity. Try using an insert if you aren’t already, or going up and down a flange size to see if that helps. (For example, if you tried a 19mm insert with a 24mm flange, you may want to try a 21mm insert or a 17mm insert.) It’s all about experimenting to find the fit that works for you — sort of like finding the right bra size.

When should I measure?

You should measure one hour before or after breastfeeding or pumping.

Are my nipples weird?

Every nipple is unique in size, shape, elasticity, and the stimulation that is required to trigger letdown. If you have an inverted nipple, extra nipples, or scarring, that may impact letdown as well. A lactation consultant can answer any questions you have about your nipples!

How often should I measure my nipples?

We recommend you measure frequently (and any time that you feel discomfort or see a change in output) to make sure that you’re still using the right size.

Do I need to measure both sides?

Yes! Remember, your nipples are sisters — not twins. You may find that they vary in size so much that you need to use a different flange or flange and insert combination per side.

How long will my flanges and inserts last?

You should replace your pump parts every three months for optimal performance, even if you don’t see any obvious evidence of damage. Normal wear and tear can impact suction and output! Having extras on hand is helpful in the event that you lose a part, too.

What should I do if I find that I’m in between flange sizes?

If you fall between sizes, we recommend sizing down. If you measure larger than 25mm, we recommend applying a baby-safe lubricant to your nipple (to prevent any rubbing), and then trying our 27mm flange to see if it’s comfortable.

I'm still confused about how to measure myself. Can I meet with someone before I choose my inserts?

Yes! You can purchase The Willow Perfect Fit Bundle and book a virtual sizing session with a Sizing Specialist. They can help you measure your nipples, evaluate your flange fit if you’re unsure about it, and answer any questions you have about how to optimize comfort and output. You’ll also receive a Flange Fit Bundle with additional flanges and inserts to try. (Remember, we recommend sizing one hour before or after feeding to avoid swollen tissue — so try to keep that in mind when booking time for your session!)

If I was sized by a Lactation Consultant, do I still have to measure myself?

Yes! Depending on when you were sized by an LC, you may have experienced swelling after delivery or during the end of your pregnancy. Sizing is key to optimal output and comfort, so it’s important to revisit your measurements to ensure they’re accurate for the stage that you’re in.

Is sizing for a wearable pump different from a traditional pump?

Yes! Sizing differs because of how wearable pumps sit on the breast, which is why we typically recommend a more snug flange fit. (Also, remember that sizing can differ from one wearable pump to the next!) We recommend adding 1-3mm to your nipple measurements (or 0-2mm if you’re pumping with Willow 3.0) to find the right fit.

What is The Willow Perfect Fit Bundle?

The Willow Fit Check is a sizing service that uses your nipple measurements to recommend a bundle of inserts (or a flange and insert combo) that may help optimize comfort and output. In addition, it provides you with access to a 30-minute virtual session with a Willow Sizing Specialist. They can help you take your measurements if you’re not sure how to use an at-home measurement tool, and answer any other questions you have about fit.

The content provided here is for informational purposes only and is not intended to replace the advice of medical professionals. It should not be used to diagnose or treat medical conditions or problems. Please contact your healthcare provider with questions or concerns.
Which pump is right for you?
Which pump is right for you?

Which pump is right for you?

Which pump is right for you?

Which pump is right for you?

Which pump is right for you?