Pool volume calculator (litres)

Illustration.

Work out your litres

How the sum works

A pool's volume is its surface area times its average depth. For a rectangle: length × width × average depth, in metres, gives cubic metres; each cubic metre is 1,000 litres. Average depth is the shallow end plus the deep end, divided by two. An 8 by 4 metre pool, 1.0 m deep at one end and 1.8 m at the other, holds 8 × 4 × 1.4 = 44.8 m³, about 44,800 litres.

Round pools use π × radius² for the area; oval and kidney pools are close to an ellipse, π × half the length × half the width. Freeform pools are best measured as a few rectangles added together.

Maple's tip

Measure the water, not the shell. Use the length and width at the waterline and the depth from the waterline to the floor, and your doses will be closer.

Why your pool's volume matters

Every chemical dose is worked out per litre. If you guess the volume and guess low, you under-dose chlorine and the pool can turn green; guess high, and you overdose, wasting money and making the water uncomfortable or unsafe. Knowing the litres also tells you how long your pump needs to run to turn the water over, how much salt a salt pool needs, and what size heater or filter suits the pool.

Measuring your pool

1. Length and width: measure at the waterline, not the outer edge of the coping. For a round pool, measure the diameter across the middle.
2. Shallow depth: measure from the waterline to the floor at the shallow end.
3. Deep depth: measure from the waterline to the floor at the deepest point.
4. Average depth: add the shallow and deep depths and divide by two. For a pool with a long shallow section and a short deep hole, the average sits closer to the shallow depth, so measure a few points and average them.

A long tape measure and a pole marked in centimetres make it easy. Measure on a calm day so ripples do not throw the depth off.

Worked examples

Rectangle: 8 m long, 4 m wide, 1 m at the shallow end and 1.8 m at the deep end. Average depth is 1.4 m. Volume is 8 x 4 x 1.4 = 44.8 cubic metres, or 44,800 litres.

Round: 5 m across (2.5 m radius), 1.2 m deep throughout. Area is about 3.14 x 2.5 x 2.5 = 19.6 square metres. Volume is about 19.6 x 1.2 = 23.6 cubic metres, or about 23,600 litres.

Oval or kidney: 7 m long and 3.5 m wide at the widest point, average depth 1.3 m. Area is about 3.14 x 3.5 x 1.75 = 19.2 square metres. Volume is about 19.2 x 1.3 = 25 cubic metres, or about 25,000 litres.

Freeform and odd-shaped pools

For a freeform pool, break it into simple shapes: a rectangle for the main body, a half circle for a rounded end, a smaller rectangle for a ledge. Work out each shape's volume and add them up. It will not be perfect, but it will be close enough for dosing. Spas attached to pools share the water, so add the spa's volume if it overflows into the pool.

Typical backyard pool sizes

Many Australian backyard pools hold somewhere between 30,000 and 60,000 litres; plunge pools and small courtyard pools can be well under 20,000 litres, and large family pools can exceed 70,000. These are only rough guides: measure yours.

Checking your number

If you add salt to a salt pool and the salt reading rises less or more than expected, your volume estimate may be off. Some owners check volume this way: add a known amount of salt, wait for it to mix, and compare the change in reading with what the volume predicts. Your pool builder's plans, if you have them, may also list the volume.

Keep it handy

Write your pool's volume on the inside of the pump shed door or in your phone, so it is there whenever you dose. The calculator on this page remembers your pool size in your own browser too.

Above-ground and inflatable pools

Above-ground pools have straight sides, so their volume is simpler: area times the water depth. For a round above-ground pool, use pi times the radius squared times the depth; for an oval, pi times half the length times half the width times the depth. Inflatable pools are usually smaller and their sides slope, so the label's capacity is a good starting point. Always measure the water depth, which is often below the top rim.

Litres, gallons and cubic metres

Australian pool chemicals are dosed in litres and grams, so work in metres and litres. One cubic metre is 1,000 litres. If a manual or product uses US gallons, one US gallon is about 3.79 litres.

How the water level changes the volume

Pools lose water through evaporation and backwashing and gain it from rain, so the true volume changes a little. For dosing, use the volume at your normal water level, halfway up the skimmer opening.

Using the volume

Once you know the litres, use them for every dose in the pool chemical calculator, for setting salt levels in a salt pool, and for working out how long to run your pool pump. Measure once and keep the number somewhere you will see it.

Common measuring mistakes

1. Measuring the outer coping instead of the water's edge, which overstates volume.
2. Using the deepest point as the depth for the whole pool.
3. Forgetting the volume of a spa or large step area.
4. Mixing feet and metres between measurements.

Why calculators differ

Online pool calculators can give slightly different answers because they treat depth differently. Some use the simple average of the shallow and deep ends; others weight the shallow section more for pools with a long flat area. For dosing, a figure within about ten per cent is fine; for buying a large amount of salt or minerals, measure carefully and add in stages.

Volume and safety

A correct volume also matters for safety. Shocking a pool with a dose worked out for a much larger volume can leave chlorine far too high for swimming, while under-dosing leaves germs unchecked. Always retest after dosing, before anyone swims.

A quick check by depth

As a rough check, a rectangular pool's volume in litres is about its area in square metres times its average depth in metres times 1,000. A 9 by 4.5 metre pool with an average depth of 1.4 metres is about 9 x 4.5 x 1.4 x 1,000, or roughly 56,700 litres. If the calculator gives a very different number, recheck your measurements and units.

Common questions