How we work it out
Every calculator does exact arithmetic and rounds only the figure you read. Each section gives the formula, where its numbers come from and a case worked out by hand, which the calculator must match to the last digit.
Pool volume
Gallons = the surface of the shape × the average of the two depths, at 231 cubic inches a gallon. A round pool is π × diameter² / 4, an oval π × length × width / 4.
An estimate for a pool with a straight slope between the ends. Measure inside the walls.
Worked examples
- A 32 × 16 ft rectangle, 3 ft 6 in and 8 ft deep: 512 ft² × 5.75 ft = 2,944 ft³, and 2,944 × 1,728 ÷ 231 = 22,023 gallons
- A round pool 24 ft across, 4 ft deep: π × 24² ÷ 4 = 452.4 ft², × 4 ft = 1,809.6 ft³, and × 1,728 ÷ 231 = 13,536 gallons
Source
Pool salt
Pounds = gallons × (target − reading) × 8.34 ÷ 1,000,000, since a gallon of water weighs about 8.34 lb. Hayward’s AquaRite manual gives 2,700 to 3,400 ppm, 3,200 the best.
Use pool salt over 99% sodium chloride. Wait 10 to 14 days before adding salt to new plaster.
Worked example
- 20,000 gallons from 2,400 to 3,200 ppm: 20,000 × 800 × 8.34 ÷ 1,000,000 = 133.4 lb
Source
- Hayward AquaRite owner’s manual, 092538 Rev D
Chlorine
1 ppm is 1 mg per liter. The grams of available chlorine = gallons × 3.785 × (target − reading) ÷ 1,000, then divided by the strength on the label: 10.24 fl oz of 12.5% liquid per ppm per 10,000 gallons.
Strengths from the labels: cal-hypo at least 65% and dichlor 56% available chlorine (EPA-registered labels). Follow the label of your product.
Worked example
- 20,000 gallons from 1 to 3 ppm with 12.5% liquid: 10.24 fl oz × 2 ppm × 2 = 41.0 fl oz
Sources
- Calcium hypochlorite label, EPA Reg. No. 1258-1232
- Dichlor label, EPA Reg. No. 7124-31
Pump size
Flow = gallons ÷ 360 minutes, never below 36 gpm: the residential filtration rate of ANSI/APSP/ICC-15, which the 2021 ISPSC references.
Size the pipes for that flow too: the standard keeps water under 8 ft per second in the return and 6 in the suction.
Worked example
- 20,000 gallons turned over in 6 hours: 20,000 ÷ 360 = 55.6 gpm