Why pool volume actually matters
Pool volume isn't a trivia figure. It's the number that everything else about running a pool is dosed against: chlorine, salt, acid, algaecide, and whatever a pool shop hands you to fix a water problem. Every one of those products is measured per thousand litres or per cubic metre, and a wrong volume figure means every dose you add from that point on is wrong too, either under-treating the water or overdosing it. Most pool volume tools online return the number in US gallons, because most of them are built for an American audience, which is exactly backwards for anyone standing at an Australian pool shop counter holding a bag of stabiliser dosed in litres per thousand.
This calculator works in litres and cubic metres, the units your test strips, your pool shop and your dosing instructions actually use, and it covers the four shapes most backyard pools fall into: rectangular, round, oval, and kidney or freeform.
Measuring a rectangular or oval pool
A rectangular pool is measured at the waterline, not the coping or the tiled edge, since that's the actual water surface the volume is based on. Length and width both matter here, and it's worth measuring at more than one point if the pool isn't perfectly square, which a lot of older pools aren't, and using the larger figure if the two ends differ.
An oval pool uses the same two measurements, the longest length and the widest width, because an oval is mathematically close enough to an ellipse for this purpose, and the area of an ellipse comes from exactly those two figures. It's an approximation, not an exact match to your pool's actual curve, but it's a close one, and it's the same approach every pool volume estimate uses for this shape, professional or otherwise.
Round pools
A round pool needs one measurement: the diameter at the waterline, taken straight across through the centre. If you're not confident you've found the true centre, measure across in two or three different directions and use the largest reading, since measuring slightly off-centre will always understate the true diameter, never overstate it.
Kidney and freeform pools
A kidney-shaped or freeform pool doesn't have a formula of its own, because no two are the same curve. The practical way to estimate one, and the way this calculator does it, is to treat the pool as if it had two straight ends: measure the width at the widest point, the width at the narrowest point, and the distance between them, and use those three figures. It's an approximation rather than an exact measurement, which is true of every method for a genuinely irregular shape, including the ones pool builders use on a quote. It gets you close enough for dosing purposes, which is what the volume figure is actually for.
If your pool has a genuinely complex shape, an attached spa, a beach entry, or several distinct sections at different depths, the most reliable approach is to break it into two or three simpler shapes, rectangle, oval or kidney, work out each section separately using this calculator, and add the volumes together. That takes longer than one pass but gives a far more honest number than forcing an odd shape into a single set of measurements.
Constant depth versus a shallow and deep end
Plenty of backyard pools, especially smaller plunge pools and above-ground pools, are one consistent depth from end to end, and that's the simpler of the two depth options here. Most in-ground pools built for swimming laps or diving slope from a shallower end to a deeper one, and for those, use the shallow and deep end option rather than guessing at one number in between.
The maths behind the sloping option is straightforward and exact for a floor that slopes evenly the whole way from one end to the other: the average of the shallow and deep readings gives the true average depth across the whole pool. If your pool has a flat shallow section, a slope, and then a flat deep section, rather than one continuous slope, this will be a close approximation rather than an exact figure, and it'll tend to slightly understate the true volume, since a flat deep section holds more water than a straight-line average between two end points would suggest.
Measure both depths at the waterline down to the pool floor, not to the top of the coping, and measure the deep end at its actual lowest point, which on some pools is a small sump or drain area rather than the main floor of the deep end.
What the litre figure is actually for
Once you've got a volume figure, it's what goes into working out how much chlorine, salt, stabiliser or acid to add, and how a test result translates into a dose. A pool shop water test will typically hand you a set of readings and a dosing instruction based on your pool's volume in litres, so having an accurate figure on hand before you go in, rather than guessing it on the spot, gets you a more reliable dose recommendation. It's also the figure a pool builder or plumber will ask for if you're getting a heater, a chlorinator or a pump sized for your pool, since equipment is rated to handle a specific volume of water efficiently.
If your pool volume changes, because you've added a section, changed the water level the pool normally sits at, or you're not confident in an old figure from when the pool was first built, it's worth re-measuring rather than relying on a number that's years old, particularly if you've had any work done on the pool shell or coping since.
A worked example
Take a rectangular pool 8 metres long and 4 metres wide, sloping from a shallow end at 1 metre to a deep end at 1.8 metres. Surface area is 32 square metres. Average depth, taking the midpoint of the shallow and deep readings, is 1.4 metres. Multiply the two and the volume comes out to 44.8 cubic metres, or 44,800 litres. That's the figure you'd hand a pool shop for a water test, or use to work out a salt or chlorine dose from a product's per-thousand-litre instructions.
Get the depth wrong by even a small amount and the error compounds across the whole volume. Reading the deep end as 1.5 metres instead of 1.8 metres on the same pool drops the average depth to 1.25 metres and the volume to 40,000 litres, a gap of nearly 5,000 litres from one measurement being read at the wrong point. On a dosing calculation, a gap that size is the difference between a correctly balanced pool and one that's been under or over-dosed without anyone realising why the readings don't match what was added.
Common measuring mistakes
The most frequent one is measuring at the coping or the tiled edge rather than the actual waterline, which slightly overstates both length and width, since the coping usually overhangs the water by a small margin all the way around. It's a small error on its own, but it compounds with any error in the depth reading, and the two together can push a volume estimate further out than either mistake would on its own.
The second is treating a pool with a shallow ledge or bench seat as if it were a simple slope from one end to the other. A ledge holds a section of the pool at a shallow, constant depth rather than sloping continuously, and averaging straight from the shallow end to the deep end without accounting for it will understate the volume, since that shallow section holds less water than a smooth average would suggest across its length. For a pool with a significant ledge or bench, it's worth treating that section separately, as its own shallow rectangle, and adding it to the sloped section's volume rather than averaging across the whole thing in one pass.
The third is not re-measuring after the water level has been topped up or dropped. Pools lose water to evaporation and splash-out over summer, and if yours regularly sits a few centimetres below where it was when it was first filled, the true working volume is slightly less than the figure from the original build measurements. It rarely matters for day-to-day dosing, but it's worth knowing if a water test result seems consistently off from what the numbers on a product label suggest it should be.
When to bring in a pool professional
Working out your pool's volume and using it to guide everyday dosing, chlorine, pH, salt, is well within reach for most pool owners once they've got an accurate litre figure. A persistent water problem that won't clear with normal dosing, algae that keeps coming back, equipment that's underperforming, or anything involving the pool shell, coping or surrounding paving, is a job for a licensed pool professional. Getting a second opinion on a stubborn water chemistry issue is usually cheaper than working through a chemical cabinet's worth of products on guesswork, and a pool shop with your actual volume figure in hand can usually diagnose a lot faster than one working from a rough estimate.
If you run a pool servicing business, our page on websites for pool servicing covers what makes a trade site convert searches like this one into real bookings, and our full set of calculators covers a wide range of other figures a trade business runs on day to day.