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 is one of the most common causes of 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. Most pumps come with 24 mm flanges, but many people need something smaller.

How to measure your nipple for a flange

1. Use millimeters. A nipple ruler (many pump brands have printable or free ones) or a ruler with a millimeter scale works. 1 inch is 25.4 mm.
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 your starting flange size. The calculator above picks the closest common size and shows the alternatives.

Flange size chart

Nipple diameter	Starting flange size	Also try
11 mm	13 mm	15 mm
13 mm	15 mm	17 mm
15 mm	17 mm	19 mm
17 mm	19 mm	21 mm
18 mm	21 mm	–
19 mm	21 mm	–
20 mm	24 mm	–
21 mm	24 mm	25 mm
22 mm	24 mm	25 mm
23 mm	25 mm	27 mm
25 mm	27 mm	28 mm
26 mm	28 mm	30 mm

Flange sizes usually come in steps of 2 to 4 mm, and sizes from different brands can fit slightly differently. 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 and tugging, but not pain.
• Your breast feels softer after pumping and output is steady.

Signs it’s too big: a lot of areola gets pulled in, swelling or a ring around the nipple after pumping, pain, or less milk than expected. Signs it’s too small: the nipple rubs or gets stuck against the tunnel walls, redness, blisters or cracks on the sides of the nipple.

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.