Work out your pool volume in gallons and the chemicals to buy

How to calculate pool volume in gallons

Multiply the water surface in square feet by the water depth in feet, then by 7.48 to turn cubic feet into US gallons. The surface depends on the shape:

• round: 0.785 × diameter × diameter;
• rectangular: length × width;
• oval: oval pools are a rectangle with a half circle at each end, so (length - width) × width + 0.785 × width × width.

Use the depth of the water, from the floor to the water line, not the height of the wall. If the floor slopes, enter the shallow end and the deep end: the calculator takes the average, which is exact when the floor drops evenly from one end to the other.

Fill time is the volume divided by the hose flow. Every hose and tap is different, so time it: open the tap as you will to fill the pool and see how long a 5 gallon bucket takes. If it fills in 37.5 seconds, your hose gives 8 gallons a minute, the value the calculator opens with.

A worked example

In feet (US). The calculator opens with an 18 ft round above-ground pool, 52 in wall, filled to 4 ft.

• Surface: 0.785 × 18 × 18 = 254.5 sq ft
• Volume: 254.5 × 4 = 1,018 cubic feet, or 7,614 gallons
• Fill time at 8 gallons a minute: 7,614 ÷ 8 = 952 minutes, 15 h 52 min
• Shock at opening, 1 lb (16 oz) per 10,000 gallons: 12.2 oz, so one 5 lb pack
• pH decreaser at 16 oz per 10,000 gallons (the HTH dose for pH 8.0-8.4): 12.2 oz, one 5 lb pouch
• 3 in tablets, 1 per 10,000 gallons a week: 0.76 a week and 10 for 12 weeks, so one 8 lb bucket of 8 oz tabs (16 tablets)

Every inch of water in this pool is 158.6 gallons.

In metres (UK and elsewhere). A 4.57 m round pool with 1.10 m of water has 16.4 m² of surface and holds 18.04 m³, or 18,043 litres. At 15 litres a minute it fills in 20 h 3 min. With Bayrol Chlorifix at 50 g per 10 m³ the first chlorination takes 90 g; bringing the pH from 7.6 down to 7.4 with pH-Minus (100 g per 10 m³ for each 0.1) takes 361 g; with 200 g tablets at one per 20 m³ a week, 12 weeks need 11 tablets.

Pool volume chart: gallons by size

Figures from the calculator. For another size, enter your diameter, or length and width, and the water depth.

Pool	Water depth	Gallons	Cubic feet
Round, 12 ft	4 ft	3,384	452
Round, 15 ft	4 ft	5,288	707
Round, 18 ft	4 ft	7,614	1,018
Round, 21 ft	4 ft	10,364	1,385
Round, 24 ft	4 ft	13,536	1,810
Oval, 12 × 24 ft	4 ft	7,693	1,028
Oval, 15 × 30 ft	4 ft	12,020	1,607
Oval, 18 × 33 ft	4 ft	15,693	2,098
Rectangle, 16 × 32 ft	3.5 to 6 ft (average 4.75)	18,193	2,432

Shock, chlorine tablets and pH decreaser: label doses and pack sizes

The starting doses come from product pages and can be changed in the advanced options. Enter the ones on the pack you buy: the dose and the volume it treats, usually 10,000 gallons.

• Shock. HTH Chlorine Granules (calcium hypochlorite) give 1 lb per 10,000 gallons as the standard dose for routine treatment and 2 lb for cloudy water or early algae. They come in 5, 18, 40 and 50 lb sizes.
• 3 inch chlorine tablets. HTH says to start with one tablet per 10,000 gallons a week and adjust it to keep free chlorine between 1 and 4 ppm, with the tablets in a floater, feeder or skimmer basket. The Pool Season tablets sold in an 8 lb pail weigh 8 ounces each, so a pail holds 16.
• pH decreaser. HTH pH Down has a table instead of a single dose: for 10,000 gallons, 16 oz when the pH reads 8.0 to 8.4 and 24 oz above 8.4. In the calculator, the number of doses is how many times you add the label dose. HTH asks you to wait at least 48 hours before retesting.

The CDC recommends a pH of 7.0 to 7.8 in home pools and free chlorine of at least 1 ppm, or at least 2 ppm in pools that use cyanuric acid or stabilized chlorine such as dichlor and trichlor. 3 inch tablets are trichlor, so the 2 ppm figure is the one that applies.

Common mistakes

• Using the wall height. An 18 ft pool with a 52 in wall and 4 ft of water holds 7,614 gallons; with 52 in the result would be 8,249 gallons and every dose 8% higher.
• Treating an oval as an ellipse. The formula 0.785 × length × width fits a true ellipse. On a 15 × 30 ft oval with straight sides it gives 10,575 gallons instead of 12,020, 12% short.
• Mixing up US and UK gallons. The calculator uses US gallons (3.785 litres). An imperial gallon is 4.546 litres, so UK labels in gallons need converting, or switch the calculator to metric.
• Reading the dose without the volume. "16 oz" means nothing without "per 10,000 gallons": check the volume on the label.
• Throwing tablets into the pool. HTH says not to: on a vinyl liner a tablet bleaches the surface. Use a floater, feeder or skimmer basket.