Flange Size Calculator: How to Measure Your Breast Pump Flange Size

The flange is the funnel-shaped part of a breast pump that fits over your breast, and its size matters more than most people expect. The wrong flange size can cause painful pumping and lower milk output. This flange size calculator gives you a starting size for each side from a simple nipple measurement. It works with any pump brand.

Flange sizes are measured in millimeters and refer to the width of the tunnel your nipple goes into. Many pumps come with a standard medium flange of about 24 mm, but one size doesn’t fit everyone.

How to measure your nipple for a flange

1. Use millimeters. A nipple ruler or a ruler with a millimeter scale works; 1 inch is 25.4 mm. You can print our nipple ruler: it has a straight millimeter ruler, a circle gauge from 11 to 26 mm and a credit-card box to check it printed at true size.
2. Measure before pumping. Pumping can make your nipple swell temporarily. Some people find it easier to measure after gently rolling the nipple so it stands out; just be consistent.
3. Measure the base of the nipple, not the areola. Measure straight across where the nipple meets the darker skin around it.
4. Measure both sides. It’s common for the two nipples to be different sizes.
5. Add 2 to 4 mm. That gives the range to try. The Cleveland Clinic notes that most people find this the most comfortable fit, but it is a rule of thumb, not an evidence-based formula. The calculator above picks the smallest common size in the range and shows the alternatives.

Flange size chart

Nipple diameter	Range to try (nipple + 2–4 mm)	Starting flange size
11 mm	13–15 mm	13 mm
13 mm	15–17 mm	15 mm
15 mm	17–19 mm	17 mm
17 mm	19–21 mm	19 mm
18 mm	20–22 mm	21 mm
19 mm	21–23 mm	21 mm
20 mm	22–24 mm	24 mm
21 mm	23–25 mm	24 mm
22 mm	24–26 mm	24 mm
23 mm	25–27 mm	25 mm
25 mm	27–29 mm	27 mm
26 mm	28–30 mm	28 mm

The starting size is the smallest common flange size in the range. The list of common sizes the calculator uses (13, 15, 17, 19, 21, 24, 25, 27, 28, 30, 32 and 36 mm) is our own selection from sizes pump makers and insert brands sell; some brands also make sizes in between. Sizes from different brands can fit slightly differently, and inserts can make a large flange smaller.

Signs your flange fits well

• Your nipple moves freely in the tunnel, without rubbing the sides.
• Little or no areola is pulled into the tunnel.
• Pumping is comfortable: you feel suction but no pain.
• You can express milk effectively, and your nipple isn’t swollen, cracked or bruised after pumping.

Signs it’s too big: a lot of areola gets pulled in, swelling or a discolored or sore ring around the areola, pain or a tugging feeling, or little milk even with more suction. Signs it’s too small: the nipple doesn’t move freely in the tunnel, pain or tenderness, cracks or blisters, a purple nipple or a pale ring at its base, or little milk.

There’s no exact formula that works for everyone. The calculator gives you a starting point; if pumping still hurts, try the next size, check your suction setting (comfortable, not as strong as possible), and ask a lactation consultant for a fitting.

Why flange fit matters

A good fit protects your nipples and helps the pump remove milk well, which protects your supply. Pain while pumping is not something you have to push through: see our guide to sore nipples. If your output is lower than you hoped, a better flange fit is often the first thing to check, before trying power pumping. Our pumping guide covers pump types, cleaning and pumping at work, and our pumping schedule guide helps you plan your sessions.

If nothing seems to fit, or pumping is painful whatever size you try, an IBCLC lactation consultant can measure you and check your pump: see our breastfeeding support directory.