Measure your nipple across the base, in millimetres, and pick your pump. You get the flange size to start with — in the sizes your pump’s maker actually sells, by the maker’s own rule — every other published rule beside it, and the fit check that decides between two sizes. The rules do not agree: Medela adds 4 mm, Spectra 2–3 mm, Cleveland Clinic 2–4 mm. You see all of them.
A 17 mm nipple
About 19–21 mmCleveland Clinic’s rule: nipple + 2 to 4 mm
What each published rule says
Then check the fit while you pump
A measurement is where fitting starts, not where it ends — the maker’s own guide and how pumping actually feels decide. Pain, cracks, blisters, a white or purple nipple, or milk that will not come: see a lactation consultant (IBCLC) or your doctor. This tool is not medical advice.
Rules and sizes checked 2026-09-25.
Quick answer: measure your nipple — not the areola — straight across its base in millimetres, then add a few millimetres. How many depends on whose rule you follow: Medela adds 4 mm and takes the closest size it makes, Spectra adds 2–3 mm, Cleveland Clinic says 2–4 mm, and Lansinoh says one size up. So a 17 mm nipple starts at about 19–21 mm. Then the fit while you pump decides: the nipple centred and moving freely, no rubbing, and little or no areola pulled in.
How to use it
Type your measurement in millimetres (17, or 3/4 in if your ruler is in inches), and the other side too if it is different. Pick your pump, and you get the starting size that brand makes, by that brand’s own rule — Elvie and Momcozy publish full charts, which are used as printed. Below the answer, every published rule is set side by side for your measurement, and the fit check tells you whether to go up or down a size. The result is in the link, so Copy link keeps it.
How to measure your nipple for a flange
Medela’s instructions, which every other maker’s echo: “use a ruler or measuring tape to figure out the diameter size of your nipple at the base (across middle) in millimeters (mm). Remember not to include your areola.” Measure each side — many people need a different size per breast — and measure again every so often: Medela says your size “could change over the duration of your pumping experience.” Nipples also swell while pumping, on average 2–3 mm (Medela), which is the reason every rule starts larger than the measurement.
Why the rules disagree — and why you see all of them
Each maker writes its rule for its own flanges, which come in different steps: Spectra in 4 mm steps (16, 20, 24, 28, 32 mm), Medela in 3 mm steps (18 to 30, then 36), Ameda in steps as fine as 1.5 mm. A rule of “add 2–3 mm” only works if a size exists in that window. When your brand makes nothing inside its own range, the finder shows the size on each side and starts you on the roomier one, then tells you what to look for to come down — because the fit, not the arithmetic, is the final answer.
Signs the flange is the wrong size
Too small (Cleveland Clinic): pain or tenderness in the nipple, a nipple that does not move in the tunnel, cracks or blisters, a purple nipple or a pale ring at its base. Too big: the areola pulled into the tunnel, a tugging pain, gaps between the breast and the flange, or a sore or discoloured ring around the areola. Right (Medela): comfortable during pumping, the nipple moves freely, and milk flows freely after let-down.
What this tool will not do
It does not diagnose anything. Pumping should not hurt; if it does — or if you see cracks, blisters or a white or purple nipple, or the milk will not come — see a lactation consultant (IBCLC) or your doctor, who can check the fit in person.
Pumping guides on Best & Right
Choosing the pump itself: the best-rated breast pumps. Storing what you pump: breast milk storage bags — and bottles for breastfed babies for feeding it.