What size pool cover do you need?
A pool cover that is too small leaves water exposed, which defeats the whole purpose, while one that is far too big is awkward to handle, drags on the ground and wears out faster. The right size starts with the surface area of your pool, which depends on its shape: a rectangle is length times width, an oval is pi times length times width divided by four, and a round pool is pi times the diameter squared divided by four. Once you know the water area, add about ten percent for overhang, because a cover needs extra material beyond the waterline to sit on the coping, to be anchored, and to cope with wind lift. The calculator above does both steps, including the shape maths, so you only need your pool's dimensions.
Measuring is where most cover mistakes happen. Measure the pool at its longest and widest points, at the waterline, not at the outer edge of the coping, and measure twice, because pools are rarely as regular as they look on a plan. Freeform pools with curved ends are the hardest: measure the maximum length and width and treat the pool as an oval, which usually gives a safe, slightly generous result, or order a custom cover traced from the pool's outline. For a rectangular pool with rounded corners, use the full length and width and let the extra material bunch slightly at the corners, which is harmless for a floating solar cover but matters for a rigid safety cover, where the manufacturer needs exact corner radii.
Solar covers, safety covers and winter covers
Not all covers are sized the same way, because they do different jobs. A solar cover, also called a bubble cover, floats directly on the water and is cut close to the waterline size, with only a small overhang, so that it heats the water and cuts evaporation. A safety cover is anchored to the deck around the pool and must be larger than the water, extending well onto the coping so its straps can be fixed, and it is usually made to measure. A winter debris cover is the loosest of all: it sits over the whole pool, overlaps the deck by a generous margin, and is held down by water bags or a cable, so it needs the most extra material. If you buy a stock-size cover, check which type it is before comparing its stated size with your pool.
Solar covers deserve a closer look because they are the most common purchase and the most misunderstood. The bubbles face down into the water, trapping heat and reducing evaporation by up to ninety percent when the cover is on, which is a huge saving on heating bills and water top-ups. The cover does not need to be a perfect fit to work well: a small gap at the edges loses little heat, but a cover that is badly undersized and leaves a strip of open water along one side loses much of the benefit. Trimming a slightly oversized solar cover with household scissors is easy and normal, which is another reason to err on the generous side when ordering.
How much water and heat does a cover save?
An uncovered pool loses a surprising amount of water to evaporation, commonly several millimetres per day in warm, windy weather, which for an 8 by 4 metre pool means tens of litres a day and thousands of litres over a season. A cover cuts that loss dramatically, and because it also keeps the water warmer overnight, the heater runs less. The combined saving on water, heating energy and chemicals, since less top-up water means less chemical rebalancing, often pays for a solar cover within a single season. Safety and winter covers save less heat but protect against debris and accidents, which is a different kind of value: a clean pool in spring needs far less work than one that has collected a winter of leaves.
Wind is the enemy of a well-fitting cover. A floating solar cover on a windy site can bunch up at one end, leaving water exposed and letting the cover blow onto the deck, where it can be damaged. A cover reel helps enormously, because it makes putting the cover on and taking it off a one-person job, and a cover that is easy to use gets used, while one that is a wrestling match gets left folded in a corner. For windy sites, choose a slightly heavier gauge solar cover, or a cover with a hem and eyelets that can be tied down, and always store the cover out of direct sun when it is off the pool, since ultraviolet degrades the bubble material over time.
Measuring tricky shapes
Kidney and freeform pools defeat simple formulas, so use the tracing method for an accurate result. Lay a lightweight sheet or a set of taped-together bin bags over the pool, mark the outline, and measure the traced shape's maximum length and width, or take the tracing to a cover supplier for a custom cut. Alternatively, divide the pool into sections on paper, rectangles and semicircles, calculate each section's area and add them up. This section method is accurate enough for a solar cover, where a small error is harmless, but for a safety cover, where the fit is structural, a professional measurement is worth paying for, since a safety cover that does not anchor correctly does not protect anyone.
Above-ground pools have their own sizing logic. Round above-ground pools are sized by diameter, and the cover must include the top rail width, so a 5 metre pool usually takes a cover stated as 5 metres or slightly larger, with an elastic hem or a cable that grips under the rail. Oval above-ground pools need both the length and the width, and the cover size is stated for the pool model rather than the water surface. Check whether the stated size is the pool size or the cover size before ordering, because manufacturers differ, and a cover stated as 5 metres may be made to fit a 5 metre pool with overlap already included, or may be exactly 5 metres of material with no overlap at all.
Cover care, lifespan and replacement
A solar cover typically lasts three to five seasons, less in harsh sun or if it is left on the ground, and more if it is stored on a reel in the shade. The first sign of failure is bubbles that flatten or flake, which reduces heating performance and sheds plastic into the pool. Rinse the cover with fresh water occasionally, especially after heavy chemical dosing, and never leave a solar cover on the pool during shock chlorination, since high chlorine attacks the material. Winter and safety covers last much longer, often eight to fifteen years, but their straps, springs and anchors wear and should be inspected each season, since a safety cover is only as strong as its weakest fixing.
When replacing a cover, re-measure the pool rather than ordering the same size as before, because pools shift, coping is replaced and memory is unreliable. Keep the old cover as a template if it fitted well: lay it on the new material and cut around it, which is the simplest way to reproduce a good fit for a freeform pool. And if you are buying your first cover, order slightly large rather than slightly small. A cover that is too big can be trimmed in minutes; a cover that is too small cannot be stretched and must be returned.
- Measure length and width at the waterline, twice.
- Add about 10% overhang beyond the water area.
- Solar covers float on the water; safety covers anchor to the deck.
- Err generous: an oversized cover trims easily, an undersized one does not.
- Use a reel and shade storage to make the cover last.
Measure your pool, pick its shape, and let the calculator turn those numbers into a cover size with the overhang built in. A cover that fits is a cover that gets used, and a cover that gets used saves water, heat and cleaning time all season long.