If your nipple measures 16mm in diameter, you’ll typically need a flange size between 19mm and 21mm, depending on the brand. The general rule is to add 2 to 4mm to your nipple measurement, which puts you well below the 24mm flanges that come standard with most breast pumps. Getting the right size matters more than many new pumpers realize: a properly fitted flange is more comfortable, protects your skin, and can noticeably increase how much milk you express.
Why the Standard Flange Won’t Work
Most breast pumps ship with a 24mm or 27mm flange. For a 16mm nipple, that’s significantly too large. When a flange is oversized, your areola gets pulled into the tunnel during suction, which causes pain, tugging sensations, and swelling around the nipple after pumping. You may also notice gaps between your breast and the flange, and milk output often drops even when you crank up the suction. Many people assume they need more suction when output is low, but the real problem is frequently the fit.
How Brands Size Differently
There’s no universal flange sizing standard across brands, which is why you’ll see a range of recommendations for a 16mm nipple.
Medela recommends adding 4mm to your nipple diameter. For a 16mm nipple, that puts you at their 21mm shield. Spectra recommends adding 2 to 3mm, which points to a 19mm or 20mm flange. Both approaches are valid, and the difference comes down to how each brand designs their tunnel shape and suction mechanics. A good starting point is to try sizes in the 19 to 21mm range and pay attention to how your nipple moves inside the tunnel.
What a Correct Fit Looks and Feels Like
With the right flange, the sides of your nipple should lightly touch the walls of the tunnel while still gliding back and forth with each suction cycle. Pumping should be comfortable, not painful, and milk should flow easily. According to guidance from the University Hospital Southampton, a flange closest to your actual nipple size tends to feel best and gets the most milk out. Many people also find they can pump for a shorter time and still get the same volume, or more, compared to using an oversized flange.
If your nipple can’t move at all inside the tunnel, the flange is too small. Signs include sharp pain, cracks or blisters on the nipple, purple discoloration, or a pale ring at the base. If your areola is being sucked in and you see swelling afterward, it’s too big.
Using Inserts Instead of Buying New Flanges
You don’t necessarily need to buy a whole new set of flanges. Silicone flange inserts slide into your existing 24mm shield and narrow the tunnel to a smaller diameter. Insert kits commonly come in a range of sizes from 13mm all the way to 22mm, and many include a specific 19mm or 20mm option that would work well for a 16mm nipple. These are affordable, widely available, and let you experiment with small size differences without committing to a full flange purchase each time.
How to Measure Accurately
Your flange size is only as good as your measurement. Measure the diameter of your nipple at its widest point, not including the areola. Do this after stimulating the nipple so it’s erect, since that’s the state it’ll be in while pumping. You can use a printable nipple ruler, a millimeter measuring tape, or one of the card-style guides with graduated holes that let you find the smallest circle your nipple fits through comfortably.
It’s also worth measuring both sides. Many people have slightly different nipple sizes on each breast and may need different flange sizes for each.
If Your Nipple Tissue Is Elastic
Some nipples stretch significantly during pumping, pulling further into the tunnel than their resting measurement would suggest. If this sounds like you, measure at the widest point, which may be the base rather than the tip. You might also benefit from a slightly snugger fit than the standard formula suggests, since a tighter tunnel helps prevent the tissue from stretching too far.
Flange inserts made of silicone can help hold elastic tissue back so only the nipple enters the tunnel. Some brands also make angled or bowl-shaped flanges designed specifically for this issue. One thing to avoid: lubricating your nipples or flanges with oils like coconut oil, which can increase stretch and actually reduce how efficiently milk is removed.
Finding Your Size in Practice
Start with a 19mm or 21mm flange (or the equivalent insert for your pump) and pump for a full session. Watch how your nipple behaves in the tunnel. If it moves freely with gentle contact on the sides, milk flows well, and you’re comfortable, you’ve found your size. If something feels off, try one size up or down. Flange fitting is partly trial and observation, and your size can even change over the course of your breastfeeding journey as tissue adapts to regular pumping.