The Two-Inch Rule Is Compensating for a Weak Insert

The short answer: a 20×20 insert for an 18×18 cover. That is the standard advice and it is right for almost every insert you can buy, but it is worth knowing why, because the rule is a workaround rather than a law. Size is a proxy for fill: a 20×20 insert has more material in it than an 18×18, so it fills the corners and the cover looks plump instead of saggy. What actually decides the result is fill weight in grams, and almost no listing publishes it. Which is why one insert manufacturer states its 20×20 suits a 20×20 cover, while every cover shop tells you to size up. Both are describing what they sell honestly.

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Why the rule exists, and where it breaks

Every shop that sells pillow covers gives the same instruction: buy the insert two inches larger. An 18×18 cover takes a 20×20 insert, a 20×20 takes a 22×22, and so on up the chart.

It works, and it is worth understanding what it is actually doing, because the sizes vary more than the confident charts suggest. One shop says one to three inches depending on the cover size. Another says one to four. A third says two inches for squares and the same size for lumbars. A fourth builds its covers deliberately undersized, so what it calls a 17×17 cover is sold as fitting an 18×18 insert.

Those cannot all be the same instruction, and the reason they differ is buried in a sentence on one of those same sites.

The sentence that explains it

One retailer, having published a full size chart, adds this almost as an aside: every manufacturer’s fill weights are different, and there is unfortunately no one size fits all. They mention having compared five different manufacturers’ inserts to arrive at their recommendations.

That is the whole thing. The number on an insert tells you the dimensions of the fabric shell. It tells you nothing about how much is inside it, and how much is inside it is what makes a pillow look full.

So a 20×20 insert from one maker arrives plump and holds its corners, and a 20×20 from another arrives limp with the fill settled in the middle. Same label, different product.

Which means the rule is compensating

Sizing up two inches is a way of buying more fill without knowing how much fill you are buying. Take a bigger shell, get more stuffing by default, and the surplus fabric compresses into a smaller cover and forces the fill outward into the corners.

The proof is what happens when the insert is genuinely well filled. One insert manufacturer states that its 20×20 is recommended for 19×19 or 20×20 covers, which directly contradicts every cover shop. It is not wrong. Its insert is dense enough that it does not need the two-inch handicap.

So the practical answer stays the same, because you usually cannot tell how dense an insert is before it arrives. Size up two inches, and if the listing happens to publish a fill weight, use that instead.

Which leaves the size chart itself, and it is worth having in front of you rather than working from the eighteen-inch example alone.

Find your insert in 3 taps

Your cover size, the look you want, and where the pillow lives. Those three give you a size and a fill type.

The full size chart

Measure the cover seam to seam rather than trusting the label, since finished sizes vary between makers. Then find your row.

Two rows deserve attention. The 20×20 row is where most people go wrong, because putting a 20×20 insert into a 20×20 cover feels obviously correct and produces exactly the soft empty corners the whole exercise is trying to avoid.

And the lumbar row is where the sources genuinely split. Several say same size, one says an inch up. Same size is the safer default, because a long narrow shape has nowhere to redistribute surplus fill, so an oversized insert bulges in the middle instead of filling the ends.

Feather or polyester, and why corners are the test

Once the size is settled, the fill decides whether the pillow looks like the catalogue photograph or merely full. The difference shows up in one specific place.

Feather and down has give

A typical feather insert is 95 percent feather to 5 percent down, and the useful property is that the fill moves and stays where you put it. One retailer describes it exactly: feather and down have more give than polyester, so you can position the fill into the pillow cover corners for a proper fit.

That is why a feather pillow reads as square and a polyester one reads as slightly round. It is also why feather takes the karate chop dent and holds it, while polyester springs back within seconds.

The costs are real. Feather flattens under use and needs plumping, quills can work through a loosely woven shell, and it is more expensive. Look for a shell described as down-proof or tightly woven, which is what keeps the quills in.

Down alternative recovers

Polyester microfibre does the opposite. It resists being pushed into a corner and springs back toward the middle, which loses you the sharp square shape, and it recovers from being sat on without any attention at all.

On a sofa that people actually use, that trade is usually worth making. A decorative bed pillow that nobody compresses is where feather earns its price.

Down alternative is also the answer with pets or allergies, since there are no quills to shed and most are machine washable.

The corner test, in ten seconds

Push your thumb firmly into one corner of the finished pillow and let go. If the corner stays filled, you have enough fill and it is the kind that holds. If it collapses back, the insert is either too small or too lightly filled, and no amount of arranging fixes it.

The one that publishes its fill weight

Size: 20×20. For an 18×18 cover. Set of two.

The argument of this whole article is that fill weight decides the result and almost no listing tells you what it is. This one does: 850 grams, or 1.87 pounds, and the maker states that is twenty percent more than comparable inserts. Whether or not that comparison is exact, publishing the number at all puts it in a small minority.

It is down alternative microfibre rather than feather, so it recovers from being sat on without needing plumping, which suits a sofa that gets used. It is also the right choice with pets or allergies, since there are no quills to work through the shell.

One useful detail: the fill can be added to or taken out through the opening, so if the pillow comes out firmer than you wanted, you can adjust it rather than buying a different size.

Worth it or skip: Worth it for sofa pillows and for anyone who wants to compare fill on a number rather than a photograph. Skip it if you specifically want the karate chop dent or sharp square corners, which need feather, since polyester springs back out of a crease.

Check the 20×20 down alternative pair on Amazon

Three more, all 20×20 for an 18×18 cover

Feather and down, for corners and the chop. Size 20×20, set of two. Ninety-five percent duck feather to five percent down in a thick cotton shell, medium firm. This is the fill that has give, so it can be pushed into the corners and stay there, and it holds a karate chop dent that polyester releases in seconds. The maker includes the recovery instruction on the listing: fluff it a few times or give it a minute of cold air in the dryer before judging it, since it ships vacuum packed. See the feather and down pair →

Natural feather down that states the rule itself. Size 20×20, set of two. Worth including partly for what the listing says: the maker recommends choosing an insert one to two inches larger than the cover for a fuller appearance, which is the two-inch rule stated by somebody selling inserts rather than covers. Feather fill, and the same caveats apply, meaning regular fluffing and a preference for a down-proof shell. See the natural feather down pair →

A clump-resistant single, if you need one rather than two. Size 20×20, single insert. Note the count, since almost everything in this category ships in pairs and it is easy to order one by mistake. This is synthetic down in a polyester cotton blend, described as clump-resistant, which addresses the specific failure where polyester fill migrates and sets in lumps after a few washes. Sold for the chop look, though feather does that better. See the single clump-resistant insert →

Once the insert is right, the cover has its own rules. Washing a decorative pillow depends entirely on what is inside it, which I go through in the cover is not the problem, the filling is.

Already bought one and it looks wrong? 2 taps

The quiz above is for choosing. This one starts from the pillow on your sofa, and the second question usually explains it.

People also ask about pillow inserts

What size insert do I need for an 18×18 pillow cover?

A 20×20 insert. That two inch step is what every cover shop recommends and it is right for almost every insert you can buy, because the surplus fabric compresses into the cover and forces fill outward into the corners. The step is not universal though: covers of 16×16 and below usually want only one inch, covers of 24×24 and above want two to three, and lumbars generally want an insert the same size as the cover.

Why do my throw pillows look flat?

Usually one of three things. The insert is the same size as the cover, which leaves nothing to push into the corners. The insert is the right size but underfilled, which is the case where size stops being a useful proxy and fill weight becomes the specification that matters. Or it is brand new and still compressed from vacuum packing, in which case fluffing it by hand or giving it a minute in the dryer on cold air brings it back to full loft. Check that last one before concluding anything, since a compressed insert looks two sizes too small.

Feather or down alternative for throw pillows?

Feather for looks, down alternative for use. Feather and down has give, so the fill can be pushed into the corners and stays there, which is what makes a pillow read as square rather than round, and it holds the karate chop dent that polyester releases within seconds. Down alternative springs back toward the middle instead, which loses the shape and gains recovery: it bounces back from being sat on without any plumping. So feather for decorative bed pillows, down alternative for a sofa people use, and down alternative with pets or allergies since there are no quills to shed.

Can an insert be the same size as the cover?

For lumbars, yes, and several retailers specifically recommend it to avoid a long narrow pillow bulging in the middle. For squares, only if the insert is unusually densely filled. One insert manufacturer states its 20×20 is recommended for 19×19 or 20×20 covers, which directly contradicts the cover shops, and it is not wrong: a well-stuffed insert does not need the two inch handicap. The problem is that you cannot usually tell how dense an insert is before it arrives, which is why the rule exists.

Pillow insert FAQ

How do I measure my cover?

Seam to seam, laid flat, rather than trusting the label. Finished sizes vary between makers and some deliberately build covers undersized: one shop sells what it calls a 17×17 cover as fitting an 18×18 insert. Measuring takes ten seconds and removes the guesswork that produces most of the wrong purchases.

Can I go three inches up for extra plumpness?

Better not to on a standard square. One retailer warns that over-filling puts stress on the seams and stretches the fabric over time, and the cover is the expensive part while the insert is not. Three inches is appropriate on large covers of 24×24 and above, where the proportion is different. On an 18×18, stick to two.

Why is fill weight so hard to find?

Because most makers do not publish it, and its absence is informative. Where it appears it looks like 850 grams or 1.87 pounds for a 20×20, and a manufacturer proud of the number tends to print it prominently. When two inserts look identical in photographs and only one states a weight, that is a reasonable tiebreaker.

Do inserts come in pairs?

Usually, and not always, which catches people out. Most listings in this category ship two, but singles exist and the difference is easy to miss when the photographs show a pair. Check the count before ordering, particularly if you are buying to fill several covers at once.

Can I wash a pillow insert?

Down alternative usually yes on a gentle cycle, and feather often yes with more care, but check the label since shells vary. The part that matters is the drying: fill clumps as it dries and sets in position, so tumble on low with dryer balls and take it out to break up the clumps by hand halfway through. That is the same principle as drying a duvet, at a smaller scale.

How do I get the karate chop dent?

Feather and down only, since polyester springs back out of a crease within seconds. Hold the pillow at both sides, plump it upward, then strike down into the centre of the top edge with the side of your hand. Feathers hold the crease. If the dent vanishes as you watch, you have polyester and no technique will change that.

The pillow insert cheat sheet

Free printable

The Cozy Home Starter Checklist

A room usually looks unfinished for small technical reasons rather than stylistic ones, and the wrong insert size is a good example. This printable walks you through a home room by room, catching the details that quietly decide how a space reads.

Get the free Cozy Home Starter Checklist

Keep reading

• The Cover Is Not the Problem. The Filling Is
  Once the insert is right, washing it depends entirely on what is inside. Includes the squeeze test for an unlabelled pillow.
• Cozy Bedding Ideas: The Complete Guide to a Dreamy Bed (2026)
  How many pillows a bed actually needs, and how the layers sit together rather than competing.
• Hotels Wash the Layer That Touches You. Nothing Else
  The same principle applied to bedding: the washable outer layer protects the filling you cannot easily clean.
• New Sheets Are Not Stiff. They Are Coated
  Another case where the number on the label, this time thread count, is not the specification that decides the result.
• Cotton Velvet and Polyester Velvet Are Not the Same Fabric
  If the pillows are going on a velvet sofa, the fabric underneath has its own set of rules.

Buy the size, but understand the number

A 20×20 insert for an 18×18 cover. That is the answer, and it is worth knowing that it is a workaround rather than a physical law, because it tells you what to do when it fails.

Size is standing in for fill. A bigger shell has more stuffing in it by default, and the surplus compresses into the cover and pushes outward into the corners, which is what makes a pillow look plump. What actually decides the result is fill weight in grams, and almost no listing publishes it. That is why one insert maker can honestly say its 20×20 suits a 20×20 cover while every cover shop tells you to size up: theirs is dense enough not to need the handicap, and you have no way to know that before it arrives.

So use the chart, and when a listing does publish a fill weight, use that instead. Then pick the fill for the job rather than the price. Feather has give and holds a corner and takes the chop; polyester springs back and survives a sofa. Neither is better.

And fluff the thing before you judge it. They arrive vacuum packed and looking like a mistake, and a minute of cold air in the dryer is the difference between a return and a pillow you like.