How many litres does my pool hold?
Before the garden hose runs for hours, it pays to take a quick look at the volume. It determines how long filling takes, what the water costs, how much treatment product you need and how large the filter should be. The good news: two simple formulas cover the common pool shapes.
The quick-start examples in the volume calculator include a rectangular pool, a round pool, a rain barrel and a water tank. Simply adjust the measurements to your own pool.
Rectangular pool: length × width × water depth
Mathematically, a rectangular pool is a cuboid. Its volume is V = l · w · h. Make sure to use the actual water depth for h, not the height of the wall — usually 10 to 15 cm at the top stay empty.
Example: a pool 8 m long, 4 m wide with 1.5 m of water holds 8 · 4 · 1.5 = 48 m³. Since one cubic metre equals 1,000 litres, that is 48,000 litres.
If your pool has a shallow and a deep end, use the average depth: (shallowest + deepest point) ÷ 2. For 1.0 m and 2.0 m that is 1.5 m. For a pool with a step it is better to split it into two cuboids and add the volumes.
Round pool: π × radius² × water depth
Round above-ground pools are cylinders: V = π · r² · h. Product specifications usually give the diameter, which you need to halve first. A round pool with a 4.5 m diameter and 1.2 m of water has r = 2.25 m and therefore V = π · 2.25² · 1.2 ≈ 19.1 m³, about 19,100 litres.
Careful with oval pools: they are not circles. As an approximation, treat them as a rectangle with two semicircles at the ends, or multiply the ellipse area π · a · b (a and b are the semi-axes) by the water depth. The area calculator gives you the ellipse area.
Rain barrel and water tank
A rain barrel is roughly a cylinder too. A barrel with a 60 cm diameter and 90 cm height holds π · 30² · 90 ≈ 254,500 cm³ — about 254 litres. Because barrels are often slightly bulged or tapered, the real capacity differs by a few percent.
Box-shaped water tanks are calculated like a rectangular pool. A tank with inner dimensions of 1.2 × 1.0 × 1.0 m holds 1.2 m³ = 1,200 litres. Measure on the inside, because wall thickness and frame can easily account for ten percent in small containers.
Weight: one litre of water weighs one kilogram
Water is heavier than many people think. One litre weighs about one kilogram, so one cubic metre is roughly one tonne. The 48,000-litre pool puts 48 tonnes on its base slab, the round pool about 19 tonnes. Spread over 15.9 m² of floor area, that is about 1.2 tonnes per square metre. For a pool on a terrace, balcony or roof, have the load-bearing capacity checked by a professional first. The calculator shows the weight of the water directly below the volume.
Estimating fill time and water cost
Depending on water pressure, a garden hose delivers about 10 to 20 litres per minute. At 15 litres per minute, filling the 48,000-litre pool takes about 53 hours; the round pool is full after just over 21 hours.
For the cost, multiply the volume in cubic metres by your utility's price for drinking water and wastewater. If the combined price is, say, €4 per cubic metre, filling the large pool costs about €192 and the round pool about €76. Some municipalities waive the wastewater fee for pool water if you use a separate meter — ask your local authority.
Dosing treatment products correctly
Dosage instructions for pool treatment almost always refer to 1 m³ or 10 m³ of water. With the exact volume you avoid over- and under-dosing. Example: 20 g per 10 m³ means about 38 g for 19.1 m³. Recalculate after large top-ups, because only the water actually in the pool counts.
Conclusion
Calculating pool volume is not rocket science: rectangular pools are cuboids, round ones are cylinders. What matters is the correct water depth, the halved diameter and the conversion 1 m³ = 1,000 litres. With the result you can plan filling, cost, treatment and structural load on a solid basis.