How to Measure Your Flange Size (and Pick the Right Insert)

Before you start

Measure before pumping or feeding, when the nipple is at its resting size. If you can, measure again right after a session: the difference tells you how much the nipple swells, and that matters for fit.

Use the silicone ruler from your kit, or any ruler with millimetre marks. Good light helps more than you would think.

Measure the nipple, not the areola

Place the ruler at the base of the nipple, where it meets the areola, and read the width straight across. Do not include the darker skin around it. If you are using the circle cut-outs, find the smallest circle that slides over the nipple without pressing.

Add two to four millimetres

Your flange tunnel should be a little wider than the nipple, so it can move freely as the pump pulls. A common starting point is nipple width plus 2–4mm:

• 11–12mm nipple → 13 or 15mm insert
• 13–14mm nipple → 15 or 17mm insert
• 15–16mm nipple → 17 or 19mm insert
• 17–18mm nipple → 19 or 21mm insert
• 19–20mm nipple → 21mm insert or the 24mm shield itself

Check the fit while you pump

A good fit looks like this: the nipple moves freely in the tunnel, little or no areola is drawn in, and nothing rubs. Afterwards the nipple should look round, not pinched or white at the tip.

If the areola is pulled deep into the tunnel, try one size down. If the nipple rubs on the sides, go one size up. This is why our kits carry two of every size — you are meant to experiment.

Sizes change

Many mothers measure differently in the first two weeks, again once supply settles, and sometimes between the two sides. Re-measure whenever pumping starts to feel different. If pain continues whatever size you try, an IBCLC can look at your latch and fit in person.