To find your flange size, measure the diameter of your nipple at its widest point (usually the base) in millimeters, then add 1 to 3 mm. That final number is your starting flange size. Most nipples measure between 15 and 25 mm across, putting the ideal flange somewhere in the 16 to 28 mm range, though sizes outside that are completely normal.
How to Take the Measurement
You’re measuring just the nipple, not the areola (the darker skin surrounding it). Find the widest point, which is typically right where the nipple meets the areola at its base. Use a flexible measuring tape, a ruler with millimeter markings, or a printable nipple ruler (many pump brands offer these for free on their websites). If you have a pair of calipers, those give the most precise reading.
Place the ruler or tape across the base of your nipple and read the diameter in millimeters. Write it down. Then add roughly 1 to 3 mm to that number. The extra space accounts for the slight swelling that happens during suction and lets your nipple move freely inside the flange tunnel without rubbing.
For example, if your nipple measures 17 mm across, you’d start with a 19 or 20 mm flange.
When to Measure
Nipples expand during a pumping session, sometimes noticeably. A flange that looks like a perfect fit in the first minute can feel too tight five minutes later. Because of this, it helps to measure before pumping, use that number as your starting point, and then check the fit again partway through a session. If your nipple is pressing hard against the tunnel walls toward the end, you may need to size up.
It’s also worth measuring both sides. Many people have slightly different nipple sizes, and using two different flange sizes is more common than you’d think.
What a Correct Fit Looks Like
When the flange size is right, your nipple should gently touch the sides of the tunnel and glide a little bit back and forth with each suction cycle. There should be a small amount of space around the nipple, but not so much that large portions of your areola get pulled in. Pumping shouldn’t hurt once your letdown starts. You might feel tugging or pressure, but sharp pain, pinching, or a burning sensation all point to a sizing problem.
Signs Your Flange Is Too Small
If the flange is too small, your nipple rubs against the sides of the tunnel with every suction cycle. This friction causes redness, soreness, and sometimes cracking at the base of the nipple. You’ll usually feel it right away as a pinching or scraping sensation. Over time, the constant rubbing can damage skin and make pumping something you dread.
Reduced output is another clue. When the nipple is compressed inside a too-tight tunnel, milk ducts can’t empty efficiently, and you’ll get less milk per session than your supply would otherwise allow.
Signs Your Flange Is Too Large
A flange that’s too big pulls excess areola tissue into the tunnel along with the nipple. You’ll see a noticeable amount of the darker skin getting sucked in with each cycle, and the area around the base of your nipple may look swollen or puffy after you finish. Discomfort tends to show up as a deep aching sensation rather than the sharp pinching of a too-small flange. You may also notice swelling or redness around the areola after sessions.
Too-large flanges can also reduce output because the suction is spread across a wider area of tissue instead of being focused on the nipple and the milk ducts directly behind it.
Sizing With Elastic Nipple Tissue
Some people have nipple tissue that stretches significantly under suction. If you measure correctly, add the standard 1 to 3 mm, and still see a lot of areola being pulled into the tunnel, elastic tissue is likely the reason. Standard sizing advice doesn’t always work here.
The approach for elastic tissue flips the usual logic. Instead of adding millimeters, start with a flange that matches your nipple measurement exactly. If pumping is still painful or you’re still seeing excess tissue in the tunnel, try going down another 1 to 2 mm. The snugger fit holds the stretchy tissue in place and keeps the areola from being drawn in.
Silicone flange inserts can also help. These fit inside a standard hard-plastic flange and create a cushioned, closer fit that gently holds areola tissue back. They’re a practical option if your pump brand doesn’t sell flanges small enough or if you’re between sizes.
Sizing Can Change Over Time
Your flange size isn’t necessarily fixed for your entire pumping journey. Nipple size can shift during the postpartum period, especially in the first few weeks as your milk supply regulates. Weight changes, hormonal shifts, and even how frequently you pump can all affect nipple diameter over time. If pumping suddenly becomes uncomfortable after weeks of being fine, remeasure before assuming something else is wrong. A quick check with a ruler can save you a lot of frustration.