Pool Volume Calculator

The formula, by shape

Every version is plan area times average depth, converted to gallons. The only thing that changes is how you get the plan area.

Rectangle: length × width × average depth × 7.48

Oval: length × width × average depth × 0.89 × 7.48

Round: diameter² × average depth × 0.785 × 7.48

Kidney or freeform: (widest width + narrowest width) × length × average depth × 0.45 × 7.48

The 7.48 is the constant that turns cubic feet into US gallons — a cubic foot holds 7.48052 gallons.

The shape multipliers are area corrections. A rectangle uses its full bounding box, so its factor is 1.0. An ellipse fills π/4 of its bounding box, which is 0.785, and the 0.89 used for oval pools reflects the stadium shape most "oval" pools actually are — straight sides with rounded ends — which fills more of the box than a true ellipse. The 0.45 for a kidney is an empirical figure the pool trade has used for decades; it already includes the halving implied by adding two widths together.

Average depth is where sloped pools are handled

A pool with a shallow end and a deep end is not calculated in two pieces. For a floor that slopes uniformly, the average depth is simply:

Average depth = (shallow depth + deep depth) ÷ 2

That works because the volume above a straight-sloping floor is exactly the volume of a level pool at the mean depth — the wedge you lose at the shallow end equals the wedge you gain at the deep end.

Where it stops working is a hopper bottom: the constant-depth shallow section, then a sharp transition slope, then a flat deep section. That profile is not a straight slope, and the two-point average understates the volume, typically by five to ten percent. If your pool has a distinct break in the floor, measure the depth at three or four points along the length and average those instead.

A worked example

A 32 ft by 16 ft rectangular pool sloping from 3½ ft at the shallow end to 8 ft at the deep end, with an 8 hour turnover target.

• Average depth: (3.5 + 8) ÷ 2 = 5.75 ft
• Cubic feet: 32 × 16 × 5.75 = 2,944 cu ft
• Gallons: 2,944 × 7.48 = 22,022 gallons
• Litres: 83,350
• Surface area: 32 × 16 = 512 sq ft
• Turnover flow: 22,022 ÷ (8 × 60) = 46 GPM
• Weight of the water: about 92 tons

Why the number has to be right

This is not an academic figure. Almost every decision you make about the pool afterwards is expressed *per gallon*, so an error in the volume propagates into every one of them.

Chemical dosing. Every product on the shelf is labelled in the form "X ounces per 10,000 gallons raises Y by Z". Calcium hypochlorite, dichlor, muriatic acid, soda ash, calcium chloride, cyanuric acid, algaecide — all of them. If you believe your 22,000 gallon pool holds 18,000, every dose you make is twenty percent light, your chlorine never holds, and you spend the season chasing algae. Believe it holds 28,000 and you are over-dosing acid, driving the pH down and the alkalinity out, and corroding the heater.

Turnover and pump sizing. The health-code convention for residential pools is one complete turnover every 8 hours, and public pools are held to 6 or less. Flow required = volume ÷ (turnover hours × 60). Get the volume wrong and you buy the wrong pump, which either fails to circulate or runs the system above its plumbing's design velocity.

Filter sizing works from the same flow figure, expressed as gallons per minute per square foot of filter area, so it carries the same error.

Heater sizing and run cost. Raising the water one degree takes about 8.34 BTU per gallon, so a 22,000 gallon pool needs roughly 183,000 BTU per degree of rise before you account for surface losses. The surface area figure matters here too — most of a pool's heat loss is evaporation off the top, which is why a cover is the single most effective thing you can do about heating cost.

Draining and refilling is priced per thousand gallons on your water bill, and hauled water is priced by the tanker load.

Getting a better number on an odd pool

Freeform pools with tanning ledges, swim-outs, benches and beach entries defeat the simple multipliers. Two practical approaches: divide the pool into recognisable pieces on the plan — a rectangle plus a semicircle plus a wedge — calculate each and add them; or, if the pool is being filled from empty, put a meter on the hose and read it. A metered fill is the only truly accurate answer, and it is worth writing on the inside of the equipment cabinet door where the next person will find it.