Solar cover size calculator: what size solar cover do I need?

A solar cover (a bubble cover or solar blanket) does two jobs. It stops evaporation, which the US Department of Energy calls by far the largest source of energy loss from a pool, and it lets sunlight through to warm the water. To work, it has to cover the whole water surface and float flat on it, so size matters: too small leaves a strip of open water at the edges, too big rides up the walls and lets wind under it.

How the solar cover calculator works

• Round pools: buy the cover sold for your pool’s size. Round covers are made to the nominal pool size; Intex, for example, sells its 18 ft cover as designed for 18 ft diameter pools.
• Oval pools: the same: an oval cover sold for, say, a 15 × 30 ft oval.
• Rectangles: you need at least length × width at the water line. Covers are commonly sold in 2 ft steps, so the calculator rounds each side up to the next even foot (or half metre); check the brand’s list. Anything left over gets trimmed.
• Kidney and freeform pools: buy a rectangle as long as the pool and as wide as its biggest lobe, lay it on the water, and trim it to the outline. The trimmed area is the rectangle minus the kidney (0.45 × (A + B) × length).

rectangle sheet = (length rounded up) × (width rounded up)
trim = sheet area − water surface area
water saved = surface × evaporation per week × weeks × share stopped × share of time covered
heat kept = pounds of water saved × 1,048 BTU per lb (DOE, 80°F water)

What a cover saves

How much water that is depends heavily on climate. The Southern Nevada Water Authority puts an average Las Vegas in-ground pool at 48.6 gallons per square foot per year, about 78 in of water, or 22,842 gallons for a 470 sq ft pool. That is a hot, dry desert; most places lose much less. Use your own weekly loss, from a bucket test or the pool evaporation calculator, rather than a desert figure. The calculator’s default of 1 in a week is only an example.

Heat follows water: every pound that evaporates from 80°F water carries 1,048 BTU out of the pool, according to the DOE. A gallon is 8.34 lb, so each gallon the cover keeps in the pool is about 8,700 BTU you don’t have to put back with a heater or wait for the sun to supply. Turn that into money with the pool heating cost calculator.

Clear or opaque?

The DOE says a pool absorbs 75–85% of the sun’s energy that strikes it. A transparent bubble cover cuts that by 5–15%, an opaque one by 20–40%. So a clear solar cover is the better choice for warming the water in sunshine; an opaque cover blocks more light, which slows algae but warms less.

Tips and mistakes

• Measure the water, not the deck. The cover floats on the surface, inside the walls or coping. Measure inside to inside at the water line.
• Trim after it has sat in the sun. New covers come folded and can shrink or relax a little; let it lie on the pool for a day before cutting, then cut in small passes with sharp scissors.
• Don’t confuse it with a winter cover. Winter covers are sized much bigger, with a full 4 ft of overlap for above-ground pools and 5 ft for inground in Swimline’s catalog, because they are tied down over the edge. A solar cover sized like that will not lie flat.
• Use it. The savings depend on the cover being on. A reel or a helper makes the difference between nightly use and a cover that lives in the shed.

Working out a heater too? The pool heater size calculator shows how much faster a covered heat-up goes, and the worked example on how long to heat a pool 10 degrees explains why losses make the real time longer. Need the surface area of an odd-shaped pool? The pool volume calculator shows it alongside the gallons.