How breast pump flange sizing works
A flange (also called a breast shield) is the funnel-shaped piece that fits over the nipple and areola during pumping. Its tunnel diameter has to match the nipple closely enough that the nipple can move freely with each suction cycle, but not so loosely that too much areola is pulled in or so tightly that the nipple rubs the tunnel wall. Getting that fit right affects comfort and how efficiently milk is removed.
How the recommended size is calculated
Lactation consultants and pump manufacturers use a simple, widely taught rule: measure the nipple's diameter at its base, in millimeters, then add a small clearance margin. For a standard rigid flange, add 3 to 4 millimeters; for a flexible or silicone-insert flange, which compresses slightly during use, add only 1 to 3 millimeters. This calculator applies that rule to your measurement and reports the resulting range.
Reading your result
The "recommended size" is the midpoint of that range, and the "sizing range" shows the min and max in millimeters. Because commercially available flanges only come in specific sizes (commonly around 13, 15, 17, 19, 21, 24, 27, and similar millimeter increments depending on the brand), the calculator also rounds to the closest widely sold size from that list — treat it as a starting point to try, not a guaranteed match for every brand's exact sizing.
When to consult a professional
This tool applies the standard sizing rule to your measurement; it does not replace a hands-on assessment. If pain, reduced milk output, or visible changes to the nipple continue after trying the recommended size, an IBCLC lactation consultant can do an in-person fitting and check for other causes.