How to Choose Breast Pump Flange Size
Choosing the correct breast pump flange size is one of the most important steps you can take for a comfortable, efficient pumping experience. Yet, it’s something many pumping moms overlook — often leading to pain, poor milk output, and frustration.
Your flange (also called a breast shield) plays a crucial role in how well your breast pump works. When it fits properly, it allows your nipple to move freely without rubbing or pulling too much areola into the tunnel. The result? Better suction, less discomfort, and more milk.
In this guide, you’ll learn how to measure your nipple correctly, how to pick the right flange size, and how to recognize the signs of a poor fit. We’ll also walk through a helpful sizing chart and simple fixes for common flange issues, so you can pump confidently and comfortably at every stage of your breastfeeding journey.
Prefer to watch instead? Check out my full video guide on choosing the right flange size here:
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What Is a Breast Pump Flange?
A breast pump flange — also known as a breast shield — is the cone-shaped part of your pump that fits over your breast. Its main job is to create a secure seal around your nipple and areola so that the pump can extract milk through gentle suction.
The flange guides your nipple into a tunnel during pumping, acting as a barrier to protect your breast tissue while your pump mimics the natural rhythm of your baby’s latch. It’s a simple but essential part of your breast pump setup.
Flanges are not one-size-fits-all. They come in a range of sizes, typically from 21mm to 36mm, measured by the diameter of your nipple. Choosing the correct flange size ensures that your pump can do its job without causing unnecessary strain on your body.
Why the Right Flange Size Matters
The size of your flange can make or break your pumping experience. A proper fit isn’t just about comfort — it directly affects how much milk you can express and how well your body responds to the pump.
When the flange is too small, it can compress your nipple, leading to pain, blocked ducts, and lower milk output. When it’s too large, it may pull in too much of your areola, which can reduce suction efficiency and cause swelling or soreness.
Here’s why getting the fit right is so important:
- Supports full and efficient milk removal
- Helps maintain or boost your milk supply
- Prevents nipple trauma, chafing, or bruising
- Reduces the risk of clogs, mastitis, or inflammation
Finding the right flange size sets the foundation for a successful pumping routine — one that supports both your physical comfort and your breastfeeding goals.
How to Measure Your Nipple Size for the Right Flange Fit
Finding the correct flange size starts with measuring your nipple accurately — and having the right tools makes the process much easier. You can use a soft measuring tape or a ruler marked in millimeters. Many pump brands, including Momcozy have a free flange sizing tool that comes with your pump, however you can also buy nipple measuring rulers online. A mirror can also be helpful to check your positioning as you measure.
For the most accurate measurement, begin by stimulating your nipple slightly or pumping for 30 to 60 seconds until it becomes erect. Then, measure the diameter of your nipple at the base — right where it meets the areola — in millimeters. Be sure to measure only the nipple itself and not any part of the surrounding areola. Since your nipples may vary slightly in size, it’s a good idea to measure both.
Once you have your measurement, add 3 to 4 millimeters to determine your starting flange size. For example, if your nipple measures 17mm, a 21mm flange is likely a good starting point. If you’re unsure or in between sizes, you can use a nipple sizing chart or download a printable flange ruler from your pump brand’s website to help guide you. Some moms also find silicone nipple sizing tools more accurate and easier to use than paper rulers.
It’s important to remember that your nipple size can change over time, especially in the early postpartum weeks. That’s why it’s helpful to remeasure periodically — particularly if you notice discomfort, friction, or a drop in milk output. Taking a few minutes to check your size can make a big difference in your comfort and pumping success.
Breast Pump Flange Size Chart (Based on Nipple Diameter)
| Nipple Diameter (mm) | Recommended Flange Size (mm) |
|---|---|
| 13 – 15 mm | 17 – 19 mm |
| 16 – 17 mm | 20 – 21 mm |
| 18 – 19 mm | 22 – 23 mm |
| 20 – 21 mm | 24 – 25 mm |
| 22 – 23 mm | 26 – 27 mm |
| 24 – 26 mm | 28 – 30 mm |
| 27 – 29 mm | 32 – 36 mm |
Tip: Measure the diameter of your nipple (excluding the areola) and add 3–4 mm to determine your starting flange size. A proper fit improves comfort and helps you express milk more efficiently.
Finding Your Perfect Flange Fit
Once you’ve measured your nipple and selected a starting flange size, the next step is testing how it fits during an actual pumping session.
Here’s how to assess and fine-tune your fit:
- Center the flange over your nipple, making sure your nipple sits comfortably in the tunnel before turning on the pump.
- Begin pumping on a low suction setting and observe how your nipple moves.
✦ Your nipple should move freely without rubbing against the sides.
✦ Minimal areola should be drawn into the tunnel — just enough for a gentle seal. - Watch for any signs of discomfort, pinching, redness, or decreased milk flow — these may indicate the flange is too small or too large.
- Adjust as needed:
- Size up if your nipple feels tight, rubs the sides, or turns white.
- Size down if too much areola is pulled in or suction feels weak.
- Test over multiple sessions, since breast tissue can vary in firmness and fullness throughout the day. What feels fine in the morning might need adjusting later on.
Remember, the goal is a pain-free, efficient pumping experience with steady milk flow and no nipple trauma. Don’t be afraid to try more than one size — many moms need to experiment before finding their ideal fit.
Signs Your Flange Size Isn’t Right
How can you tell if your breast pump flanges aren’t the right size? Your comfort and pumping efficiency depend on using the correct flange size, and there are clear signs when something’s not quite right.
Watch for these key indicators during your pumping sessions:
- Your nipples rub against the sides of the tunnel, causing friction and discomfort – this means the flange is too small.
- You see excess areola being pulled into the tunnel, suggesting the opening is too large.
- Your nipples don’t move freely in the tunnel or appear white/discolored during pumping.
- You’re experiencing pain or pinching sensations, which isn’t normal with proper fit.
If you notice any of these signs, it’s time to reassess your flange size and make adjustments for better pumping success.
Common Problems and Solutions for Flange Sizing
Even with careful measurements and proper testing, you might still encounter some common issues when using breast pump flanges.
When problems arise, you’ll want to identify and address them quickly to guarantee comfortable, effective pumping sessions.
Here are the most common flange-fitting issues and their solutions:
- Nipple rubbing against tunnel sides: Size up to the next flange size, as your current one is too small.
- Excessive areola being pulled in: Switch to a smaller flange size to prevent tissue damage.
- Pain during pumping: Check for proper centering and try lubricating the flanges with coconut oil.
- Poor milk output: Verify your flange size again, as an incorrect fit can reduce milk expression.
If you’re still experiencing discomfort or reduced milk flow, it might not just be your flange — worn pump parts can also affect performance. Here’s how often you should replace your pump parts.
FAQs About Breast Pump Flange Sizing
If your flange is too small, it can cause your nipple to rub against the sides of the tunnel, leading to pain, redness, and even nipple trauma. You may also experience clogged ducts or a drop in milk output because the milk isn’t being removed effectively.
Yes. If your flange is too large, it may pull in too much of the areola, which can reduce suction efficiency and lead to swelling, soreness, or bruising. A poor seal may also make it harder to fully empty your breast.
A properly fitting flange allows your nipple to move freely in the tunnel without touching the sides. Only a small portion of the areola should be pulled in, and you should feel comfortable — not sore or pinched — during pumping. Milk should flow steadily without pain or friction.
The most commonly used flange size is 24mm, but every mom is different. The right size for you depends on your nipple diameter, not your breast size. That’s why measuring is essential.
Yes, nipple size can fluctuate during your breastfeeding journey — especially in the early postpartum weeks. It’s a good idea to remeasure if you experience pain, reduced output, or physical changes in your breasts.
Yes, if your nipples measure differently, you may need two different flange sizes. Always fit each breast individually for the best comfort and performance.
Final Thoughts on Choosing the Right Flange Size
Finding the correct breast pump flange size may seem like a small detail, but it can make a big difference in your comfort, milk output, and overall pumping experience. From preventing nipple pain to helping you maintain your milk supply, the right fit truly matters.
By taking the time to measure your nipple accurately, test different flange sizes, and watch for signs of a good fit, you’ll set yourself up for pumping success — whether you’re exclusively pumping, building a freezer stash, or pumping occasionally.
Remember, your perfect flange size can change over time, so don’t hesitate to reassess if things start to feel off. Your body is unique, and your pumping gear should support your comfort every step of the way.
Not sure which breast pump is right for you? Be sure to check out our helpful guide on how to choose a breast pump — packed with tips to find the best fit for your lifestyle, budget, and feeding goals.
