Breast Pump Flange Size Calculator
Enter the diameter of your nipple at its base - not including the areola - to find the flange (breast shield) size that will fit comfortably and pump most effectively. The calculator adds the standard 2-4 mm clearance and tells you which common flange sizes to try first. Switch between millimeters and inches anytime.
What is a breast pump flange and why does size matter?
A breast pump flange (also called a breast shield) is the funnel-shaped cup that fits over your nipple and areola to create the vacuum seal needed for milk expression. The tunnel portion, the cylindrical opening that your nipple slides into, must be sized precisely to your nipple. If the tunnel is too narrow, the nipple is compressed during suction and cannot move freely, causing pain, friction marks, reduced milk output, and sometimes bruising or blistering. If it is too wide, the areola gets drawn into the tunnel along with the nipple, creating a different kind of friction and discomfort and an incomplete seal. Getting the size right is one of the single most effective steps for comfortable, productive pumping.
How to measure your nipple diameter accurately
Measure the diameter of your nipple at its very base, which is where the nipple meets the areola, not across the areola itself. Use a ruler or the printable ruler tools that many pump brands provide. Stimulate the nipple gently first so it is erect, since a relaxed or inverted nipple may give a reading that is too small. Measure both nipples if there is any difference in size; most pumps allow each side to be fitted independently. Record your measurement in millimeters for the most accurate calculation, though this calculator will convert from inches if that is what you have. Nipple size can change during the course of breastfeeding, particularly in the first weeks, so remeasure if your flange starts feeling different from how it felt initially.
Signs your flange fits correctly vs. signs it does not
A correctly fitted flange feels comfortable from the first session onward. Your nipple moves freely back and forth in the tunnel with a small gap visible on each side. Only 2-3 mm of areola or less is drawn in at the peak of suction. There is no pain, redness, or white/purple discoloration of the nipple. Milk flows well and the breast feels softer after pumping. Signs the flange is too small include: nipple rubs the tunnel walls, nipple turns white or purple, friction marks or soreness around the nipple base, and low milk output despite adequate suction. Signs the flange is too large include: large amounts of areola being pulled in, a tugging sensation across the whole breast, gaps around the nipple, swelling after pumping, and milk that is not fully removed.
Elastic nipples and other fit challenges
Elastic nipples are a common reason why the standard +3 mm formula does not work right away. An elastic nipple may measure 18 mm at rest but expand to 24 mm or more once suction begins, meaning a 21 mm flange quickly feels tight. The fix is to start at the lower end of the range and size up only if you notice restriction during pumping, not at rest. Flat or inverted nipples often benefit from a breast pump that draws the nipple out before milk expression begins; the flange size should still be based on the diameter of the nipple once it is erect and extended. If your nipple is asymmetric or irregularly shaped, measure the widest dimension and use that as your base figure.
Common flange sizes by brand
| Brand | Available sizes (mm) | Standard included size |
|---|---|---|
| Medela | 13, 16, 19, 21, 24, 27, 30, 36 | 24 mm |
| Spectra | 20, 24, 28, 32 | 24 mm |
| Motif Luna | 21, 24, 27, 30 | 24 mm |
| Willow | 17, 21, 24, 27 | 24 mm |
| Elvie | 21, 24, 28 | 24 mm |
| Lansinoh | 21, 25, 28, 32 | 25 mm |
| Haakaa | 21, 24, 27 | 24 mm |
| Generic / universal | 13-40 (varies by retailer) | 24 mm |
Flange sizes available across major breast pump brands. Most brands include a 24 mm flange as standard. Sizes outside this table may require ordering separately.
Frequently asked questions
How many millimeters should I add to my nipple measurement?
Most lactation consultants recommend starting with 3 mm of clearance. So if your nipple is 18 mm across, begin with a 21 mm flange. If the nipple feels restricted after a few sessions, try 22 mm. If the areola gets pulled in, try 20 mm. The goal is to find the smallest size in which the nipple moves freely and pumping is comfortable.
What is the most common flange size?
The most widely used flange size is 24 mm, which is the standard flange included with most breast pumps. However, research and clinical experience suggest that many pumping parents actually need a smaller size, closer to 19-21 mm, because nipple diameters are often smaller than assumed. The fact that 24 mm is the default does not mean it is the correct size for everyone.
Can I use the same flange size for both sides?
Not necessarily. Nipple size often differs slightly between left and right, and some people need a different flange on each side. Measure both nipples separately and fit each side independently if they differ by more than 1-2 mm.
How do I know if my flange is too small?
If the flange is too small, the nipple is compressed against the tunnel wall during suction. Signs include: pain or soreness while pumping, a white or purple ring around the base of the nipple, friction marks or chafing, and lower milk output than expected. Moving up 1-2 mm usually resolves these issues quickly.
How do I know if my flange is too large?
If the flange is too large, a significant portion of the areola is pulled into the tunnel along with the nipple. Signs include: a tugging or pulling sensation across the breast rather than just at the nipple, visible areola swelling inside the tunnel, gaps around the breast where the seal is incomplete, and feelings of incomplete emptying. Dropping down 1-2 mm usually corrects this.
Does nipple size change during breastfeeding?
Yes, it can. In the early weeks of pumping the nipple can become slightly more elastic or larger due to repeated suction exposure. Some parents find they need to reassess their flange size after the first few weeks. If pumping that was comfortable becomes painful, or if milk output drops unexpectedly, remeasuring is worthwhile.
Are flange inserts or silicone inserts an alternative to buying a new flange?
Yes. Silicone inserts, also called flange inserts or reducers, fit inside a standard 24 mm flange and effectively reduce the tunnel diameter to 15, 17, 19, or 21 mm depending on the insert. They are a lower-cost way to get a smaller fit without buying a whole new flange, and many parents find them at least as comfortable as the hard plastic equivalents.