If I could fix just one thing for the moms who come to me frustrated with pumping, it would be flange size. It's the single most overlooked variable in pumping comfort and output — and almost every pump ships with a flange that's too big for the average nipple. The good news: measuring takes two minutes, and the fix is inexpensive.
What a flange is (and what it does)
The flange (or "breast shield") is the funnel-shaped part that fits over your nipple and areola. During a pumping cycle, your nipple is drawn into the tunnel. The tunnel needs to be just wide enough for your nipple to move freely — without pulling in a lot of areola and without rubbing the sides.
Signs your flange is the wrong size
- Too big: too much areola is pulled into the tunnel; you feel tugging, rubbing or pinching.
- Too small: your nipple rubs the sides of the tunnel and turns white or feels painful.
- Either way: low output, sore or cracked nipples, and a nipple that looks blanched after pumping.
How to measure your nipple
Measure the diameter of your nipple only — the base, not including the areola — in millimeters, ideally right after a pumping or nursing session when it's at its fullest.
- Use a ruler or a printable nipple ruler.
- Measure straight across the base of the nipple in mm.
- Add roughly 2-3 mm to that number to choose your flange or insert size.
So if your nipple measures 15 mm, you'll likely be comfortable with a 17 mm opening. Most standard pumps ship with 24 mm or 28 mm shields — which is why so many people are, without realizing it, sized wrong.
Both sides can differ
It's completely normal for your left and right nipples to be different sizes. Don't be afraid to run two different insert sizes at the same time.
The easy fix: inserts instead of new shields
You don't need to buy a whole new set of shields. Silicone inserts drop into your existing 24 mm flange and step the opening down to a smaller size, so you can fine-tune the fit cheaply.
Shop: TheRelievoo All-Size Flange Inserts (10-pack, 13-21 mm) — $37.90 — free USPS tracked shipping, 30-day money-back and a lifetime warranty. Independently lab-tested for quiet, comfortable use.
A multi-size pack lets you test a couple of sizes and keep the ones that feel right. If you pump with a wearable like the TheRelievoo Wearable Breast Pump, correct sizing matters even more, because a good seal is what makes a hands-free, in-bra pump comfortable.
Re-check your size over time
Nipple size can change across your breastfeeding journey — often getting slightly smaller after the early engorged weeks. If output dips or comfort changes, re-measure. It's a two-minute check that can transform your sessions.
Frequently asked questions
How do I know my flange fits correctly?
Your nipple should move freely in the tunnel with only a little areola drawn in, and you should feel no pain.
What size flange do most people need?
Many people are more comfortable in the 15-21 mm range, smaller than the 24-28 mm shields most pumps include.
Can I use inserts with any pump?
Inserts are designed to fit standard 24 mm shields, which covers the majority of popular pumps, including wearables.
Get the fit right and everything else about pumping gets easier — more milk, less pain, fewer tears.
Written by Jessica Nolan, IBCLC — International Board Certified Lactation Consultant (Austin, Texas). Jessica is a mom of two who has supported 500+ American families with breastfeeding, pumping and the return-to-work transition. She is a member of the United States Lactation Consultant Association (USLCA) and works alongside local La Leche League USA groups.
Clinically reviewed by Dr. Rachel Adams, MD, FAAP (Austin, Texas). This article is educational and is not a substitute for advice from your OB-GYN, pediatrician or IBCLC.
