To measure your flange size for a Spectra breast pump, you need to measure the diameter of your nipple in millimeters, then add 2 to 3 mm. That final number is your flange size. For example, if your nipple measures 16 mm across, you’d look for a 19 or 20 mm flange. The process takes just a few minutes, but getting it right makes a real difference in comfort and milk output.
How to Measure Your Nipple
You’re measuring only the nipple itself, not the areola (the darker skin around it). The goal is to find the widest point of your nipple’s diameter. For most people, that’s the base where the nipple meets the areola, but for some it may be closer to the tip.
The easiest tool is a printable nipple ruler, which several pump brands offer as free downloads. These typically have a series of circles you cut out and slide over your nipple to find the closest match. Print at 100% scale on standard 8.5 x 11 inch paper, or the sizing will be off. A small metric ruler or digital caliper also works. Hold it across the base of your nipple and read the measurement in millimeters.
Measure when your nipple is at rest, not erect or cold. Do not measure while pregnant, as your nipples will change size during that time. And always measure both sides. Your breasts may not be the same size, which means you could need a different flange for each.
Calculating Your Flange Size
Once you have your nipple measurement, add 2 to 4 mm. That range gives you a starting point, not a guaranteed perfect fit. A Cleveland Clinic specialist describes it like shoe shopping: you need to try sizes and see what actually works. If your nipple is 17 mm, your likely flange size falls between 19 and 21 mm. Spectra’s own guidance suggests adding 2 to 3 mm specifically.
Spectra flanges come in several standard sizes, typically ranging from 20 mm to 28 mm, with smaller and larger options (like 16 mm and 32 mm) available as accessories. If your calculated size falls between two available options, it’s generally better to try the smaller one first, since a too-large flange causes more problems than one that’s slightly snug.
What a Correct Fit Looks Like
When you pump with the right size flange, two things should happen. First, your nipple moves freely inside the flange tunnel without rubbing against the sides. Second, only a small amount of areola gets pulled into the tunnel during suction. You should feel a tugging sensation, but not pinching or pain.
If the flange is too small, your nipple will drag against the tunnel walls. This creates friction, leads to soreness, and can reduce how much milk you express. You may notice your nipple looks white or blanched after pumping, which signals the tissue is being compressed.
If the flange is too large, too much areola gets sucked in. This feels uncomfortable, can cause swelling at the base of the nipple, and often means poor milk removal because the suction isn’t focused where it needs to be. You might also notice your nipple bouncing around loosely in the tunnel rather than centering naturally.
Sizing With Silicone Flanges or Inserts
If you’re using silicone flanges or flange inserts instead of the standard hard plastic flanges that come with most Spectra pumps, you’ll likely need to size down. Silicone is softer and more flexible, so it conforms to your skin differently. Many pumping parents report needing to drop 1 to 2 mm compared to their hard plastic size. If you wear a 19 mm hard plastic flange, for instance, a 17 mm silicone flange may be a better match.
Flange inserts are a cost-effective alternative to buying entirely new flanges. They sit inside your existing flange and reduce the tunnel diameter. This is especially useful if your Spectra pump came with 24 mm flanges (a common default size) and you need something smaller. Inserts are available in small increments, making it easier to fine-tune the fit.
Elastic Nipple Tissue
Some people have elastic nipple tissue, meaning the nipple stretches significantly during pumping, sometimes elongating well beyond what you’d expect from the resting measurement. If your nipple seems to fill the entire tunnel or gets pulled deep into the flange, elasticity may be the issue, even if your static measurement suggested the right size.
With elastic tissue, a smaller flange often works better because it limits how far the nipple can stretch. This protects the tissue and can actually improve milk output. Avoid using oils like coconut oil on or around the nipple before pumping, as lubrication can increase stretch and reduce pumping efficiency. A lactation consultant can help identify elastic tissue and recommend the right combination of flange size, inserts, and technique.
When to Remeasure
Your flange size isn’t permanent. Nipple size can change over the course of your pumping journey, particularly in the early postpartum weeks as swelling goes down and your body adjusts to regular pumping. If you notice new discomfort, decreased output, or visible changes in how your nipple looks after a session (redness, creasing, blanching), it’s worth remeasuring and trying a different size. Many people go through two or three flange sizes before finding what works long-term.