Flange Size Measurement: How to Measure at Home
To find your flange size, measure the width of each nipple tip (not the areola) in millimetres, before pumping, after gently touching or tugging the nipple so it stands out. That number is a starting point. The right flange is the one where the nipple moves freely in the tunnel, pumping is comfortable and milk flows easily. This guide covers how to measure, how to read the result, how to check the fit and what to do when the numbers do not match how pumping feels.
TL;DR
- Measure the width of the nipple tip, not the areola, on each side separately, before you pump.
- Use a ruler or measuring tool marked in millimetres, with the zero at one edge of the nipple.
- Treat the measurement as a starting point. Try two or three sizes and judge by comfort and milk flow.
- A correct fit lets the nipple glide in the tunnel without rubbing, and pumping should not hurt.
- Fit can change over time, so check again if comfort or output drops.
Table of Contents
- What flange size actually means
- What you need to measure
- How to measure your nipple for a flange, step by step
- From measurement to flange size
- How to check the fit
- Signs the flange is too small
- Signs the flange is too large
- When your measurement falls between sizes
- Why your size can change
- Our view: measure, then trust the test
- Flange sizes for Wren pumps
- A quick home routine
- FAQ
- Sources
What flange size actually means
The flange (also called a breast shield) is the funnel-shaped part that sits on the breast. Its size is the internal diameter of the tunnel, the narrow part the nipple moves into, given in millimetres. Common sizes sit in steps of a few millimetres, for example 17 mm, 21 mm, 24 mm or 25 mm, depending on the pump brand. Sizes are not identical across brands, so a 21 mm flange from one maker is not guaranteed to feel the same as another.
The size matters because the nipple moves in and out of the tunnel during pumping. The NHS University Hospital Southampton Flange FITS guide says the best fit should be comfortable and produce plenty of milk, and that a flange that is too small or too large can reduce output. The same guide recommends judging the result by your own comfort and flow, not by the measurement alone.
Many pumps ship with one standard size in the box. That size suits some people and not others, which is why measuring and testing is worth ten minutes.
What you need to measure
You do not need special equipment.
- A small ruler or measuring tape marked in millimetres. A clear plastic ruler lets you see the nipple through it.
- A printable flange sizing chart or a sizing tool from your pump maker, if you have one. Some brands include these.
- A mirror, or a partner or friend, if you find it hard to line things up.
- Your phone, to note the numbers for each side.
A piece of paper with a hole of known size can work as a rough check, but a ruler is simpler and gives you an actual number.
How to measure your nipple for a flange, step by step
The method below follows the NHS Southampton Flange FITS guide.
- Get comfortable. Sit somewhere warm and relaxed, with your bra off.
- Help the nipple stand out. Gently touch or tug the nipple so it protrudes.
- Place the ruler. Put the zero mark next to one edge of the nipple tip. The tool does not need to press on or touch the nipple.
- Read the width. Look across to the opposite edge of the nipple tip and note the number in millimetres.
- Measure the tip only. Do not include the areola, the darker skin around the nipple.
- Repeat on the other side. Write down both numbers separately.
Measure each side separately because left and right nipples can differ. It is common to need two different flange sizes, one per breast.
Before or after pumping?
Measure before pumping. The guide uses the pre-pump measurement to estimate which sizes to try. A nipple can swell during or after a session, so a measurement taken straight after pumping may read larger than your starting width and lead you to a flange that is too big.
Worked example
Say you measure 16 mm on the left and 18 mm on the right. You would not pick a single size on the numbers alone. You would choose a few flanges around each figure, test them, and probably end up with a different size per side, or one size that works acceptably on both. The numbers narrow the search. The test decides.
From measurement to flange size
How much larger than your nipple the flange should be is where guides disagree. Advice you will see online ranges from adding a couple of millimetres to adding a few more. The NHS guide does not rely on a single formula. It suggests trying hard-plastic flanges in a few sizes: one a little smaller than your nipple width, one about the same and one a little bigger, then comparing comfort and milk output.
The reason is practical. The nipple needs space to move without rubbing the tunnel walls hard, but not so much space that areola is drawn in. Your own nipple tissue, how firm or elastic it is and how it responds to suction all affect where that sits, so a table of sizes cannot replace a trial.
A sensible way to use your measurement:
| Your measurement | What to try first |
|---|---|
| Matches a flange size on your pump | That size, plus the next size up and next size down |
| Falls between two sizes | Both neighbouring sizes, then compare |
| Differs left to right | Test each side separately, and expect to possibly use two sizes |
| Smaller than the smallest size in the box | An adaptor or smaller flange from your pump maker |
If your pump brand publishes a sizing chart, use that for the brand’s own size labels, because the same millimetre figure can be built differently from one maker to the next.
How to check the fit
Once you have a flange to try, check how it behaves on the breast. The Southampton guide describes this check:
- Centre the nipple in the tunnel.
- Start on a low vacuum setting.
- Watch the nipple while the pump runs.
A correct fit looks like this: the nipple touches the sides of the tunnel lightly and glides gently back and forth. It feels comfortable, and milk comes out easily.
If you use a wearable pump you cannot see the nipple as easily during a session. Do your fit test with the flange held against the breast before you fit the pump into the bra, or check your bra position afterwards. Our guide to pumping under clothes covers the fit side of that.
Signs the flange is too small
According to the NHS Flange FITS guide, a flange that is too small restricts the nipple’s movement, and you may get less milk or none. Look out for:
- The nipple does not move freely in the tunnel.
- The nipple rubs on the tunnel walls and feels pinched or sore.
- Milk output is lower than expected, or stops.
Signs the flange is too large
The same guide says that when a flange is too large, pumping may hurt, the nipple may swell and milk output may fall. Areola can be drawn into the tunnel and swell as well. Signs include:
- Pain during or after pumping.
- A lot of areola pulled into the tunnel.
- A swollen nipple after the session.
- Output that drops without another explanation.
None of these proves the size is the only problem. Suction strength, flange angle, pump parts and the position of the pump all play a part. If pumping hurts, lower the suction before anything else, and see our guide to comfortable suction strength.
When your measurement falls between sizes
This happens a lot. The guide’s advice is to try two or three hard-plastic flange sizes and compare comfort and milk output. It also suggests trying different materials and speaking to an experienced lactation professional if you are not sure.
In practice:
- Try each size for a full session, not just a few seconds.
- Change one thing at a time. If you swap flange and suction together, you will not know which made the difference.
- Note comfort and how much milk you expressed.
- Keep the size that is comfortable and gives easy flow.
If you want to check the fit with your breastfeeding team, ask your midwife, health visitor or local infant feeding team. Many NHS trusts publish their own expressing guidance, and Southampton’s expressing milk page explains why correctly fitted equipment matters.
Why your size can change
The size that works in the first week may not be the size that works in month three. Nipples and breasts change as milk supply settles, and regular pumping itself can change how the nipple responds. The simplest rule is to measure again, and re-test the fit, whenever pumping becomes uncomfortable or output drops without a clear cause. Our page on a wrong flange size and how to fix it walks through that check.
If you are pumping and also nursing, soreness can come from latch as well as flange. Reviewing breastfeeding latch positions can help you separate the two. For soreness itself, see our notes on sore nipples while breastfeeding and how to soothe them, and the postpartum breast care guide.
Our view: measure, then trust the test
We make wearable pumps, so we see flange fit questions all the time. Our view is simple.
A measurement gets you to the right neighbourhood. It does not name the answer. The people who end up happiest with their pump are usually the ones who measured, tried two or three sizes, and then went with what felt comfortable, not the number on the ruler.
That is why a wearable pump should let you swap flanges. Each Wren pump comes with three different flanges that can be swapped in or out to help you get a good fit, and we sell separate sizes so you are not stuck if the standard one does not suit you. Our guide on choosing the right flange for a Wren pump explains how the sizes work with the pump.
One more point. With a wearable pump, bra fit matters as much as flange fit. A flange that is the right size but sits off-centre will feel wrong. Check the nipple is centred first, then judge the size. Our notes on pumping bras for wearables help with that.
— Paul
Flange sizes for Wren pumps
Wren’s standard flange is 25 mm. If it does not sit right, there are two alternative sizes: the 17 mm breast shield adaptor and the 21 mm breast shield adaptor. A spare 25 mm breast shield is available too.
If you are choosing a pump, see the full range of Wren breast pumps, or browse spare parts if you already have one and need a different flange size. Current prices are on each product page.
A quick home routine
Here is a short routine you can follow in one sitting.
- Measure both nipples before pumping, using the steps above, and write down the numbers.
- Pick the flange sizes around each number: one smaller, one close, one larger.
- Run a full session with the first size on a low vacuum, noting comfort and milk output.
- Repeat with the other sizes, changing only the flange.
- Choose the size with the best comfort and easiest flow, per side.
- Re-check any time pumping becomes sore or output drops.
If you are still not sure after this, ask your midwife, health visitor or an experienced lactation professional to watch you pump. They can see what the nipple is doing in the tunnel, which is hard to judge on your own.
FAQ
Do you measure your nipples before or after pumping?
Before. The NHS Southampton guide uses a pre-pump measurement, taken after gently touching or tugging the nipple so it protrudes, to estimate which sizes to try. Measure each side separately, because the two can differ.
Do you measure the nipple or the areola?
The nipple tip only. The areola is the darker skin around it and should not be in the measurement. If a lot of areola is drawn into the tunnel while you pump, the flange may be too large.
Will a smaller flange get you more milk?
Not necessarily. The NHS guide says both a flange that is too small and one that is too large can reduce output. The aim is a size that lets the nipple move freely, feels comfortable and lets milk flow easily.
What if my measurement falls between two sizes?
Try both neighbouring sizes, and perhaps one more, for a full session each. Compare comfort and milk output and keep the one that does best. If you are unsure, an experienced lactation professional can check the fit with you.
Can I use two different flange sizes?
Yes. Nipples can differ left to right, so some people use a different size on each side. Measure and test each breast separately.
Sources
- The Flange FITS Guide for optimal comfort, efficiency and milk yield, University Hospital Southampton NHS Foundation Trust
- Expressing milk, University Hospital Southampton NHS Foundation Trust
