Everything you need to know about perfecting your flange fit — the secret to more comfort and milk output. 

When it comes to wearable pumps, size really matters. Wearable breast pumps aren’t one-size-fits-all, and they often need to fit more snugly than their traditional counterparts. This is critical for optimal performance — that’s right, size impacts milk output!

Here’s why: a poorly-fitting flange can impact stimulation, which can then affect the hormonal rush of oxytocin that triggers letdown and milk production. In turn, your output can take a dip. 

If your flange is too large, you’ll have less pressure on the alveoli (this is where milk is produced and stored in your breast!) 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, compressing milk ducts and leading to restricted emptying. 

Beyond output, the right flange fit can determine whether pumping feels comfortable or painful. Using the wrong size can lead to nipple abrasion, pinching, and pulling, and damage your breasts over time if you’re not careful. 

For this reason, measuring your nipples before you start pumping and whenever you feel pain or discomfort or notice a change in the amount you’re pumping is SO important. It’s not just a nice-to-do — it’s a must-do if you plan on pumping with a wearable.

Below, we’re diving into everything you need to know about finding the right flange fit — how to measure your nipples, why you should experiment with sizing inserts, how to troubleshoot common sizing issues, when to seek help from a lactation consultant if you’re struggling to find your fit, and more. 

What is a Breast Pump Flange?

A breast pump 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. Flanges exist in different sizes because nipples vary in size from person to person and even from breast to breast. We like to say that they’re sisters — not twins! 

Nipples also vary in size throughout your pumping journey — you may have one measurement when you’re newly postpartum and a totally different measurement once you’ve established your supply and gotten into a feeding routine. This is totally normal, and in fact, it’s to be expected. Here are a few other interesting nipple facts: 

• Nipple fact #1: Nipples can change size from morning to night, before 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 a letdown, 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 comfortable fit that results in optimal output.  

Most wearable pumps are compatible with flanges in a variety of sizes. For example, 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 also available in 21mm and 27mm. 

Breast pump sizing inserts, made of flexible silicone, will always sit inside our 24mm flange — they’re designed to create an even more snug and comfortable fit. They are recommended for women with nipples measuring smaller than 21mm and are available in sizes 13mm, 15mm, 17mm, 19mm, and 21mm.  

Based on years of sizing data, we know that most moms find the best fit using a 17mm 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!)

The Importance of Finding the Right Breast Pump Flange Size

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. This can also lead to painful conditions like plugged ducts and even mastitis, which can throw your feeding plan off track.

In the longer term, an improperly fitted flange can reduce three important things: stimulation, the hormonal rush of oxytocin that triggers letdown and milk production, and milk output and capacity. In other words, your supply could drop. 

“Oxytocin is blocked by pain and fear,” pediatrician Dr. Laurie Jones says. “If your flanges don't fit well, and your brain knows it, that limits how soon the oxytocin is released, how much is released, and whether there will be second and third bursts of oxytocin as you pump. The fit is important to be pain-free, fear-free, and dread-free.”

How to Measure Your Flange Size

There are a few different ways to measure your nipples. We recommend using a cut-out nipple sizing tool like the one provided in the Willow pump kit. (Here’s a digital version.) Alternatives include using a cloth measuring tape, a physical ruler, or a digital ruler. 

When to do it? 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, your nipples 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.

Now you’re ready to follow these steps. 

Step 1: Get Your Supplies

Get your sizing tool or measuring tape, and find a well-lit space that ideally has access to a mirror.

Step 2: Measure Your Nipple Diameter

No matter what tool or tape you’re using, you’ll want to 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 to measure your nipples with a cutout sizing tool 

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
4. Once you know your nipple size, find your flange using our sizing chart. 

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. 

How to measure your nipples with a ruler or tape measure

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

• Stimulate the nipple (you’ll need to do this to ensure it’s perked up enough for correct sizing).
• 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!).
• 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.
• Repeat with the other side. 
• Once you know your nipple size, find your flange using our sizing chart. 

Step 3: Determine Your Ideal Flange Size

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 — this is true if you pump with the Willow Go. Other pumps, like the Willow 360, need to fit a bit more snugly — you’ll want to add 0-2mm to your measurement in this case.

Measured Nipple Size	Willow Recommended Size Flange / Insert
9-11mm	13mm insert in 24mm flange
12-13mm	15mm insert in 24mm flange
14-15mm	17mm insert in 24mm flange
16-17mm	19mm insert in 24mm flange
18-20mm	21mm flange
21-23mm	24mm flange
24-26mm	27mm flange

Tips for Finding the Correct Flange Size

Once you’ve measured your nipples, here are a few things to look out for — and some important reminders. 

Signs of a good flange fit 

A good fit is a snug fit. You shouldn’t feel any pain or discomfort when your nipple is pulled into the flange, and your output should be consistent with good milk flow. 

Signs of a poor flange fit

A poor fit will likely cause pain or discomfort and negatively impact your output. (Your output can also dip if it’s been too long since you replaced your pump parts, such as the duckbill and diaphragm, so make sure to keep an eye out for that as well.)

Don’t be afraid to experiment with sizing

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 also depends 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.

Give your body time to adjust

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.

When to Seek Professional Help from a Lactation Expert

A lactation expert can be an incredibly valuable resource when finding your flange fit and beginning to pump. Look for a certified Lactation Consultant (also known as an IBCLC), Breastfeeding Counselor, or Lactation Educator — they have all been trained to assist with sizing and everything that comes along with it.

Benefits of Consulting a Lactation Consultant

You should seek professional support if you are: 

• Struggling to measure your nipples at home 
• Not sure if you need a sizing insert 
• Experiencing pain or discomfort despite using the “right” size
• Experiencing a sudden change in your milk output
  

Finding the Right Professional

If you’re pumping with the Willow 360 or Willow Go, you can book a complimentary session with a Willow Mom Coach. They are Lactation Educators and experts on all things sizing, and can help you measure your nipples, experiment with different flange and insert sizes, and optimize output through proper alignment. They’ll even help you find the right bra! (If your flange fits well, but your bra is too loose, you may still deal with less than optimal output.)

If you’re not yet pumping with Willow, there are many ways to find a lactation expert. Start by asking your pediatrician or OBGYN for a local recommendation, or explore care through a platform like SimpliFed, which offers virtual support. 

Achieving Comfort and Efficiency with the Right Flange Size

There’s a lot to know about finding your flange fit — but we promise you’ll get the hang of it! In summary:

• Wearable pumps fit differently than traditional pumps because of how wearables sit on the breast. You’ll want a more snug fit for optimal comfort and milk output. 
• Sizing isn’t one-size-fits-all. The flange that comes in your pump kit may not be the right flange for you — so measuring your nipples is critical. 
• Your nipples will change in size as you establish breastfeeding and throughout your pumping journey. Measure whenever you notice discomfort or a change in output! 
• Inserts are a great way to optimize comfort. They always fit into our 24mm flanges.
• Use a cutout sizing tool, a cloth measuring tape, or a ruler to get your nipple measurement. 
• If you’re pumping with Willow, add 0-2 or 1-3mm to your nipple measurement (depending on which Willow pump you’re using) to find your flange fit.
  

You can learn more about finding your Willow flange fit and shop sizing inserts here.

Breast Pump Flange Size FAQs

What is a Breast Pump Flange and Why Does Size Matter?

• A breast pump flange is a cone-shaped funnel that fits over the nipple and areola to create suction for milk extraction. Flanges come in various sizes because nipples vary significantly in size and shape among individuals and even between breasts. Size matters because a properly fitted flange ensures effective milk removal without causing discomfort or reducing milk output. Using an incorrectly sized flange can lead to issues like poor stimulation, decreased milk production, and even breast damage over time.

How Do I Measure My Nipple Size for a Breast Pump Flange?

• To measure your nipple size for a breast pump flange, you can use a cut-out sizing tool included in pump kits, a cloth measuring tape, or a ruler. It's best to measure when your nipples are at their average size, typically before pumping or at least an hour after. Measure the diameter of your nipple from one side of the base to the other. Most moms find success with a flange size slightly larger (1-3mm) than their actual nipple measurement to ensure a snug fit without causing discomfort.

When Should I Seek Help from a Lactation Consultant Regarding Flange Fit?

• Seek assistance from a certified lactation consultant if you experience difficulties measuring your nipples, unsure about using sizing inserts, or encountering pain and discomfort despite using what you believe to be the correct flange size. Lactation experts can provide invaluable guidance in finding the right flange and insert sizes, optimizing milk output, and ensuring a comfortable pumping experience. Professional support can significantly improve your breastfeeding journey with wearable pumps like Willow Go or Willow 360.