How this calculator works
Medela adds 4 mm to nipple diameter and selects the closest listed size as a starting fit check. Plus/Flex shields span 18 to 36 mm, Hands-Free In-Bra shields 18 to 27 mm, and inserts 15 to 21 mm, with the individual listed sizes used here. The target must fall within the selected family range. Equal distances are identified in the note, with the lower value displayed. Measure each nipple without the areola and assess fit during pumping. This guide does not apply to other manufacturers. The result is an estimate for information only and is not medical advice.
Formula
Target diameter = nipple diameter + 4 mm. Select the listed size minimizing absolute difference from the target; report both sizes when tied.
Worked example
A 15 mm nipple gives a 19 mm target and a closest Plus/Flex size of 18 mm. A 17 mm nipple gives a 21 mm target and size.
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Frequently asked questions
Does this give a final fit?
No. Check comfort, nipple movement and milk flow during pumping. Reassess if fit changes or pumping hurts.
Do I include the areola?
No. Measure the nipple at its base, excluding the surrounding areola.
Can both sides need different sizes?
Yes. Assess each separately. Confirm insert and pump compatibility in the product instructions.