Pool volume calculator

What is a pool volume calculator?

A pool volume calculator tells you how much water a swimming pool holds. It is the single most useful number an owner can know: almost every chemical you add — chlorine, algaecide, stabiliser, pH adjuster — is dosed per gallon or per litre of water, and so are the flow ratings that size a pump, a filter, or a heater. Guess the volume and you either waste product or fail to treat the water properly.

The calculator supports the three shapes that cover most residential pools:

• Rectangular — the classic straight-sided pool, including most above-ground frame pools.
• Round — circular above-ground pools and plunge pools.
• Oval — stadium-style and elliptical pools, wider in the middle than at the ends.

Pick a shape, enter its dimensions and the average depth, and the result is shown in US gallons by default. The unit selector next to the result switches it to litres, cubic metres, cubic feet, or imperial gallons, and every dimension can be typed in feet, metres, inches, centimetres, or yards — so it does not matter which measuring system your tape uses.

How does the calculator work?

Every pool shape is handled the same way: work out the surface area of the water, then multiply by the average depth to get a volume.

$V = A \times d$

What changes between shapes is only how the area $A$ is found. The calculator computes the volume internally in cubic metres and then converts it to whichever unit you select, so you never have to chain conversion factors by hand.

Write $l$ for the length, $w$ for the width, $D$ for the diameter of a round pool, and $d$ for the average depth.

Rectangular pool

The water surface is a rectangle, so the area is length times width:

$V_{rect} = l \times w \times d$

Round pool

The surface is a circle whose radius is half the diameter, giving an area of $\pi (D/2)^2$:

$V_{round} = \pi \times \left(\frac{D}{2}\right)^2 \times d$

Oval pool

The surface is an ellipse with semi-axes $l/2$ and $w/2$, whose area is $\pi \times \frac{l}{2} \times \frac{w}{2}$:

$V_{oval} = \pi \times \frac{l}{2} \times \frac{w}{2} \times d$

Notice that an ellipse fills only $\pi/4 \approx 78.5\%$ of the rectangle that would just enclose it — an oval pool of the same length, width, and depth as a rectangular one holds a little over three-quarters as much water.

Converting the result

Once the volume is known in cubic metres, the conversions are exact multiples:

• US gallons: multiply by 264.172 (1 m³ = 264.172 US gallons).
• Litres: multiply by 1000 (1 m³ = 1000 litres).
• Cubic feet: divide by 0.0283168 (1 ft³ = 0.0283168 m³). One cubic foot of water is 7.48 US gallons.

Worked examples

Example 1: Rectangular pool

A rectangular pool measures 10 m long and 5 m wide, with an average depth of 1.5 m.

$V = 10 \times 5 \times 1.5 = 75 \text{ m}^3$

Converting that volume:

$75 \times 264.172 \approx 19{,}812.9 \text{ US gallons}$

So the pool holds 75 m³, which is 75,000 litres, about 19,812.9 US gallons, or 2,648.6 cubic feet. The same pool measured with an imperial tape is roughly 32.81 ft × 16.40 ft with an average depth of 4.92 ft — enter those figures in feet and you get the identical answer.

Example 2: Round pool

A circular above-ground pool has a diameter of 5 m and an average depth of 1.2 m. The radius is $5/2 = 2.5$ m.

$V = \pi \times 2.5^2 \times 1.2 = \pi \times 6.25 \times 1.2 \approx 23.56 \text{ m}^3$

That is about 23.56 m³, or 23,562 litres, or roughly 6,224.4 US gallons (832.1 cubic feet). In imperial terms this is a pool about 16.40 ft across, filled to an average depth of 3.94 ft.

Example 3: Oval pool

An oval pool is 8 m along its long axis and 4 m across its short axis, with an average depth of 1.4 m.

$V = \pi \times \frac{8}{2} \times \frac{4}{2} \times 1.4 = \pi \times 4 \times 2 \times 1.4 \approx 35.19 \text{ m}^3$

The pool holds about 35.19 m³, or roughly 9,295.1 US gallons. Note the ellipse effect: a rectangular pool of the same 8 m × 4 m × 1.4 m would hold 44.8 m³, and $35.19 / 44.8 = 0.785 = \pi/4$, exactly as predicted.

Practical notes

• Use the average depth, not the deepest point. For a pool whose floor slopes evenly from a shallow end to a deep end, the average depth is simply the mean of the two: a pool that runs from 1 m to 2 m has an average depth of $(1 + 2)/2 = 1.5$ m. A pool with a flat shallow section, a slope, and a flat deep section is better handled by splitting it into sections, working out each one, and adding the results.
• Measure the water, not the wall. Pools are filled to somewhere around the middle of the skimmer, typically several inches below the top edge. Using the full wall height overstates the volume — measure from the floor to the actual waterline.
• Gallons drive your chemistry. Dosing instructions are written per 10,000 gallons (or per cubic metre), so an error in the volume becomes an error in every dose you ever add. It is worth measuring carefully once and writing the number down.
• Water is heavy. One US gallon weighs about 8.34 lb, and one cubic metre of water is 1000 kg — the pool in Example 1 holds roughly 75 tonnes of water, which is why decks and above-ground supports matter.
• Freeform and kidney shapes. These have no simple formula. Approximate them by taking an average length and an average width and treating the pool as an oval, or split the pool into rectangular and circular pieces and sum them. Expect a few percent of error.
• For non-pool vessels the same geometry applies — a water butt or storage tank is covered by the tank volume calculator, a circular container by the cylinder volume calculator, and a general shape by the volume calculator.

Frequently asked questions

How many gallons is a typical backyard pool?

It varies enormously with size, but the 10 m × 5 m rectangular pool in Example 1 — a common family size — holds close to 20,000 US gallons. A small round above-ground pool of 5 m diameter holds around 6,200 US gallons.

Why does the calculator ask for the average depth?

Because the volume formula multiplies the water’s surface area by a single depth figure. Most pools are not the same depth everywhere, so the average depth is the value that makes the simple formula give the right total.

Does the pool’s volume change with the water level?

Yes — the volume the calculator reports is the volume of water actually in the pool, so it depends on where the waterline sits. Enter the depth you measure from the floor to the current (or intended) waterline, not the height of the pool wall.

Are US gallons and imperial gallons the same?

No. A US gallon is 3.785 litres, while an imperial (UK) gallon is 4.546 litres — about 20% larger. Chemical dosing instructions sold in the US are written for US gallons, which is why that is the default unit here; the result’s unit selector offers imperial gallons as well.