Find your correct breast pump flange size from your nipple diameter
| Nipple diameter | Typical flange size | Offset | Notes |
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| Flange size | Fits nipple | Availability | Notes |
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A flange that doesn’t fit is the single most common reason pumping hurts. Most pumps ship with a 24 mm shield as the default, but the average nipple measures closer to 15 to 17 mm. That gap turns a routine pumping session into cracked nipples, low output, and a lot of wasted time.
The fix is measuring. Not guessing from bra size, not assuming the default will work, not picking a size because it looked right in the box. Measure the nipple diameter at the base with a ruler in millimetres, then add 2 to 4 mm of clearance. That offset is what the flange tunnel needs so your nipple can move without dragging.
This calculator handles the arithmetic. You enter the measurement, choose a fit preference, and the tool returns the closest standard size. It also shows three offset options and a chart so you can see where you sit against every other size. It’s educational, not a substitute for a lactation consultant, but it’ll get you to the right starting point.
Measure your nipple diameter in millimetres at the base, where it meets the areola. Skip the areola itself.
Type the measurement into the first field. Anything from 5 to 40 mm works.
Choose a fit preference: snug adds 2 mm, standard adds 3 mm, roomy adds 4 mm. Standard is the default.
Read the hero panel. Your recommended flange size sits at the top in large type.
Compare the three option cards below: snug, standard, and roomy. The recommended one is highlighted in green.
Check the four tiles for the nearest standard size plus the acceptable range for each offset.
Scroll the reference table to see where your measurement falls against every nipple diameter band.
Re-measure the other nipple too. Left and right are frequently different sizes.
Formula: Recommended flange = nearest standard size to (nipple diameter + offset)
The offset is 2 mm for snug, 3 mm for standard, and 4 mm for roomy. Standard is the default because it’s what most lactation consultants recommend as a starting point.
The tool doesn’t create custom sizes. It snaps your target to the nearest value in a fixed list of standard sizes: 13, 15, 17, 19, 21, 24, 27, 30, 33, and 36 mm. That’s because flanges are manufactured in fixed increments, and a 20.4 mm flange doesn’t exist outside of custom orders.
Nipple inputs are clamped between 5 and 40 mm. The tool also categorises your measurement into one of six bands (very small through extra large) and adjusts the verdict text accordingly. Small nipples under 12 mm get a note about flange inserts. Large nipples above 28 mm get a warning about limited availability and a suggestion to check your pump brand’s accessory range.
The three option cards show what each offset produces, so you can see how the recommendation shifts if you choose a looser fit.
Suppose you measure your nipple at 16.5 mm across the base.
Target with standard offset: 16.5 + 3 = 19.5 mm
Nearest standard size: 19 mm (since 19 is closer than 21)
Snug option: 16.5 + 2 = 18.5 → nearest standard is 19 mm
Roomy option: 16.5 + 4 = 20.5 → nearest standard is 21 mm
The calculator would show 19 mm in the hero panel as your recommended flange. The snug card would also show 19 mm since 18.5 rounds to the same standard size. The roomy card would show 21 mm. The acceptable range tiles would show 15.5 to 19.5 mm for snug, 17.5 to 21.5 mm for standard, and 19.5 to 23.5 mm for roomy.
Try it during a full pumping session. If the 19 mm feels tight or leaves a pale ring around the base, size up to 21 mm. If the areola gets pulled into the tunnel, drop back to 17 mm or add a sizing insert inside a 24 mm shield. Fit is partly a matter of comfort, and the measurement gets you close before you have to guess.
Measure the diameter at the base, where the nipple meets the areola, in millimetres. Stimulate the nipple first so it stands out. Don’t measure right after pumping or breastfeeding, and don’t include the areola.
A 16 mm nipple typically fits a 19 mm flange. That’s a 3 mm offset, which is the standard recommendation. If the 19 mm feels tight, size up to 21 mm.
Only for nipples that measure around 20 to 23 mm at the base. That’s a smaller share of women than the 24 mm default suggests. Measure first. A 24 mm flange on a 15 mm nipple will hurt and reduce output.
Pain, cracking, blisters, or a pinched-looking nipple after pumping are the main signs. You may also see a pale ring around the nipple base and lower milk output. Size up one step and re-check.
Areolar tissue gets pulled into the tunnel, which hurts and reduces suction. You’ll see a wide discoloured ring around the areola after pumping, and the nipple may look swollen. Size down or add a flange insert.
Sometimes. A flange one size up may work if your nipple expands during let-down. If it pulls the areola in or hurts, use a flange insert to narrow the tunnel instead of sizing down.
Silicone rings that sit inside a larger flange and reduce the tunnel diameter. A 21 mm insert inside a 24 mm shield effectively makes a 21 mm flange. They’re cheaper than buying new flanges and easier to swap.
Yes. Nipples adjust in the first few weeks postpartum, and fit can shift with each pregnancy, weight change, or supply fluctuation. Re-measure every few weeks in the early months.
Measure both and use separate flanges for each side. Left and right are often one or two millimetres different, and using the same flange on both causes problems on the larger side.
Yes. A poor fit reduces how efficiently the pump removes milk, which signals the body to produce less over time. Fixing the flange size often restores supply within a few days.
21 mm fits nipples measuring about 17 to 20 mm. 24 mm fits 20 to 23 mm. Both are common alternatives to the default. If you’re between them, use a sizing insert or try the larger one first.
Often yes. Left and right nipples are frequently different sizes. Measure both and order the size that fits each side. If you’re close, a single size may work for both.
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