The air pillow is structural, not padding

What the pillow is actually for

Not comfort, and not to hold the cover up. The Pool Factory describes it precisely: the air pillow acts as an ice compensator absorbing the ice expansion, and that expansion is what damages a pool wall, cover or liner.

Water expands as it freezes. In a closed pool with nowhere to go, that expansion pushes outward against the wall all winter. Pool Geek states the mechanism from the other side: with a pillow in place, the ice expands inward toward the pillow instead of outward into the pool walls. PoolSupplies.com makes the consequence explicit, listing pool walls splitting from ice damage as the thing being prevented.

There is a secondary benefit that gets most of the attention: a pillow raises the center of the cover so rain and snow run to the edges instead of collecting in a heavy pond in the middle. That is real, and it is why our winter cover guide notes that covers fail from water on top of them rather than from cold.

How much to inflate it: the number everyone gets wrong

The instinct is to fill it hard. Every published source says otherwise. The Pool Factory says to inflate to about 60% of capacity to allow for compression. PoolSupplies.com says to fill the pillow between half and three-quarters full to allow for expansion pressure. Blue Wave’s own guidance is to inflate until firm but not overly taut, and notes it should not be over-inflated.

The reason is the entire point of the product. A pillow that is rock hard cannot compress, and a pillow that cannot compress is just a rigid object in the middle of the ice — it absorbs nothing. Under-inflating is the safe error; over-inflating defeats the purpose and usually bursts the seams in the first hard freeze.

Sizing, and why two pillows are often better than one

The Swimline 4 by 8 is rated by its retailer as a single-pillow solution for pools up to 28 feet round and small ovals, which covers essentially every above-ground pool this site writes about. On that basis one pillow is enough.

There is a practical argument for two on larger pools, and it is about geometry rather than volume. A single pillow in the middle of a 24 or 27 foot pool leaves a wide ring of water that freezes solid against the wall before the ice ever reaches the pillow. Two pillows, spaced, give the ice a compressible target from more directions. The Blue Wave listing makes the same point — larger pools may need multiple pillows to spread pressure across the cover.

On a small pool, a 4 by 4 is proportionate and easier to position. On anything from about 18 feet upward, a 4 by 8 is the default, and two 4 by 4s are a reasonable alternative.

Getting it in and keeping it there

1. 1

   Inflate on the ground, to about 60%

   Blow it up before it goes near the water, to between half and three-quarters full. It should give easily under a hand.
2. 2

   Tie it before you float it

   Attach cord to the grommets while you can still reach them. The Pool Factory's method is to tie string through the grommets and to the top ledges, left deliberately slack.
3. 3

   Center it, loosely

   Tie it so it cannot drift to one side, but with slack. Tight strings tear grommets or snap in the first freeze, and then the pillow is under the cover doing nothing.
4. 4

   Cover over the top

   The winter cover goes over the pillow, not under it. Fit and secure the cover as normal - our cover sizing guide covers the overlap the cover needs.
5. 5

   Check it after the first storm

   A deflated pillow is the most common winter failure, and the whole protective effect goes with it. If it has gone down, a replacement mid-winter is worth the trip.

What it saves, against what it costs

A pillow is among the cheapest items in a pool close, and the only one doing structural work. The thing it is preventing — a split wall, a torn liner, a collapsed cover full of ice — is the bill itemized in our cost of a bad close guide, and it dwarfs the pillow by orders of magnitude.

It is also the item most often skipped, because the connection between a floating bag and a steel wall is not obvious until someone explains the ice. If you are closing a pool in a climate where the surface freezes at all, it belongs on the list alongside the cover, the plugs and the cover pump — all of which our winterizing guide sequences properly.

How we picked

We have not tested these pillows. Scores reflect published specifications and the honesty of what is published, and this category publishes very little: no manufacturer specification sheet, no warranty on any of the three, conflicting vinyl gauges for the same product, and valve types absent everywhere. What is consistent across every source is the inflation guidance, which is the part that decides whether the product works.

What happens on a pool without one

Ice forms from the surface down and expands as it goes. In a pool with a rigid wall and no compressible object in the middle, that expansion has one direction available to it, and the published consequence from the trade is pool walls splitting from ice damage.

The secondary failure is the cover. A flat frozen surface with no high point collects rain and snowmelt in the middle, and a winter cover holding several hundred pounds of standing water tears at its grommets or pulls its cable tight enough to deform the top rail. Our cover care guide is built around that failure, and a cover pump is the other half of the answer.

Neither failure is dramatic while it is happening. Both are discovered in April, which is why the bill lands in our cost of a bad close rather than in the autumn budget.

Tying it down so it is still there in March

A pillow that has drifted to the side of the pool is a pillow doing nothing, and it is the most common way this product fails. The published method is to use the grommets: tie string through them and to the pool’s top ledges or through the cover’s own grommets, deliberately leaving slack.

Slack is the part people get wrong. Strings pulled tight tear grommets out or snap when the ice shifts, and once either happens the pillow moves. The Pool Factory’s guidance is explicit that the ties should be a bit loose rather than taut.

A second failure worth planning for is a puncture. Pillows are cheap, and a spare on the shelf costs less than a mid-winter trip. If a pillow does deflate in January, it is worth replacing rather than writing off — there is usually a great deal of winter left, and the pillow is doing structural work for every week of it.

One pillow or two

Retailer ratings put a single 4 by 8 pillow at up to a 28 foot round pool, which covers everything this site writes about. The argument for two is geometry rather than capacity: on a 24 or 27 foot pool a single pillow in the middle leaves a wide ring of water that freezes solid against the wall long before the ice front reaches the pillow.

Two 4 by 4 pillows, spaced apart, give the ice a compressible target from more directions and cost about the same as one large one. One retailer listing makes the same point, noting that larger pools may need multiple pillows to spread pressure across the cover.

For a pool under about 18 feet, one pillow sized to the pool is plenty, and a 4 by 4 is easier to position and tie than a 4 by 8 in a small space.

Where it sits in the closing sequence

The pillow goes in after the water has been lowered and the chemistry balanced, and before the cover. That order matters: inflating and tying a pillow through a fitted cover is a job nobody enjoys, and lowering the water afterwards leaves the pillow hanging on its strings.

The rest of the sequence — cold water first, then chemistry, then plugs, then pillow, then cover — is in our winterizing guide, and the cover itself needs to be sized with the water drop in mind, which our cover sizing guide covers. Get the pillow in at the right point and the whole close takes an afternoon.

What people use instead, and why it is worse

The common substitutes are a partly inflated inner tube, a few lengths of pool noodle, or a sealed plastic drum. All of them are compressible, and all of them will do something.

What they do not have is the surface area. A 4 by 8 pillow presents a wide, low, compressible face across the middle of the pool, which is what lets an advancing ice sheet push inward over a broad front rather than concentrating against one small object. A single inner tube in a 24 foot pool is a small target in a large sheet of ice.

The second problem is the cover. A pillow also lifts the center so rain and snowmelt run to the edges; an object that sits low in the water does not, and the standing-water failure our cover care guide describes happens anyway. Given that a purpose-made pillow costs less than most winterizing chemicals, the substitution rarely makes sense.