If you're pumping and something feels off (pain, low output, persistent clogs, sore nipples between sessions, suction that pulls in too much tissue), there's a high chance the issue isn't the pump or your body. It's the flange size.
Most pumping parents are wearing flanges that are too big. The flanges that came with your pump are almost certainly not the right size for you. And the difference, when you fix it, is dramatic.
So if nobody has handed you this information yet, here it is.
Why this is the #1 unfixed pumping problem
Major pumps (Spectra, Medela, Motif, the wearables) ship with 24mm and 28mm flanges. Those numbers were standardized decades ago, before nipple sizing was understood as variable. The actual distribution of nipple sizes in adults runs from about 13mm to 30mm, with the median much smaller than 24mm.
Translation: the standard kit covers maybe the largest 30% of users. If you're not in that group, the box you opened contained none of the right parts.
Most parents don't know this. They struggle for weeks or months, blame the pump, blame their supply, blame their bodies, and eventually give up on pumping. It's not the pump. It's the fit.
What "right size" actually means
A flange's job is to draw the nipple (and only the nipple, not the surrounding areola) into the tunnel using suction. The right size:
- Pulls the nipple cleanly into the tunnel
- Leaves a small amount of clearance around the nipple as it moves in and out
- Pulls minimal areola in
- Doesn't rub the sides of the tunnel painfully
- Doesn't compress the base of the nipple as it enters
A flange that's too big pulls in too much areola, kneading the surrounding tissue with each cycle. This causes pain, blocks ducts (because the milk-producing tissue is being squeezed instead of stimulated), and reduces output.
A flange that's too small pinches the nipple as it moves in the tunnel. This causes tip pain, can damage tissue, and reduces output.
Most parents err big, because that's what came in the box. A few err small, because they read about flange-fit issues online, ordered something, and went too far the other way.
How to measure
You're measuring nipple diameter (the distance straight across the base of the nipple, not including the areola), in millimeters.
You'll need:
- A clear ruler or, ideally, a printable nipple measurement chart (free PDFs available from lactation supply sites like Pumpables and Legendairy Milk)
- A few minutes when your nipples are in their relaxed state (not erect, not after a feed)
The measurement:
- Look straight on at the nipple
- Measure the diameter at the base of the nipple where it meets the areola, not the tip
- Take the largest measurement in any direction (nipples are often slightly oval)
- Don't include the areola; only the protruding nipple
A more accurate read is to measure after a pump session, when the nipple is fully drawn out. That's the actual size your flange needs to accommodate during use.
Sizing from the measurement
The general rule:
Flange size = nipple diameter + 1 to 4 mm
So a 17mm nipple needs a 17mm to 21mm flange. Most fitters recommend starting at the lower end (nipple + 1–2mm) and only sizing up if the fit is too tight in motion.
Common sizes available from third-party sellers: 13, 15, 17, 19, 21, 24, 27, 28, 30mm. (The 24/27/28 standard kit sizes are the largest options, not the medians most people imagine.)
The most-commonly-needed real-world size in the EP community? 15mm to 19mm. Far smaller than the 24mm that came in the box.
After a fit change, output usually jumps within 2–3 days. Logging session-by-session volumes in Tottli makes the difference visible: if you don't see a change, the size still isn't right or there's another issue.
Asymmetry is normal
Many people have one nipple a different size than the other. It's worth measuring both. If they're more than 2mm apart, you may want different flange sizes on each side. Yes, this is annoying. Yes, the pump session will look mismatched. The output difference is usually worth the visual asymmetry.
Signs the fit is wrong
Even before you measure, a few signs the flange size is off:
Too big:
- Areola pulls into the tunnel along with the nipple
- Pain at the base of the nipple or in the surrounding tissue
- Persistent clogs in the same spot
- Output is low for the time spent
- Skin around the areola is red, irritated, or chafed
- The nipple "sloshes" loosely in the tunnel
Too small:
- Pain at the tip of the nipple
- White ring around the nipple after pumping
- The nipple looks compressed or pinched as it moves
- A burning sensation during pumping
- Reduced output
- Cracked, sore tip
Just right:
- Pumping is comfortable, not painful
- Output is steady
- Nipple moves freely in the tunnel
- Minimal areola pulled in
- After the session, the nipple is the same color as before
Where to get other sizes
Standard pump manufacturers don't make flanges in all sizes. Third-party brands have filled the gap:
- Pumpables (the "Liquid Shield" silicone insert and full flanges)
- Maymom (rigid hard-plastic flanges in many sizes)
- Beaugen (silicone soft flange inserts that fit inside larger flanges)
- Lacteck, Legendairy Milk, and others
Inserts are a popular option: they're soft silicone shapes that drop inside your existing 24mm/28mm flange and convert it to a smaller size (e.g., a 17mm). Cheaper than buying entirely new flange sets and useful when you're between sizes.
Some pump manufacturers (Spectra, Motif) sell smaller flanges directly. Medela's hard plastic line tops out at 24mm, but their PersonalFit Flex line goes down to 21mm.
Wearable pumps (Elvie, Willow, Momcozy) usually require pump-specific inserts or replacement cups in different sizes. Check the manufacturer's site.
The fitting process, in practice
If you're starting fresh:
- Measure. Both sides, before and after a session.
- Order two sizes: the calculated size and one size up. Inserts are cheap; this is worth doing.
- Pump a session with each. Feel which is more comfortable; note which produces more output.
- Repeat for 2–3 sessions before deciding. A single session can be misleading.
- Track the output difference. If size 17 produces 4 oz where size 24 produced 2 oz, you have your answer.
If you're pumping established and not getting what you expected:
- Re-measure first. Nipple size can change postpartum and may have shrunk from when you started.
- Try one size down from where you are.
- Keep notes for at least 3–4 sessions before evaluating.
When fit isn't the only issue
If you've fixed flange size and pumping is still painful or low-output, other things to check:
- Suction setting. Higher isn't better. The setting should be the highest one that's still comfortable.
- Cycle/speed setting. Most pumps have multiple modes (let-down, expression). Cycling between them mimics a baby's pattern and often increases output.
- Lubricant. A small amount of lanolin, coconut oil, or a designed pumping lube on the flange reduces friction. Many EPers swear by this.
- Pump parts. Old, stretched valves and membranes lose suction efficiency. Replace per the schedule.
- Pump motor. Heavily-used motors lose suction over time. A 12-month-old daily-use Spectra is not the same machine it was new.
A lactation consultant who specializes in pumping (not all do) can fit and troubleshoot in one visit. Many take insurance.
What changes when the fit is right
The first time someone gets the right flange size, the most common reaction is "oh, this is what it's supposed to feel like." Reports from newly-fitted pumpers:
- Output goes up, sometimes 50–100% in the first session
- Pumping stops hurting
- The skin around the nipple stops being red or irritated
- Persistent clogged ducts resolve
- Pumping sessions feel shorter
- Confidence about the whole feeding plan goes up
It's not a small fix. It's the difference between thinking "I'm bad at pumping" and "I'm a pumping parent and I have what I need."
What this comes down to
If you're pumping and something feels off, fix the flange first before you change anything else. Measure, get the right size, and try inserts if a full new flange feels like too much. The pump that came with a 24mm in the box is almost never the right size for the body using it. A $15 silicone insert can change everything.