How many gallons in a 21 ft round pool?

A 21 ft round sits between the popular 18 and 24 ft sizes, and it lands almost exactly on 10,000 gallons — which means chemical labels written “per 10,000 gallons” apply to it nearly as printed.

The 10,000-gallon pool

With about 346 sq ft of surface, each inch of water is about 216 gallons. Filled to 46 in on a 52 in wall that comes to about 9,900 gallons — close enough to 10,000 to use label doses directly.

• 48 in wall: filled to 42 in it holds about 9,070 gallons. 54 in wall at 48 in: about 10,360.
• Dosing: the worked pages for 10,000 gallons fit this pool — how much chlorine for a 10,000 gallon pool, how much salt for a 10,000 gallon pool and how much stabilizer for a 10,000 gallon pool.
• Metric: about 37,600 litres.
• Base and liner. A 21 ft pool needs a level base about 21 ft across; liners are sold for 21 ft rounds in 48, 52 and 54 in wall heights, so order by both numbers.
• Evaporation: a quarter-inch a day in hot weather is about 54 gallons, or roughly 1,600 gallons over a month — worth a solar cover.
• Above-ground vs inground. A 10,000 gallon inground pool usually has a deep end and a much smaller footprint; this pool spreads the same water over a wide, flat floor, so it warms faster in sun and loses more to evaporation.

Next sizes: how many gallons in a 24 ft round pool and how many gallons in an 18x48 pool.