Breast Pump Flange Sizing Guide: How to Measure for the Perfect Fit (17–28mm)
If pumping feels pinchy, sore, or you're emptying slowly and ending up with less milk than you'd hoped, the culprit is very often the same simple thing: your breast pump flange size is wrong. It's one of the most overlooked parts of pumping, yet getting the fit right can be the difference between a frustrating session and a comfortable, productive one. The good news is that flange sizing isn't complicated once you know what to measure and what a good fit actually feels like.
In this guide I'll walk you through exactly how to measure your breast pump flange at home, share a simple sizing chart you can use as a starting point, and help you troubleshoot the tell-tale signs of a flange that's too big or too small. We'll also cover why so many mums need something other than the standard 24mm flange, and how a pump that includes a range of inserts (like the Relievoo wearable) makes it far easier to dial in your fit without buying extra parts.
Why the right breast pump flange size matters for comfort and output
The flange (sometimes called the shield or funnel) is the cone-shaped part that sits against your breast, with the tunnel your nipple is drawn into during pumping. When the size is right, your nipple moves freely in and out of that tunnel, the suction works efficiently, and your milk ducts can drain properly. When it's wrong, you get friction, pinching, blocked drainage, and reduced output — and over time that can knock your supply and leave you sore.
A correctly fitted flange should draw the nipple into the tunnel freely, without pulling in lots of areola and without pain. You want a little movement, not your nipple rubbing the sides or a big chunk of areola being sucked in. Comfort is the headline goal, but it's also closely tied to how well you empty — and a well-emptied breast is what protects your supply.
It's about your nipple, not your bra size
Here's the part that surprises most mums: flange sizing has nothing to do with your cup or bra size. It's based purely on the diameter of your nipple — not your areola, and not your breast. Two mums with very different breast sizes can need the exact same flange, and one mum can even need two different sizes for her left and right side. So please don't assume the size in the box is "your" size. The standard pump usually ships with a 24mm flange, but a great many mums actually fit smaller (around 17–21mm) once they measure.
How to measure your breast pump flange size at home
You only need a couple of things: a ruler or measuring tape marked in millimetres (mm), or a printable nipple ruler. Measure when your nipple is at its everyday resting state rather than fully erect, and measure both sides separately. Here's the step-by-step:
- Get set up. Find a private, comfortable spot and have your mm ruler ready. Good lighting helps.
- Measure the nipple only. Place the ruler across the base of your nipple — where the nipple meets the areola — and measure straight across the diameter (the width). Do not include any of the surrounding darker areola.
- Note the number in millimetres. For example, your nipple base might measure 18mm across.
- Measure the other side. Repeat on your other breast — it's normal for the two to differ by a millimetre or two, and occasionally more.
- Add a few millimetres for the flange size. Your nipple needs a little room to move inside the tunnel, so the recommended starting flange is generally your measured nipple diameter plus roughly 2–3mm. So an 18mm nipple often starts around a 20–21mm flange or insert.
- Test and adjust. A measurement gives you a starting point, not a final answer. Pump for a few minutes, watch how your nipple moves, and adjust up or down based on comfort and how freely the nipple glides (more on the signs below).
A quick tip: it's easiest to measure just after a feed or pump, when your nipple is naturally a bit more drawn out, and to take the measurement at the base rather than the tip.
Breast pump flange sizing chart
Use the chart below as a starting point. Find your measured nipple diameter, then look across to the suggested flange or insert size. Remember to add a couple of millimetres of room — that's already factored into the suggestions here — and always let comfort and free nipple movement be the final word.
| Measured nipple diameter | Suggested flange / insert size |
|---|---|
| Up to ~14mm | 17mm insert |
| ~15–16mm | 19mm insert |
| ~17–18mm | 21mm insert |
| ~19–21mm | 24mm flange (standard) |
| ~22–24mm | 27mm flange |
| ~25–26mm | 28mm flange |
| 27mm and above | Larger sizes available (30mm+) |
As you can see, the popular 24mm flange suits a fairly narrow band of nipple sizes. Many mums measure smaller and do far better with a 17, 19 or 21mm insert, while others need a 27mm breast pump flange or a 28mm breast pump flange for a comfortable, free-moving fit. There's nothing "wrong" with needing a larger or smaller size — it's simply anatomy, and the whole point of a flange sizing chart is to get you to your comfortable size faster.
Signs your flange is the wrong size
Numbers are a guide, but your body is the real test. Once you're pumping, watch what your nipple is doing inside the tunnel and notice how you feel. These signs will tell you whether to size up or down.
Signs your flange is too big
- A lot of your areola is being pulled into the tunnel, not just the nipple.
- Your nipple rubs or the areola feels tugged and uncomfortable.
- You notice a "donut" of redness or swelling around the base of the nipple afterwards.
- Output feels slow because the extra tissue is interfering with good suction.
If that sounds familiar, try a smaller flange or insert.
Signs your flange is too small
- Your nipple rubs painfully against the sides of the tunnel.
- The tip of the nipple turns white, looks blanched, or feels pinched.
- There's friction, a burning sensation, or visible redness on the nipple itself.
- Your nipple can't move freely in and out — it feels stuck or squeezed.
In that case, try sizing up to the next flange or insert.
What a good fit feels like
With the right size, your nipple glides smoothly in and out of the tunnel with the rhythm of the pump, only a small amount (or none) of your areola is drawn in, and the whole thing is comfortable — no pinching, rubbing, whitening or pain. Your breasts feel softer and well-drained afterwards. If pumping ever genuinely hurts, that's your cue to stop and reassess the fit rather than push through.
How Relievoo's included 17–24mm inserts help you dial in the fit
One of the most practical features of the Relievoo Wearable Breast Pump is that it doesn't lock you into a single size. It comes with a standard 24mm shield plus 17mm, 19mm and 21mm inserts in the box, so you can step down to the size that genuinely suits you without ordering extra parts or waiting for a delivery. For the many mums who measure on the smaller side — which, as the chart shows, is a lot of us — that range covers the most common fits straight away.
Because the Relievoo is a double, cordless, in-bra wearable, you can also fit each side independently if your left and right differ slightly — pop a 19mm insert on one side and a 21mm on the other, for instance. With 4 modes and 9 suction levels (up to 300 mmHg max), you can pair the right flange with a suction level that feels comfortable rather than relying on raw power to compensate for a poor fit. It's TGA approved, BPA-free and made from food-grade silicone, runs quietly at under 35 dB, and each cup holds up to 180ml/6oz — so once your fit is sorted, you can pump discreetly almost anywhere.
If you measure outside the included insert range — say you're sitting in 27mm breast pump or 28mm breast pump territory — that's still completely normal, and it just means you'll want to source the larger flange size that matches your measurement. The principle is exactly the same: comfort, free nipple movement, and minimal areola drawn in. You can always reach for the right starting point in the chart above and fine-tune from there, and the Relievoo wearable pump comes with a 30-Day Money-Back guarantee and Free Lifetime Warranty on the motor, plus free AU shipping in 3–5 days, so you've got room to get the fit right.
Frequently Asked Questions
How do I measure my breast pump flange size at home?
Use a ruler marked in millimetres and measure the diameter (width) across the base of your nipple only — not the areola. Then add roughly 2–3mm to allow your nipple room to move inside the tunnel. Measure both breasts separately, as it's normal for them to differ slightly, and treat the result as a starting point you fine-tune by comfort.
What size breast pump flange is "standard"?
Most pumps ship with a 24mm flange as the default, which is why so many mums assume that's their size. In reality, plenty of mums fit smaller — around 17 to 21mm — while others need a larger 27mm or 28mm flange. The right size depends entirely on your nipple diameter, not your bra or breast size.
How do I know if my flange is too big or too small?
If a lot of areola is being pulled in, your nipple rubs, or you see a ring of redness around the base, the flange is likely too big — try a smaller size. If your nipple rubs painfully against the sides, looks blanched or white at the tip, or feels pinched, it's likely too small — try sizing up. A good fit lets the nipple glide freely with little or no areola drawn in, and no pain.
Does Relievoo come with different flange sizes?
Yes. The Relievoo Wearable Breast Pump includes a 24mm shield plus 17mm, 19mm and 21mm inserts, so you can match most common nipple sizes straight out of the box and even fit each side differently if needed — no extra parts to buy.
Can my left and right breast need different flange sizes?
Absolutely. It's very common for one nipple to be slightly larger than the other, so it's worth measuring each side. With a double wearable pump like the Relievoo, you can use a different insert on each side to keep both comfortable and pumping efficiently.
What if I measure larger than 24mm?
That's perfectly normal — many mums need a 27mm, 28mm or larger flange. Use the sizing chart as your starting point and source the matching size, then fine-tune by comfort and how freely your nipple moves. The goal is the same at every size: free nipple movement, minimal areola drawn in, and no pinching or pain.
