The right breast pump flange size is typically 4 to 6 millimeters larger than your nipple diameter at rest. Most pumps ship with a 24mm flange, but nipple sizes vary widely, and using the wrong size can cause pain, tissue damage, and reduced milk output. Here’s how to find your correct fit.
How to Measure Your Nipple
You’re measuring the diameter of your nipple at its base, where it meets the areola. Don’t include the areola itself. Measure before pumping or breastfeeding, when your nipple is at rest and not swollen or elongated.
Many flange brands include a printable ruler with an H-shaped cutout that you fold and slide around the base of the nipple. You can also use a standard millimeter ruler or a piece of string that you hold up to a ruler afterward. What matters is getting a measurement in millimeters. If your nipple base measures 16mm across, for example, you’d look for a flange in the 20 to 22mm range.
Standard Flange Sizes
Flanges and flange inserts are available from 10mm all the way up to 40mm. The most common sizes sold are 21mm, 24mm, 27mm, and 30mm. Since most pumps come with a 24mm flange by default, many people assume that’s the universal size. It isn’t. A significant number of people need something smaller or larger.
If your nipple measurement plus 4 to 6mm doesn’t land on an exact flange size, round to the nearest available option and check the fit using the signs below.
Signs Your Flange Is Too Small
A flange that’s too small forces your nipple against the sides of the tunnel with every suction cycle. The friction creates a recognizable set of problems:
- Rubbing or dragging of the nipple against the tunnel walls
- Bruising or cracking at the base of the nipple
- Painful pinching or squeezing during pumping
- White or discolored nipple after you remove the flange (a sign of restricted blood flow)
- Clogged ducts from incomplete milk removal
- Dropping milk output over time
If your nipple looks compressed or blanched white after a session, size up.
Signs Your Flange Is Too Large
An oversized flange pulls areola tissue into the tunnel along with the nipple. This spreads the suction over a wider area, which means less of it targets the nipple itself. The result is inefficient milk removal: your body doesn’t get the signal to keep producing, and supply can gradually decline. You may also notice areola swelling, discomfort around the breast tissue (rather than the nipple), or a sensation that the pump just isn’t “grabbing” effectively.
A well-fitting flange allows your nipple to move freely in the tunnel with a small amount of space around it. You shouldn’t see much areola tissue being pulled in, and pumping shouldn’t hurt.
What to Know About Elastic Nipples
Some nipples stretch significantly more than average during suction. This is called having elastic nipples, and it happens across all nipple sizes and shapes. If your nipple elongates dramatically in the tunnel and starts rubbing the sides or the end, a standard hard-plastic flange in your measured size may not work well even if the diameter is technically correct.
For elastic nipples, measure the widest part of your nipple carefully. For some people that’s the tip rather than the base, and this distinction can change which size you need. Silicone flanges or flanges with longer tunnels often help because they cushion the stretch and prevent the nipple from expanding too far into the tunnel walls. Bowl-shaped or crater-shaped flanges are designed to hold back surrounding breast tissue so only the nipple enters the tunnel, which improves the seal and comfort.
Using Flange Inserts to Adjust Size
If your pump came with a 24mm flange and you need something smaller, you don’t necessarily need to buy a whole new flange set. Silicone flange inserts fit inside your existing flange and reduce its internal diameter. They’re available across the full 10mm to 40mm range, so you can fine-tune in small increments without replacing hardware.
Inserts are also useful if you pump with multiple devices. Rather than buying correctly sized flanges for each pump, one set of inserts can adapt any standard flange to your size. They’re typically inexpensive and easy to clean alongside your other pump parts.
Your Size Can Change Over Time
Nipple size isn’t fixed throughout your pumping journey. Swelling during the early postpartum weeks, changes in breast tissue as supply regulates, and even weight fluctuations can shift your measurement. If pumping becomes uncomfortable after weeks or months of working fine, re-measure rather than pushing through the pain. Many people need to size down after the initial postpartum period as swelling resolves, and some need different sizes for each side.