How to Measure Flange Size

To measure flange size, measure the diameter of your nipple at the base in millimeters using a ruler or measuring tape, exclude the areola entirely, and add the specific allowance published by your pump manufacturer. At New Parent Picks, we evaluate infant gear by comparing published manufacturer specifications, user manuals, and sizing documentation across feeding systems.

Because breast pump brands publish different sizing formulas, the same nipple measurement points to different flange sizes depending on the pump you own. Measuring both breasts with a physical ruler helps you match the shield tunnel to your specific anatomy.

Measuring Your Nipple Step by Step

Measuring your nipple requires taking the diameter across the base in millimeters while leaving out the surrounding tissue. You will need a standard millimeter ruler or a flexible sewing measuring tape.

1. Position the ruler across the base: Place the zero mark of the millimeter ruler against one edge of your nipple base, directly where the raised nipple tissue meets the areola.
2. Measure straight across the center: Measure across the widest point of the center of the nipple to the opposite edge of the base.
3. Exclude the areola completely: Count only the raised nipple tissue. Do not include the darker circular skin of the areola in your millimeter count.
4. Measure both sides: Measure the left and right breasts separately. Spectra notes that the two sides may not be the same size, so each breast gets its own measurement.
5. Record the number in millimeters: Write down the exact measurement for each side before applying your pump maker’s sizing math.

Do not measure your nipples while you are pregnant. Spectra’s guide says not to measure your nipples while pregnant, so wait until after the birth.

Why Flange Size Is a Pump Fit Question

Flange sizing is a mechanical equipment fit question that determines how your breast tissue interacts with the collection container during a pumping session. Breast pump flanges, also called breast shields, are the part of the pump that sits against the breast.

Both Medela and Spectra publish sizing guides, and both describe a good fit the same way: the nipple moves freely in the tunnel. Spectra adds that not much areola should be drawn into the tunnel. If you are comparing primary pump units, our guides to the best breast pump options and the breakdown between Spectra vs Medela compare the two brands.

Converting Your Measurement: Medela vs Spectra Allowances

You cannot apply a single universal formula to your nipple measurement because Medela and Spectra instruct parents to add different millimeter allowances. Each brand calculates the space needed for tissue expansion differently.

According to the official Medela breast shield sizing guide, you must add 4 mm to your measured base diameter to find your starting size. You then choose the closest available breast shield size in Medela’s lineup. For example, if your nipple base measures 15 mm across, you add 4 mm to reach 19 mm. Because 19 mm is not among Medela’s listed sizes, you select the closest available size, which is 18 mm. Medela produces its PersonalFit Plus and PersonalFit Flex breast shields in six distinct sizes: 18 mm, 21 mm, 24 mm, 27 mm, 30 mm, and 36 mm. Medela also lists inserts in 15 mm, 18 mm, and 21 mm.

In contrast, the Spectra flange sizing guide instructs parents to add 2 to 3 mm to their measured nipple diameter. Under Spectra’s published formula, a 16 mm nipple measurement maps to a 19 mm or 20 mm flange size. Spectra Baby USA does not publish its full list of retail flange sizes on its sizing guide page, so you must verify current size availability directly on the company’s product listings.

Sizing Feature	Medela Sizing Rule	Spectra Sizing Rule
Allowance to Add	Add 4 mm to base nipple diameter	Add 2 to 3 mm to base nipple diameter
Brand Example	15 mm nipple + 4 mm = 19 mm (select 18 mm)	16 mm nipple + 2 to 3 mm = 19 to 20 mm flange
Shield Sizes Published	18 mm, 21 mm, 24 mm, 27 mm, 30 mm, 36 mm	Sizes not listed on sizing guide (check parts listing)
Inserts Listed	15 mm, 18 mm, 21 mm	Not listed on the sizing guide
Areola Guidance	Exclude areola from measurement	Ensure minimal areola enters tunnel

Because the published allowances differ by 1 to 2 mm, a person with a 15 mm nipple measurement would start with an 18 mm shield under Medela’s rules, but would seek a 17 mm to 18 mm shield under Spectra’s guidelines. Use the arithmetic that matches the brand of pump you own.

Signs the Flange Fits

Both major pump manufacturers publish clear physical indicators to help users confirm that a chosen breast shield fits correctly during an active pumping session. Observing your tissue while the motor runs shows whether your millimeter calculation translates into a comfortable physical match.

Medela outlines three distinct criteria for a proper breast shield fit:

• Comfortable during pumping: The pumping session feels comfortable while the vacuum cycles.
• Centering and free movement: Your nipple is centered in the shield tunnel and moves freely without scraping the sides.
• Free milk flow: Milk flows freely from the breast after let-down occurs.

Spectra outlines similar equipment checks on its guide page. Spectra states that with the right size, your nipple moves freely inside the flange tunnel and not much areola tissue is drawn into the tunnel alongside it.

When to Re-Measure

Your flange size is not permanent and can shift across the weeks and months that you pump milk. Both brands treat sizing as something to recheck, not a one-time setup step.

Spectra notes in its sizing documentation that flange size may change once your milk production is well established, which Spectra puts at roughly 10 weeks in. A measurement taken in the first days after delivery may therefore need repeating once that point passes.

Medela advises parents to re-evaluate their fit after every pumping session. The manufacturer notes that nipples can expand by 2 to 3 mm during active pumping, and breasts change over the course of lactation. If a shield that fit comfortably during the first month begins to pinch or pull excess areola later on, re-measure your nipple base with your millimeter ruler to see if your allowance has shifted up or down.

Wearable and Hands-Free Pumps Use Their Own Sizes

Wearable pumps do not necessarily use the same sizes as countertop pumps, so check the listing for the model you own.

Medela publishes a separate sizing lineup for its Hands-Free In-Bra collection cups. While standard Medela PersonalFit Flex shields span six sizes up to 36 mm, Medela manufactures its Hands-Free In-Bra collection cups in four sizes only: 18 mm, 21 mm, 24 mm, and 27 mm. Those hands-free sizes stop at 27 mm, so check Medela’s listing against your own measurement.

Other wearable manufacturers publish their own distinct sizing systems and cup geometries. If you use integrated in-bra pumps, our comparison of Willow vs Elvie compares two wearable pumps. Always consult the specific instruction manual for your wearable model before ordering third-party inserts or collection parts. You can read our broader guide to the best wearable breast pump models to compare cup profiles, or review how to choose a breast pump to evaluate whether standard bottles or wearable cups match your daily schedule.

When to Ask a Lactation Consultant

This guide covers product sizing specifications and manufacturer fit guidelines only. It does not provide medical advice, diagnose physical conditions, or assess clinical lactation questions.

If pumping is persistently painful, you see tissue damage, or you have ongoing concerns about your milk output, reach out to an International Board Certified Lactation Consultant (IBCLC) or your healthcare provider. A clinical lactation specialist can evaluate your latch, assess your pumping mechanics in person, and help troubleshoot underlying physical issues that sizing charts cannot address.

You can review manual extraction tools in our best manual breast pump overview, compare motor platforms in our Spectra S1 vs S2 breakdown, or explore coverage options through our guide on getting a breast pump through insurance. Measure your nipple diameter with a millimeter ruler today and apply your pump brand’s allowance to order the correct flange size for your next session.

Frequently Asked Questions

What flange size comes with a Spectra pump?

Spectra’s official flange sizing guide does not list which specific flange sizes ship inside the box with its pumps. Check the exterior label on your pump’s packaging or consult your user manual to verify which millimeter shield sizes came with your unit.

Does flange size change over time?

Flange size can change. Spectra notes that nipple size can shift once milk production is well established, which Spectra puts at roughly 10 weeks in. Medela also notes that breasts change throughout lactation and nipples can expand 2 to 3 mm during active pumping sessions, which requires re-measuring if your current parts feel uncomfortable.

Should I measure the areola?

You should not measure the areola when sizing a breast pump flange. Medela specifies measuring the nipple’s diameter at the base and excluding the areola, and Spectra’s guide says the right flange draws in little areola. Measuring the areola would answer a different question than the one the sizing guides ask.

Do Medela and Spectra size the same?

Medela and Spectra do not size the same because they instruct users to add different allowances to their measurements. Medela instructs parents to add 4 mm to their nipple base diameter and choose the closest available shield. Spectra instructs parents to add 2 to 3 mm, which means identical nipple measurements yield different recommended shield sizes across the two brands.

Can I use a Spectra flange on a Medela pump?

Direct cross-brand compatibility between Spectra flanges and Medela pumps is not specified in the official sizing guides published by either manufacturer. Check each manufacturer’s documentation or contact customer support directly to confirm connector compatibility before attempting to swap collection hardware.

Sources

1. Medela breast shield sizing guide (medela.com)
2. Medela (medela.com)
3. Spectra flange sizing guide (spectrababyusa.com)
4. Spectra Baby USA (spectrababyusa.com)