Breast Pump Flange Size Calculator
Use this breast pump flange size calculator to turn your nipple diameter in mm into a flange size range and a good size to try first.
Updated October 22, 2026
Medical Disclaimer: This calculator provides a general estimate based on established research and is for educational purposes only, not a substitute for medical advice. Individual results can vary based on health conditions, so consult a qualified healthcare professional or registered dietitian before making significant changes to your diet, exercise, or health routine. Read our full Medical Disclaimer.
How Breast Pump Flange Size Is Calculated
The flange (also called a breast shield) is the cone-shaped part of a pump that sits on your breast. Its size is the width of the tunnel your nipple moves into, in millimeters. A breast pump flange size calculator, sometimes just called a flange size calculator, adds a small gap to your nipple measurement so the nipple has room to move without rubbing.
What Flange Size Measures
Flange size is the inside width of the tunnel, not the width of the whole cone. It is matched to the nipple only, not the areola (the darker skin around it). It says nothing about milk supply or breast size.
The Formula
Pump makers give slightly different rules. Spectra says to add 2 to 3 mm to your nipple diameter¹. Medela says to add 4 mm and pick the closest size². Cleveland Clinic, quoting a breastfeeding medicine doctor who is also an IBCLC (a board certified lactation consultant), gives a range of 2 to 4 mm³.
Flange range = nipple diameter + 2 mm, up to nipple diameter + 4 mm
Starting size = the common size in that range closest to nipple diameter + 3 mm
This is manufacturer guidance, not a tested formula. Cleveland Clinic notes there is no evidence-based formula for flange size, so the number is a starting point³.
How This Calculator Computes Your Result
Enter your nipple diameter in millimeters for one side, or both sides if they differ. The tool adds 2 and 4 mm to give your range, then lists the common sizes inside it (13, 15, 17, 19, 21, 24, 27, 30, 32 and 36 mm). It suggests the size closest to the middle of your range, picking the smaller one if two are equally close. It also shows one size down and one size up to try.
What Your Result Means
Find your nipple measurement in the first column. For half millimeters, use the calculator above.
Nipple diameter | Flange range | Starting size |
12 mm | 14 to 16 mm | 15 mm |
13 mm | 15 to 17 mm | 15 mm |
14 mm | 16 to 18 mm | 17 mm |
15 mm | 17 to 19 mm | 17 mm |
16 mm | 18 to 20 mm | 19 mm |
17 mm | 19 to 21 mm | 19 mm |
18 mm | 20 to 22 mm | 21 mm |
19 mm | 21 to 23 mm | 21 mm |
20 mm | 22 to 24 mm | 24 mm |
21 mm | 23 to 25 mm | 24 mm |
22 mm | 24 to 26 mm | 24 mm |
23 mm | 25 to 27 mm | 27 mm |
24 mm | 26 to 28 mm | 27 mm |
25 mm | 27 to 29 mm | 27 mm |
26 mm | 28 to 30 mm | 30 mm |
27 mm | 29 to 31 mm | 30 mm |
28 mm | 30 to 32 mm | 30 mm |
29 mm | 31 to 33 mm | 32 mm |
30 mm | 32 to 34 mm | 32 mm |
Brands make sizes in different steps. Medela, for example, makes 18, 21, 24, 27, 30 and 36 mm shields, plus a 15 mm insert², so pick the size your own pump offers that falls in your range. Many pumps come with a "medium" flange of about 24 mm³. If your nipple measures under 20 mm, the flange in the box may be bigger than you need.
How to Measure Correctly
Measure before you pump or feed, when the nipple is at rest. Gently rolling it between your fingers for a few seconds helps it stand out³.
Use a ruler with millimeter marks, or a printable sizing gauge from your pump brand¹ ³. If you print a gauge, print it at 100% scale and check it against a real ruler.
Measure straight across the base of the nipple, through the middle. Do not include the areola² ³.
Measure both sides. They are often a little different, and each side can have its own flange¹ ⁵.
Spectra advises not measuring during pregnancy¹, since hormones change nipple size during pregnancy and after birth³. Size can also shift as your milk supply settles, around 10 weeks after birth, so measure again if pumping starts to feel different¹.
How to Check the Fit While Pumping
Your measurement gets you close. How the flange feels with the pump running tells you the rest, because nipples swell during pumping, by 2 to 3 mm on average and up to 8 mm in some cases².
What you see or feel | What it may mean |
Nipple sits in the center and moves freely, little or no areola pulled in, milk flows, pumping feels comfortable | Good fit |
Nipple rubs the sides of the tunnel, turns purple, shows a pale ring at its base, or feels sore | Flange may be too small |
A lot of areola is pulled into the tunnel, the nipple or areola swells, or a sore ring forms | Flange may be too large |
These signs come from Medela, Cleveland Clinic and the IBCLC-written Flange FITS Guide² ³ ⁵. Low milk output can point to either problem³. The Flange FITS Guide suggests trying two or three sizes during one session to compare how each feels⁵.
Why Newer Advice Leans Smaller
Some lactation consultants now fit flanges closer to the nipple than the 2 to 4 mm rule. In a 2024 pilot study, parents pumped for a week with standard sized flanges and a week with smaller, closely fitted ones⁴. The standard flanges gave less milk (about 15 g less on average) and were less comfortable⁴. The study was small, and the smaller sizes were chosen by measuring the nipple tip, not the base. Still, if your starting size feels loose, trying the next size down is reasonable.
Flange Range and Starting Size by Nipple Diameter
The shaded band is the 2 to 4 mm range. The line is the common size the calculator suggests first.
- Starting size
- Flange range
Show data table
| Nipple diameter (mm) | Starting size (mm) | Flange range (mm) |
|---|---|---|
| 12 | 15 | 14–16 |
| 13 | 15 | 15–17 |
| 14 | 17 | 16–18 |
| 15 | 17 | 17–19 |
| 16 | 19 | 18–20 |
| 17 | 19 | 19–21 |
| 18 | 21 | 20–22 |
| 19 | 21 | 21–23 |
| 20 | 24 | 22–24 |
| 21 | 24 | 23–25 |
| 22 | 24 | 24–26 |
| 23 | 27 | 25–27 |
| 24 | 27 | 26–28 |
| 25 | 27 | 27–29 |
| 26 | 30 | 28–30 |
| 27 | 30 | 29–31 |
| 28 | 30 | 30–32 |
| 29 | 32 | 31–33 |
| 30 | 32 | 32–34 |
A Note on Accuracy
This calculator gives a starting point, not a final answer. The 2 to 4 mm rule comes from pump makers and clinicians, not a tested formula. Your nipple changes size during pumping and in the weeks after birth, and a small measuring error can shift your size by one step. If pumping hurts at any size, a lactation consultant (IBCLC) can watch you pump and help you find the right fit.
Frequently Asked Questions
How do I measure my nipple for flange size?
Measure across the base of your nipple in millimeters before you pump, without the areola. Then add 2 to 4 mm to get your flange range.
What flange size is normal?
There is no single normal size, because flanges are matched to your own nipple. Many pumps come with about a 24 mm flange, but sizes run from about 13 mm to over 30 mm³.
How do I know if my flange is too big or too small?
If the nipple rubs the tunnel walls, the flange may be too small. If a lot of areola is pulled into the tunnel or the areola swells, the flange may be too big² ³.
Is a breast pump flange size calculator accurate?
It is a good first guess, but the rule is based on manufacturer guidance rather than research³. How the flange feels and how well milk flows while pumping are the real test.
Sources
Flange Sizing Guide, Spectra Baby USA
Breast Shield Sizes, Medela
How To Find Your Breast Pump Flange Size, Cleveland Clinic, 2026
Flange Size Matters: A Comparative Pilot Study, Anders et al., 2024
The Flange FITS Guide, Mesite Frem, 2023