What size cover you need, and the overlap that decides it

The one rule that prevents the common mistake

Winter covers are ordered by the size of the pool, not by the dimensions of the fabric. Pool Mate’s product line makes the convention visible: a cover for a 24 foot round pool is sold as a 24 foot cover and measures 28 feet, with a published 4 feet of overlap.

In The Swim publishes the same convention on its product pages, describing a 4 ft overlap that allows for water drop and easy installation. The Pool Factory puts the industry range at an additional three to four feet of material.

So if you own an 18 foot round pool, you buy an 18 foot cover. Measuring your pool and ordering fabric to match is the error, and it produces a cover that will not reach around the top rail once the water is down.

Why the extra material exists

Two reasons, and both are about winter rather than storage.

The water drop. A pool is lowered for winter, and the cover has to reach a surface that is now several inches below where it was. In The Swim’s winter cover category describes the overlap as accommodating an 18 inch water drop; INYO Pools states that most covers carry enough material for a level reduced up to 12 inches, and that a deeper drop means buying a larger cover.

Securing. The cover has to wrap the top rail and be held there, which consumes material all the way round. A cover cut exactly to the pool diameter has nothing left to grip with.

How far to lower the water is a separate decision, and it depends on your pool. Industry guidance for freezing climates is four to six inches below the skimmer and no further; Bestway specifies a few inches below the inlet or outlet valves for its year-round pools. Our winterizing guide and draining guide cover both cases.

Measuring a pool that is not a neat circle

Round pools are straightforward: the nominal size on the box is the size to order. Where it gets awkward is ovals and anything that has been in the ground for a decade.

1. 1

   Measure the pool, not the cover you have

   An existing cover may be the wrong size already, or may have shrunk and stretched. Measure the pool itself, outside edge to outside edge of the water.
2. 2

   For ovals, take both dimensions

   Ovals are sold by two numbers - length by width - and both have to match. An oval cover on an oval pool is not interchangeable with a round one of similar area.
3. 3

   Measure twice, at right angles

   Old pools settle out of round. If two diameters differ, order for the larger one; a cover slightly too big can be worked with, one slightly too small cannot.
4. 4

   Check your top rail width

   Wide top rails consume overlap. A pool with 7 inch rails uses more of the margin than one with 5 inch rails before the cover reaches anything to grip.
5. 5

   Decide about the water drop first

   If you plan to lower more than about 12 inches, published guidance says you need a larger cover than standard overlap allows.

What holds it on

The primary attachment on an above-ground winter cover is a cable and winch, and it usually comes in the box — The Pool Factory states that all of its winter covers include a cable and tightening device, and In The Swim lists winch and cable with its above-ground covers. The cable runs through the cover hem, round the pool below the top rail, and is tightened with a small ratchet.

Everything else is wind management, and it is supplementary rather than alternative:

Wind is what destroys winter covers, not cold. A cover that lifts and whips tears at its grommets and its seams, and a cover with folds collects standing water — which is the failure our cover care guide is built around, and the reason a cover pump belongs on the closing list.

Leaf nets, and the second cover question

If your pool sits under trees, the cover sizing question has a companion: a leaf net goes over the winter cover, is sized the same way, and is lifted off with the leaves on it in late autumn. It is the difference between a manageable spring and a cover full of rotting leaf mulch, as our leaf season guide covers.

Order it to the same pool size as the cover. Like the cover, a net is sold to fit the pool rather than the fabric dimension, and it needs its own overlap to be secured over the top of an already-covered pool.

Taking it off again, and what that implies for sizing

A cover that was a struggle to fit is a cover that will be a struggle to remove, usually in April with a winter’s worth of water and leaf mulch on top of it. That is worth thinking about when you are choosing between a snug fit and a generous one.

The published overlap exists partly for this. Three to four feet of material means the cover can be loosened, walked round and drawn off in one direction rather than fought over the top rail. It also means a cover that still fits next year if the pool settles slightly or you lower the water a little further.

Two habits make spring easier. Pump the standing water off before you touch the cable — our cover pump roundup covers the tool for that — and take a leaf net off separately in autumn rather than leaving everything to be lifted at once. Our green pool opening guide covers what to do if the cover comes off and the news is bad.