Flange Size Calculator

Learning how to measure flange size starts with the nipple, not the breast: measure the diameter of the nipple at its base, while erect, in millimetres (a ruler or caliper works; do not include the areola). Add 2-4 mm to that number and pick the nearest available flange size. Worked example: a 17 mm nipple + 3 mm = 20 mm target, which matches a 20 mm Spectra flange exactly. Always measure both sides separately - most people need two different sizes - and re-measure if pumping becomes painful or output drops, because nipple size changes over time.

Spectra flange sizes are 16, 20, 24, 28 and 32 mm, and Spectra pump kits ship with 24 mm + 28 mm flanges in the box. Worked example: a 21 mm nipple + 3 mm = 24 mm target, which matches the 24 mm Spectra flange exactly. If your target falls below 16 mm, third-party inserts that fit inside a 24 mm Spectra flange (for example 17 or 19 mm) are the usual next step before buying a whole new pump kit.

The Medela flange size chart (PersonalFit) is: 21 mm (S), 24 mm (M), 27 mm (L), 30 mm (XL) and 36 mm (XXL). Medela's official fitting bands are: nipple up to 17 mm -> 21 mm (S); up to 20 mm -> 24 mm (M); up to 23 mm -> 27 mm (L); up to 26 mm -> 30 mm (XL); up to 32 mm -> 36 mm (XXL). Worked examples: a 22 mm nipple falls in the up-to-23 band, so the chart gives 27 mm (L); a 16 mm nipple falls in the up-to-17 band, so the chart gives 21 mm (S).

Willow's official chart maps nipple size to flange or insert: 9-11 mm -> 13 mm insert, 12-13 mm -> 15 mm insert, 14-15 mm -> 17 mm insert, 16-17 mm -> 19 mm insert (all inserts are used inside a 24 mm flange), 18-20 mm -> 21 mm flange, 21-23 mm -> 24 mm flange, 24-26 mm -> 27 mm flange. Worked examples: a 15 mm nipple -> 17 mm insert in a 24 mm flange; a 22 mm nipple -> 24 mm flange. Wearables fit snugger than traditional pumps, so Willow recommends adding only 1-3 mm to the nipple measurement.

Flange too big symptoms include: a lot of areola being pulled deep into the tunnel, the nipple moving freely but milk output lower than expected, the areola looking swollen or feeling sore after a session, and air leaking around the flange with a poor seal. If most of your symptoms match this list, try one size down or a sizing insert. Persistent pain or falling output deserves a check with an IBCLC.

Breast pump flange too small signs include: the nipple rubbing the tunnel sides while pumping, pain, friction or a burning sensation, the nipple looking white or pale (blanching) or swollen afterwards, a white ring forming at the base of the nipple, and frequent clogged ducts or dropping output with no other cause. Size up to the next flange and re-check the fit; do not keep pumping through pain - ask an IBCLC or lactation professional for a fitting.

Yes - this page includes a nipple ruler printable scale in millimetres (0-40 mm). Print it at 100% scale (no 'fit to page') and check the scale against a known reference before measuring. To convert nipple size to flange size, measure the base diameter of the erect nipple, then add 2-4 mm. Worked example: a 14 mm nipple + 3 mm = 17 mm target, which matches a 17 mm insert/flange exactly on the generic size ladder.

Standard pipe flange dimensions follow ANSI/ASME B16.5 (the b16.5 flange dimensions reference): outside diameter, minimum thickness, raised-face diameter, bolt-circle diameter, number of bolt holes and bolt-hole diameter, per nominal pipe size (NPS 1/2 to 10 in) and pressure class (150 to 2500). Worked example - 2 inch 150# flange dimensions: outside diameter 6 in (152.4 mm), 4 bolt holes of 3/4 in (19 mm) on a 4-3/4 in (121 mm) bolt circle, minimum thickness 3/4 in (19 mm, raised face included), raised-face diameter 3-5/8 in (92.1 mm).

This page is an ANSI flange size chart covering classes 150, 300, 400, 600, 900, 1500 and 2500 for NPS 1/2 through 10. The '#' (or 'lb') number is the pressure class, not a pressure rating in psi - class 150 is roughly a 285 psi service at ambient temperature. 150# flange dimensions example - 4 inch class 150: outside diameter 9 in (228.6 mm), 8 class 150 flange bolt holes of 3/4 in (19 mm), bolt circle diameter 7-1/2 in (190 mm). For the flange bolt circle diameter chart of any size, pick the size and class above. Note: minimum thickness includes the 1/16 in raised face for classes 150-300, but excludes the 1/4 in raised face for classes 400-2500.

The insert vs flange size decision is simple: sizing inserts (13, 15, 17 and 19 mm) drop inside a standard 24 mm flange to make its tunnel smaller, so they are the cheaper fix when your 24mm flange insert size target is close to 24 mm. Buy a dedicated smaller flange when the gap is large or inserts feel unstable. Worked example: a 14 mm nipple + 3 mm = 17 mm target -> a 17 mm insert (or 17 mm flange) is the exact match. This flange fitting guide for breast pumps answers 'what size flange do I need' for Spectra, Medela, Motif, Willow and generic pumps in the calculator above. Keep this flange fitting guide breast pump page bookmarked and re-measure any time your fit changes.