To find your flange size, measure the diameter of your nipple at its base in millimeters, then add about 4 mm. That final number is your starting flange size. The whole process takes less than a minute with a ruler or measuring tape, but getting it right makes a significant difference in comfort and milk output.
How to Measure Your Nipple
Place a ruler or flexible measuring tape across the widest point of your nipple at the base, where it meets the areola. You’re measuring just the raised nipple itself, not the darker areola surrounding it. Read the measurement in millimeters. Most nipples fall somewhere between 12 mm and 23 mm in diameter, though sizes outside that range are completely normal.
If your nipple isn’t perfectly round, measure at the widest point. Some people find it easier to use a printable nipple ruler (available free from most pump manufacturers) that has circles in millimeter increments. You hold it against your nipple and find the circle that matches. Either method works.
Once you have your diameter, add 4 mm. So a nipple that measures 16 mm across would start with a 21 mm flange. That extra space allows your nipple to move freely inside the flange tunnel without rubbing against the walls or leaving too much room.
When to Take the Measurement
Measure before a pumping or nursing session, when your nipple is at its resting size. After pumping, nipples swell from the suction and will give you an inflated number. That said, it’s worth checking how your nipple looks immediately after pumping too, because that tells you whether your current flange is pulling in too much tissue or compressing the nipple (more on that below).
Your nipple size can change over the first several weeks postpartum as swelling resolves and your body adjusts to regular milk removal. Remeasure if you notice a change in comfort or output, especially during the first two to three months.
What a Correct Fit Looks Like
When your flange size is right, you’ll see three things during pumping: only the nipple gets pulled into the tunnel (not the areola), the sides of the nipple lightly touch the tunnel walls, and the nipple gently glides back and forth with each suction cycle. There should be no pinching, no large gaps, and no feeling that the tissue is being squeezed or stretched.
Pumping with a well-fitted flange shouldn’t hurt. You might feel a tugging sensation, especially in the first minute, but sharp pain, burning, or soreness that lingers after the session all point to a sizing issue.
Signs Your Flange Is Too Small
A flange that’s too tight compresses the nipple against the tunnel walls. You may notice your nipple doesn’t move at all during suction, or that it turns purple or develops a pale white ring at the base. Over time, a too-small flange can cause cracks, blisters, and a noticeable drop in milk output, because the compressed tissue blocks milk ducts from draining properly.
Signs Your Flange Is Too Big
A flange that’s too large pulls areola tissue into the tunnel along with the nipple. You might see a gap between the flange and your breast, feel a deep tugging sensation, or notice swelling and discoloration around the areola after pumping. Milk output often drops here too, even if you increase suction, because the pump’s energy is being wasted on pulling in extra tissue instead of efficiently removing milk.
Sizing With Elastic Tissue
Some people have nipple tissue that stretches significantly under suction, sometimes elongating to fill the entire tunnel or expanding wider than the resting measurement suggested. This elastic tissue makes standard sizing trickier, because the nipple may measure one size at rest but behave like a different size during pumping.
If you have elastic tissue, measure at the widest point of your nipple, which might be the tip rather than the base once suction is applied. You may actually need a smaller flange than your resting measurement suggests, since a snugger fit prevents the tissue from over-stretching. Flange inserts can help here. These are small adapters that reduce the diameter of your existing flange without requiring you to buy a completely new set.
Avoid lubricating your nipple or flange with oils like coconut oil, as this can increase stretch and reduce how efficiently the pump removes milk. Some silicone flanges are shaped specifically to hold back surrounding breast tissue so only the nipple enters the tunnel, which can be a better option for elastic tissue than standard rigid flanges. Expect some trial and error, and remeasure before ordering replacement parts if your comfort or output changes.
Getting the Final Size Right
The “add 4 mm” rule gives you a reliable starting point, but flanges come in set increments (typically 21, 24, 27, and 30 mm for most brands), so you’ll round to the nearest available size. If you fall between two sizes, try the smaller one first. A slightly snug fit that allows gentle movement is generally better than one with too much extra room.
Keep in mind that your left and right nipples may not be the same diameter. Measure both. If they differ by more than 2 mm, you may need a different flange size for each side. Most pump manufacturers sell individual flanges so you can mix and match without buying two full kits.