How much water does a pool hold?

How much water does a typical pool hold?

A typical 26 × 13 ft (8 × 4 m) family pool, 3′ 11″ (1.2 m) deep at the shallow end and 5′ 11″ (1.8 m) at the deep end, holds about 11,624 gal (44 m³) of water. A 33 × 16 ft (10 × 5 m) pool at the same depths holds about 17,964 gal (68 m³), a compact 20 × 10 ft (6 × 3 m) pool about 6,076 gal (23 m³), and a 13 × 8 ft (4 × 2.5 m) plunge pool, 4′ 3″ (1.3 m) deep throughout, about 3,170 gal (12 m³).

In litres, add three zeros: the 33 × 16 ft (10 × 5 m) pool is 17,964 gal (68,000 litres), the plunge pool 3,170 gal (12,000 litres). These are the volumes you actually fill, up to where the water stands in a skimmer pool, and they come from the same geometry as every plan in the plan library.

Two things move the number more than people expect. Depth acts on the whole floor at once, so 3 7/8″ (10 cm) more across an 26 × 13 ft (8 × 4 m) pool is another 845 gal (3.2 m³). And shape: an 26 × 13 ft (8 × 4 m) oval holds about 8,982 gal (34 m³), a fifth less than the rectangle. The table below gives the water, the hours to fill it from a garden tap and the heat pump it calls for.

How do you calculate pool volume? Length × width × average depth

For a rectangle the sum is short: length × width × average depth, all in metres, gives cubic metres. Measure the water, not the hole: length and width inside the walls, and depth from where the water will stand, not from the deck or the top of the coping. In a skimmer pool the water sits roughly 5 7/8″ (15 cm) below the deck — the freeboard; in a deck-level overflow pool it is level with it.

Worked through for the 26 × 13 ft (8 × 4 m) pool: its floor runs from 3′ 11″ (1.2 m) to 5′ 11″ (1.8 m), an average of 4′ 11″ (1.5 m), so the hole holds 344 sq ft (32 m²) times 4′ 11″ (1.5 m), or 12,680 gal (48 m³). Take off the 5 1/8″ (13 cm) of freeboard the plan pages allow and the water averages about 4′ 6″ (1.37 m), which gives the 11,624 gal (44 m³) in the table. Both numbers are right, but the second is the one you fill, heat and dose.

What is the average depth of a pool with a sloped floor?

If the floor slopes evenly from end to end, the average depth is shallow plus deep, halved. Most family pools have a flat shallow third, a slope through the middle and a flat deep end, and the halfway figure still lands close, because the two flat zones roughly balance.

When the zones are unequal, weight them by length. A 32′ 10″ (10 m) pool with 13′ 1″ (4 m) of floor at 3′ 11″ (1.2 m), a 9′ 10″ (3 m) slope and 9′ 10″ (3 m) at 5′ 11″ (1.8 m) averages 4′ 10″ (1.47 m): each zone's length times its average depth, added up and divided by the total length. The halfway figure misleads most on a short, deep pocket under an old diving board, where it overstates the water by a tenth or more.

Then take off what is built into the water. At the designer's default sizes in an 26 × 13 ft (8 × 4 m) pool, a solid flight of four steps 5′ 3″ (1.6 m) wide displaces about 343 gal (1.3 m³), a 7′ 3″ (2.2 m) bench about 370 gal (1.4 m³) and a 8 × 5 ft (2.4 × 1.5 m) tanning ledge about 872 gal (3.3 m³), because it fills almost the whole depth beneath it: about 1,585 gal (6 m³) together, a seventh of the pool. Treat those as orders of magnitude, not your numbers: a ledge displaces its footprint times the solid under it, a flight of steps a little over half the box it stands in, a ladder nothing.

Every machine in the plant room and every dose in the shed is sized from one number, so take it from the water line, not the brochure.

Round, oval and kidney pools: the formula for each shape

A round pool is π × radius × radius × average depth: a pool 13′ 1″ (4 m) across has 136 sq ft (12.6 m²) of surface and, at the plan library's 3′ 11″–5′ 3″ (1.2–1.6 m) filled to a skimmer line, holds about 4,227 gal (16 m³). An oval — a true ellipse — is π/4 × length × width × average depth, about 0.785 of the rectangle around it, which is why the 26 × 13 ft (8 × 4 m) oval in the table holds 8,982 gal (34 m³) against the rectangle's 11,624 gal (44 m³).

Check which oval you have. Many builders' ovals are a rectangle with a half-circle at each end, which holds more: the straight middle (length minus width) times the width, plus one full circle as wide as the pool. Drawn that way an 26 × 13 ft (8 × 4 m) oval has 308 sq ft (28.6 m²) of surface, not the ellipse's 269 sq ft (25 m²).

Kidneys and free-form pools have no honest one-line formula, because how much of its box a kidney fills depends on how deep the waist cuts in: the 26 × 15 ft (8 × 4.5 m) kidney in the table fills under 60% of it and holds 7,397 gal (28 m³), less than the narrower oval. Split the outline into rectangles and circles and add them up, or count the squares of a 3′ 3″ (1 m) grid on a scale drawing. For a shape close to one of the designer's ten, set its length, width and depths there and read the volume it reports — worked out from the outline itself, with the steps, ledge and bench you place already taken off.

How many gallons is a cubic metre? Litres, cubic metres and US gallons

Metric is the easy system: 264 gal (1 m³) of water is a thousand litres and weighs a tonne, and European labels, datasheets and water bills all speak in cubic metres or litres.

Gallons need a question first: which ones. A British imperial gallon is about a fifth larger than a US one, so check which a figure means; every gallon on this site is a US gallon.

In the United States the same sum is done in feet: length times width times average depth, all in feet, gives cubic feet, and each cubic foot holds 7.48 US gallons. A 16x32 at the usual depths comes to about 17,171 gal (65 m³). The table below runs the common US sizes.

How long does it take to fill a pool, and what does the water cost?

A garden tap gives roughly 6.6 GPM (1.5 m³/h), the rate the plan pages assume: about eight hours for the 13 × 8 ft (4 × 2.5 m) plunge pool, 29 for the 26 × 13 ft (8 × 4 m) and nearly two days for the 33 × 16 ft (10 × 5 m). Time your own tap first: if a 3 gal (10-litre) bucket fills in 24 seconds you have that rate; if it takes 36, the fill runs half as long again.

A water delivery service fills a pool in a few hours, and patience is not the only reason to call one: a fiberglass shell is filled while it is backfilled, so the pressure stays even on both sides of its wall; well water with iron in it stains a new finish; and a slow or expensive supply makes the tap no bargain. Ask how much one load carries — a family pool can take more than one.

The fill costs your volume times the price per cubic metre on your own water bill; tariffs vary too much for a national figure to help. Many utilities also charge for sewage by the water you draw, so ask whether a pool fill counts and whether a separate garden meter is allowed. And the first fill is not the last: an uncovered pool loses 1/8–1/4″ (3–5 mm) a day to evaporation in warm weather, and each sand-filter backwash sends 79–264 gal (300–1,000 litres) down the drain.

Chemicals, heat and filtration: why everything is sized per cubic metre

Every bottle of pool chemistry gives its dose per volume of water, and the arithmetic underneath is friendly: one milligram per litre in one cubic metre is one gram. Raising free chlorine by 1 mg/l in a 11,624 gal (44 m³) pool takes 44 grams of active chlorine, more of the product depending on its strength; a salt system at about 0.4% salt needs 4 kg per cubic metre, about 175 kg to start that pool. Get the volume a tenth wrong and every dose is a tenth out.

Heat is sized the same way. A cubic metre takes 1.16 kWh per degree, so the 11,624 gal (44 m³) pool needs about 51 kWh for each one, and a heat pump for a covered pool is sized at about 0.35 kW per cubic metre: 55,000 BTU/h (16 kW) for that pool, 82,000 BTU/h (24 kW) for the 33 × 16 ft (10 × 5 m), 17,000 BTU/h (5 kW) for the plunge pool — the table's last column. The pool heating guide covers when to go a size up.

Filtration follows volume too. A private pool wants one turnover — the whole volume through the filter — every 4–6 hours, so the 11,624 gal (44 m³) pool needs about 30.8–48.4 GPM (7–11 m³/h), with the pump chosen a step above to cover the pipework. A deck-level overflow pool adds a balance tank of roughly 5–10% of its volume: another 660–1,321 gal (2.5–5 m³) for a pool this size.

How much does the water in a pool weigh?

A cubic metre of water weighs a tonne, so the 26 × 13 ft (8 × 4 m) pool holds 44 tonnes and the 33 × 16 ft (10 × 5 m) 68. In the ground that is rarely the problem: a pool, water and all, generally weighs no more than the soil dug out for it. The risk there runs the other way: an empty shell in wet ground can be lifted by the groundwater, so never drain a pool without asking the builder.

Above ground it is a different calculation. Water presses a tonne on each square metre for every metre of depth, so a terrace or roof under a 4′ 3″ (1.3 m) plunge pool carries 1.3 tonnes on every square metre before the shell and the bathers — many times what an ordinary floor or terrace is built for. A raised pool's walls take the same water sideways, twice as hard at the floor as halfway down. Both are questions for a structural engineer, before the pool is chosen.

Why the volume in a brochure differs from what you fill

A brochure, a quote and your water meter can give three different volumes for one pool, and usually nobody is lying. The commonest gap is depth: a shell's depth is quoted from the deck, so the 5 1/8″ (13 cm) of freeboard in a skimmer pool takes about 1,057 gal (4 m³) off an 26 × 13 ft (8 × 4 m). Built-in steps, a ledge and a bench take more, and a shell named for its outside dimensions, as some prefabricated ones are, is smaller inside than its name.

The edge matters too. The same 26 × 13 ft (8 × 4 m) shell holds about 11,624 gal (44 m³) with skimmers and about 12,680 gal (48 m³) as a deck-level overflow pool, whose water comes up to the brim — and then its balance tank and channel need filling as well. On fill day the meter reads a little higher still, because the pipes and the filter fill too.

So ask what a quoted volume means — brim or water line, with or without the steps, inside or outside the walls, balance tank or not — then size everything from the water-line figure and write it on the plant-room wall.

What goes wrong in practice

1. Measuring depth from the deck

   The tape starts at the top of the coping, the water stands below it, and an 26 × 13 ft (8 × 4 m) pool gains 1,057 gal (4 m³) it does not have. Every dose and heat-pump quote built on that figure comes out a tenth too big.

2. Taking the deep end as the depth

   Multiplying by the deepest point overstates a pool that runs from 3′ 11″ (1.2 m) to 5′ 11″ (1.8 m) by a fifth. Use the average, weighted by zone when the deep end is short.

3. Treating an oval or a kidney as its box

   An oval holds about four fifths of the rectangle around it, and the kidney in our plan library less than three fifths. Length × width × depth on either describes a pool that does not exist.

4. Dosing from the brochure

   A brim-full figure that ignores the steps and a ledge can sit a fifth or more above the water you actually have. Dose from it and you overshoot every time.

5. Forgetting the weight

   A plunge pool on a roof terrace puts more than a tonne on every square metre under it. Have an engineer check the structure before the pool is chosen, not after it is delivered.