The Side Sleeper
Knee pillow guides · Buying

Knee pillow fillings, explained

Nell, the Side Sleeper
Updated · Data collected · How we research

Nearly every knee pillow is solid memory foam: 19 of the 24 best sellers. It's the filling that holds a set height all night, which is the main thing a knee pillow is for. Shredded foam and fibre are softer and squash further.

"Cooling" mostly means the cover. Of the 17 pillows that claim it, only 2 do anything to the foam itself.

The 24 knee pillows in the comparison table, by filling. Best sellers on Amazon US plus two brands that sell direct, collected October 5, 2026. 1 listing contradicts itself on the filling and isn't counted.

What's inside, in cross-section

Cut a knee pillow in half and you'd find one of four things. The difference that matters is how far each one gives under the weight of your knee, and whether it comes back.

Solid memory foam: One moulded block. Sinks a little under the knee and keeps its height.

Solid memory foam
One moulded block. Sinks a little under the knee and keeps its height.

Foam with gel or vents: The same foam with a gel layer on top, or holes through it for air. gel

Foam with gel or vents
The same foam with a gel layer on top, or holes through it for air.

Shredded memory foam: Loose pieces in a casing. They shift to fit your legs, and pack down.

Shredded memory foam
Loose pieces in a casing. They shift to fit your legs, and pack down.

Polyester fibre: Soft and squashy, in stitched pockets so it doesn’t clump. Plump it up.

Polyester fibre
Soft and squashy, in stitched pockets so it doesn’t clump. Plump it up.

Seen from the foot of the bed. The circle is your top knee and the dashed line is where the surface was before it lay down. How far each one sinks is drawn from how the listings describe them, not measured: no listing gives a firmness figure.

Each filling, and what it's for

Solid memory foam

19 pillows · 3–7.2″ thick · typically $34

A single moulded block of polyurethane memory foam, cut or cast into the pillow's shape. Memory foam reacts to warmth: "Temperature-responsive memory foam softens with body heat", as the DailyCuddles Memory Foam Knee Pillow with Strap listing puts it. So it sinks slowly under your knee, takes the shape of your legs, and springs back once you get up. Because it's one piece, it can't pack down the way loose filling does, though it does give a little as it warms.

How firm a given foam is, nobody says. 9 listings call their foam "high-density" or, in Cushion Lab Side Sleeper Knee Pillow's case, "extra-dense", but none gives a density or firmness figure to compare. 6 say theirs won't flatten, again without saying for how long.

Suits: most side sleepers, and anyone who has found their number and wants a pillow that holds it.

Shredded memory foam

1 pillow · 6.5″ thick when new · typically $36

The same foam, chopped into small pieces and sewn into a casing. The pieces slide around to fit your legs, which makes it feel softer and more forgiving than a block. The catch is height: the HOMBYS Shredded Memory Foam Knee Pillow 6.5" is listed at 6.5″, but that's before your leg is on it, and loose pieces pack down under weight. Expect to plump it up.

The body pillow in the table, the TranquilRelax C-Shaped Body Pillow 47", is shredded foam too, and comes with a refill pack so you can set the firmness yourself. The shredded knee pillow's listing doesn't say whether you can open it, and its care label says to wipe it with a damp cloth.

Suits: people who find solid foam too hard, and who don't mind shaking the pillow back into shape.

Polyester fibre

2 pillows · 3″ thick (1 of 2 state it) · typically $29

Polyester fibre, the stuff in an ordinary bed pillow, sometimes sold as "down alternative". It's the softest filling and squashes easily, so its listed height tells you least about where your knee ends up. The MINUPWELL Knee Pillow (knee-to-ankle) stitches its 850 g of fibre into separate pockets ("baffle box" construction) so it stays spread out. The small HapiPoppy Soft Small Knee Pillow 11" x 7.8" has a pocket for an ice or heat pack.

The real advantage is washing. Both fibre pillows go in the washing machine whole, cover and all. And fibre has none of the new-foam smell: "Unlike memory foam", the MINUPWELL listing says, it's "completely odorless".

Suits: a smaller gap, a preference for soft over firm, or anyone who wants to wash the whole thing.

Foam with inserts

1 pillow · 5–8″ thick, adjustable · $45

Solid memory foam with removable layers, so you set the height yourself. The Coop Home Goods Adjustable Knee Pillow comes with 1″ and 2″ inserts. It's the only adjustable foam pillow among the best sellers, and it carries both a foam and an indoor-emissions certificate (more on those below).

Suits: anyone whose towel test landed between sizes, or who simply isn't sure.

"Cooling": mostly the cover

17 of the 24 best sellers say something about keeping you cool. Read the small print and 15 of them mean the cover: "breathable", "cooling fabric", bamboo-viscose blends, 3D mesh. Underneath, the filling is the same as in any other pillow.

Only 2 change the foam itself:

None of the 17 gives a temperature measurement, so treat every cooling word as a description of the materials, not a result. 7 pillows make no cooling claim at all. If you sleep hot, a pillow with vents or gel is the only kind doing anything beyond the fabric. A fibre pillow avoids foam altogether.

Washing, smell and certificates

You can't machine-wash memory foam. The makers say so: the 5 STARS UNITED Knee Pillow for Side Sleepers listing, for one, says its foam core isn't washable. For a foam pillow, the cover is what you wash: 13 of the 19 solid-foam best sellers have a removable, machine-washable one. The rest either don't say, or the cover doesn't come off. The fibre pillows are the only ones that go in the machine whole.

New foam can smell when it first comes out of the packaging, and makers seem to know it: the Vertdens Leg Pillow promises "no irritating odor on the surface", and the fibre pillows sell themselves partly on having none. If yours does, airing it out for a day before use is the usual advice.

Certificates are the one checkable thing about a filling. A few listings carry them:

The other 20 listings name no certificate. That doesn't mean the foam is bad, only that you can't check.

Fillings by the numbers

Filling Pillows Listed thickness Machine-wash Cooling in the fill Typical price
Solid memory foam 19 3–7.2″11 of 19 state it cover only (13 of 19) 2 of 19 $34
Shredded memory foam 1 6.5″ not stated 0 of 1 $36
Polyester fibre 2 3″1 of 2 state it whole pillow (2 of 2) 0 of 2 $29
Foam with inserts 1 5–8″ cover only (1 of 1) 0 of 1 $45

Thickness ranges cover the listings that state one clearly (the adjustable pillow's range is worked out from its inserts). Machine-wash counts only listings that say so. Typical price is the median, from October 5, 2026. Prices change daily. Every pillow, with its source: knee pillows compared.

Which filling for you

You want it to hold a set height

Solid memory foam. Find your number first, then pick a pillow listed at it. Most of the choice is here.

Foam feels too hard

Shredded foam or fibre. Both give more, so allow for the pillow sitting lower than its listed height.

You sleep hot

Foam with vent holes or a gel layer, or skip foam and go for fibre. A "cooling cover" alone is the same foam underneath.

You want to wash all of it

Fibre. For foam, choose one whose listing says the cover is removable and machine-washable, and consider a spare cover.

Filling is the second question. Height is the first: how thick should a knee pillow be? And if you're choosing between shapes as well, see knee pillow shapes, explained.

Quick answers

What is the best filling for a knee pillow?

For most side sleepers, solid memory foam. It’s in 19 of the 24 best sellers, and it holds a set height through the night. Fibre and shredded foam are softer and squash further, which suits a small gap or anyone who finds foam too firm.

Do memory foam knee pillows get hot?

Memory foam softens as it warms, and makers of cooling pillows say plain foam holds body heat. Of the 17 best sellers that make a cooling claim, 15 put it in the cover fabric. Only 2 change the foam itself, with vent holes or a gel layer. No listing gives a temperature test.

Can you wash a memory foam knee pillow?

Not the foam itself. Wash the cover: 13 of the 19 solid-foam best sellers have a removable, machine-washable one. The fibre-filled pillows are the ones that go in the washing machine whole.

Sources