A Guide to Finding Your Willow Flange Size

Take care of your nips, mama!


If you’re a pumping mom, you’ve probably added a new word to your vocab: flange. And while it certainly doesn’t roll off the tongue, it’s one of the most crucial components of your pumping experience. It’s the Goldilocks conundrum here: a flange that’s too tight can lead to painful nipples and extreme to mild discomfort, while a flange that’s too loose won’t provide the suction you need to get the most milk. The solution? A flange that’s just right (for you!)


Don’t worry, mama—you’ve got everything you need to ensure the perfect Willow flange fit, right here in this guide.


So, What’s a Flange and How Does It Work?


The flange is the piece that comes into contact with your breast. Its job is at once simple and extremely important: create a seal around your areola to draw in your nipple and stimulate milk flow.


Most breast pumps include external flanges that resemble funnels, but Willow is different. We built our pump so that the flange is on the inside of the pump which ensures comfort and portability.


Just Right: 3 Ways to Find The Right Flange Size For You


Willow offers three different sizes of flanges: 21mm, 24mm and 27mm (with sizing inserts in 15, 17 & 19mm) to support moms with nipple measurements between 11 - 26mm. To find the right size, there are a few different methods you can use, including measuring with our printable Willow ruler, a smartphone app or the circle cut out method.


How to Size Your Nipples with Willow’s Printable Sizing Tool


Cut around the outside of the size tool, then fold twice.

  1. Stimulate your nipple to perk it up.

  2. Slide the paper ruler against your nipple until it’s nestled in the corner. Now you should be able to read a millimeter measurement.


If your nipple measures smaller than 17mm, we now offer Sizing Inserts to fit more moms. Our sizing inserts come in 15mm, 17mm and 19mm and will fit perfectly inside the 24mm flange.


How to Size Your Nipples With a Smartphone App


Ah, the wonders of technology. If you don’t have easy access to a printer, we’ve got you, mama. All you have to do is download an app to your phone and follow the instructions. Here are five easy steps to get to the bottom of your nipple size.


  1. Download an app. We recommend Ruler 2.0 for iOS or Millimeter for Android.

  2. Set the measurement. If using the iOS app, change the measurement settings to millimeters. If using Android, select the default measurement settings.

  3. Stimulate the nipple. You’ll need to do this to ensure it’s perked up enough for correct sizing.

  4. 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.

  5. Slide measurement bar. Slide the measurement bar to the opposite side of your nipple. What you see is your nipple measurement.


Remember, we recommend using a flange that’s 2 to 4mm larger than your nipple measurement.


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 for a whole lot of pumping payout. While we wish we could suggest the exact size flange or insert to purchase, there are several factors that make the way a pump fits unique to each mom.


  1. Nipple Fact #1: Nipples change size from morning to night, before feeding and after feeding and they continue to change throughout breastfeeding. They’re magical (and mercurial!) like that.

  2. Nipple Fact #2: Elasticity is unpredictable. Some nipples stretch wider while some expand longer. Elasticity can also change depending on breastfeeding duration so our 2-4mm guideline is an estimate.

  3. Nipple Fact #3: All moms require different levels of stimulation to trigger let down. Some moms need a tighter flange while others need lots of space around the nipple so it can extend fully with each suction. Some moms can’t handle any rubbing while some moms will not get a let down unless their nipple rubs the side of the flange. We are all unique so experimenting with flange and insert sizing will allow you the opportunity to find out what works best for you.

  4. 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.


What Should I Do If I’m Between Willow Flange Sizes?


  1. If you should still fall between sizes, we recommend sizing down.

  2. If you measure larger than 25mm, we recommend applying a baby-safe lubricant to your nipple because it may rub slightly and then evaluating the fit and comfort of our 27mm flange.


If after your first few pump sessions you feel uncomfortable or don’t get the milk volume you expected, we recommend experimenting with a different size until you get it just right.

Our flanges have been tested for a use-life of 90 days. This means you should replace your flanges as well as the FlexTubes™ and containers every 90 days or if you notice signs of wear.


Tips From Our LC: How to Align Your Willow Flange Perfectly


Lower milk volume than you expected? Discomfort around your nipple when you pump? You could have an alignment issue inside the flange. Here are some tips to help:


  1. Make a small adjustment: If you’re pumping, press pause and gently break your latch. Then, slide Willow in the direction of the pinching slightly. If this doesn’t fix the pinching sensation, pause again and fully realign your breast.

  2. Look for a suction circle: If you see a suction circle on the outside of your areola, pay attention to where your nipple is aligned. If it’s not in the center, try realigning again.

  3. Stimulate: You should always stimulate your nipple before aligning your Willow to help with alignment.

  4. Check your flange size: Serious discomfort or pain is a sign that you’re likely using the wrong flange size. If your nipple touches the top, bottom or the sides of your flange, it’s most likely too small. If it doesn’t and you still feel discomfort, your flange may be too big. Revisit the sizing guide for help!

  5. Check your bra: If your flange size is correct, make sure you’re wearing a full-coverage style bra that holds Willow securely. If you can see the thumb pad on the flange, tighten your straps.


Struggling With Flange Fit? We’re Here to Help!


Still not sure which Willow flange is the right choice for you? Have more questions about your Willow?
Check out our Help Center.