How do I calculate the volume of my swimming pool?

Measure your pool's length, width, shallow-end depth, and deep-end depth, then enter them into the calculator above with the correct shape selected. It averages your two depths and applies length x width x average-depth (with an ellipse-area correction for oval pools) to give an exact volume in litres, US gallons, UK gallons, or cubic metres.

Why does an oval pool hold less water than a rectangular pool of the same length and width?

Because an oval (ellipse) shape covers less floor area than the rectangle that encloses it — about 78.5% of that rectangle's area, a ratio of pi/4. So an oval pool with identical outer length, width and average depth to a rectangular one holds roughly a fifth less water.

How many gallons of water does an average residential pool hold?

A common mid-sized rectangular pool, around 10 m x 5 m (33 ft x 16 ft) with a 1.2-1.8 m (4-6 ft) shallow-to-deep slope, holds roughly 75 cubic metres, or about 19,800 US gallons (16,500 UK gallons). Smaller plunge pools hold well under 20,000 litres; large or deep pools can exceed 150,000 litres.

How many litres does a typical Gibraltar residential pool hold?

Given the limited garden space common on the Rock, many private pools are smaller plunge pools of 15,000-40,000 litres rather than full-size 75,000-litre pools — measure your own dimensions above for an exact figure.

Pool Volume Calculator — Litres & Fill Cost | Construction Calculators

Pool Volume Calculator: Gallons, Litres and Fill Cost

Before you can dose chemicals correctly or budget a water bill, you need one number: how much water your pool actually holds. That's not obvious from the outside — a rectangular pool and an oval pool with identical length, width and depth hold noticeably different amounts of water, and almost every real pool slopes from a shallow end to a deep end rather than sitting at one constant depth. This calculator handles both the two shapes that cover the overwhelming majority of residential pools — rectangular and oval — and the shallow-to-deep slope, so the volume you get back matches the pool you actually built.

The method: average your shallow-end and deep-end depths, then treat the pool as a uniform box (or ellipse) of that average depth. A rectangular pool's volume is simply length x width x average depth. An oval pool holds less water than a rectangle of the same footprint, because an ellipse's area is smaller than the rectangle enclosing it — this calculator applies that exact correction rather than overstating an oval pool's volume. Enter your shape, length, width, and both depths below, plus an optional local water price, for an exact volume and an estimated fill cost.

How to use the pool volume calculator

Select your pool's shape: rectangular (straight sides, a true box shape) or oval (curved, rounded sides, sometimes called a 'radius' or 'lazy L' shape when approximated this way).
Enter the pool's length and width — for an oval pool, use the longest and widest points, i.e. the length and width of the rectangle that would just enclose it.
Enter the shallow-end depth and the deep-end depth separately, measured from the waterline down to the pool floor at each end. If your pool has a constant depth throughout, enter the same value for both.
Optionally enter a local water price per cubic metre if you want an estimated fill cost alongside the volume — leave it at the default if you only want the volume figure.
Read off the total volume in your preferred unit (litres, US gallons, UK gallons, or cubic metres) and the estimated cost to fill the pool from empty.

The pool volume formula, explained

Average depth is the key simplification: average depth = (shallow-end depth + deep-end depth) / 2. This works because the slope between the two ends is assumed reasonably even, so the extra water held at the deep end roughly balances the water "missing" at the shallow end.

Rectangular pool: volume = length x width x average depth — the same box-volume formula used for a tank or raised bed, since a rectangle's area is length x width at every point.

Oval pool: volume = (pi / 4) x length x width x average depth. The (pi / 4) factor (~0.7854) is the ratio between an ellipse's area and the area of the rectangle enclosing it, so an oval pool holds about 21.5% less water than a rectangular pool of the identical outer length, width and average depth.

Worked example (rectangular): a pool 10 m long and 5 m wide, with a shallow end of 1.2 m and a deep end of 1.8 m. Average depth = (1.2 + 1.8) / 2 = 1.5 m. Volume = 10 x 5 x 1.5 = 75 m^3. Converting: 75 m^3 = 75,000 litres = 19,813 US gallons (75 x 264.17) = 16,498 UK gallons (75 x 219.97).
Worked example (oval, same footprint): the same 10 m x 5 m outer dimensions and 1.5 m average depth, but oval-shaped. Volume = (pi / 4) x 10 x 5 x 1.5 = 0.7854 x 75 = 58.9 m^3 — about 16.1 m^3 (16,100 litres) less than the equivalent rectangular pool, purely because of the ellipse-versus-rectangle area difference. Converting: 58.9 m^3 = 58,900 litres = 15,563 US gallons = 12,958 UK gallons.

Pro tips and common mistakes

Measure depth from the actual waterline, not from the top of the pool coping or deck — most pools are designed to run a few centimetres below the coping, and using the coping height instead of the true waterline overstates both depths and therefore overstates volume.

As a rough Gibraltar cost example, filling the 75 m3 (about 16,500-UK-gallon) rectangular pool worked through above at a typical water rate of around GBP 3 per m3 costs roughly GBP 225 — many residential pools in Gibraltar are smaller plunge pools built into limited garden or terrace space, which will fill for proportionally less.

Don't assume a rounded or 'freeform' pool is close enough to rectangular — treating an oval pool as a rectangle overstates its volume by roughly a fifth, which throws off chemical dosing and fill-cost estimates in the same direction.
A shallow ledge, tanning shelf, or built-in bench that takes up meaningful floor area means the average-depth method will slightly overstate true water volume — for large built-in features, measure and subtract that feature's own volume separately.
Chemical products (chlorine, shock, algaecide, pH adjusters) are almost always dosed per a fixed volume, commonly per 10,000 US gallons or per 10 m^3 — knowing your pool's exact volume avoids the two most common maintenance mistakes: under-dosing and over-dosing.
1 cubic metre = 1,000 litres = 264.17 US gallons = 219.97 UK (imperial) gallons — useful for cross-checking a filling company's quote against this calculator's output.