Why Using the Correct Flange Size Matters

If you’re pumping breast milk—whether occasionally or exclusively—you’ve probably heard the term flange (also called a breast shield). It’s the part of your breast pump that fits directly over your nipple and areola. What many mums don’t realise is that using the wrong flange size can have a huge impact on both comfort and milk output.
At modernmomi, we believe that pumping shouldn’t hurt, and it certainly shouldn’t make you feel like you’re doing something wrong. Here’s why finding the right flange size is key to your pumping journey.
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What Happens When the Flange Doesn't Fit?

Too small? Your nipple may rub against the sides of the tunnel, causing pain, friction, and even damage over time.
Too large? Too much of your areola may be pulled in, leading to swelling, blocked ducts, or inefficient milk removal.
In both cases, the result can be:
• Nipple pain or trauma
• Decreased milk output
• Clogged ducts or mastitis
• Feelings of frustration or discouragement
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Everybody is Different

Just like bras or shoes, one size does not fit all. Flanges come in a range of sizes, and your perfect fit is based on your nipple size, not your breast size.
You may even need different sizes for each breast, or your size might change over time (especially in the early postpartum period, as your body adjusts). That’s why flange fitting isn’t a one-time thing—it’s something to check in with regularly.
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How to Know if Your Flange is the Right Size

When you're pumping, check for these signs of a good fit:
• Your nipple moves freely in the tunnel without rubbing.
• You don’t feel pain while pumping.
• You see steady milk flow after letdown.
• The areola stays mostly outside the tunnel.
If you're unsure, try different sizes or reach out for help. At modernmomi, we offer flange fitting advice and stock a wide range of flange sizes so you can find your perfect match.
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Download Your Free Nipple Ruler & Measuring Guide

Not sure where to start?
👉 Click here to download our Nipple Ruler and get easy measuring instructions.
Use it to measure your nipple diameter (in millimetres) and find your recommended flange size.
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📏 Guide: How to Measure Your Nipple Size

Here’s a step-by-step guide to help you find your perfect fit:

Step 1: Measure your nipple (not including the areola) after a pumping session or when it’s most relaxed.
Step 2: Use a ruler or printable nipple sizer to find your diameter in millimeters.
Step 3: Add 1–2mm to your measurement—this is your recommended flange size.
Nipple Diameter (mm) Recommended Flange Size
13–15mm 15–17mm
16–17mm 18–20mm
18–20mm 21–24mm
21–23mm 25–27mm
24–26mm 28–30mm
Note: Everyone is different—this is just a guide. Comfort is key!
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Don’t Settle for “Good Enough”

If pumping feels uncomfortable, you dread doing it, or you’re not getting the output you expect, don’t blame yourself. The problem may be as simple as the wrong flange size.
Taking the time to find the right fit can make pumping more comfortable, more efficient, and more sustainable, so you can feed your baby your way, without unnecessary stress.
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Happy pumping,
The modernmomi Team 💗