How this calculator works
A pool is a prism, so its volume is the surface area of the water times the average depth. For a rectangle the area is length × width, for a round pool it is π × radius², and for an oval it is π ÷ 4 times length × width. One cubic metre is 1,000 litres. If the floor slopes, average the shallow and the deep depths.
Worked example: a 6 × 3 m rectangular pool with water 1.0 m deep at one end and 1.6 m at the other has an average depth of 1.30 m and a surface of 18 m². Its volume is 23.4 m³, which is 23,400 litres, and the water weighs about 23.4 tonnes. A hosepipe running at 20 litres a minute takes 20 hours (0.8 days) to fill it, and at 10 litres a minute 1.6 days. At a flat 1.2 m, a 4 m round pool holds 15,080 litres, a 6 × 3 m rectangle 21,600 and a 6 × 3 m oval 16,965.
Assumptions and sources
- Assumptions used: volume = surface area × average depth, with 1 m³ = 1,000 litres; an oval is π ÷ 4 (0.7854) times a rectangle of the same length and width; the floor slopes evenly between the shallow and deep ends.
- A pool with a deeper middle section, steps or a bench needs the depth measured at several places and averaged. Freeform pools are harder: split the shape into rectangles and circles, work out each, and add them. During a hosepipe ban, filling a pool with a hosepipe is normally not allowed, so check with your water company before you start.
The formula and a worked example are shown above, so you can check any result by hand. Read how we calculate and check our numbers. Results are estimates for planning: supplier products vary, so check the size printed on the label.
Litres in common pools
Rectangular, round and oval pools of the same size, at a flat water depth of 1.2 m.
| Pool | Litres | m³ | Weight of the water |
|---|---|---|---|
| Round, 3 m across | 8,482 | 8.5 | 8.5 tonnes |
| Round, 4 m across | 15,080 | 15.1 | 15.1 tonnes |
| Round, 5 m across | 23,562 | 23.6 | 23.6 tonnes |
| Rectangular, 4 × 2 m | 9,600 | 9.6 | 9.6 tonnes |
| Rectangular, 6 × 3 m | 21,600 | 21.6 | 21.6 tonnes |
| Rectangular, 8 × 4 m | 38,400 | 38.4 | 38.4 tonnes |
| Oval, 6 × 3 m | 16,965 | 17.0 | 17.0 tonnes |
| Oval, 8 × 4 m | 30,159 | 30.2 | 30.2 tonnes |
Tips before you buy
- Know the volume before you add chemicals: the dose on the label is per litre or per cubic metre, and the wrong amount wastes money or makes the water unsafe.
- Use the depth of the water, not the height of the wall. A pool filled to the skimmer is some centimetres below the rim.
- A big pool is heavy: a litre of water weighs about 1 kg, so 20 m³ is about 20 tonnes. Check that the ground and any base can take it.
- A hosepipe takes days to fill a large pool, and during a hosepipe ban you cannot use one. Many people have the water delivered: ask your water company or a bulk water supplier.
- Time a bucket to find your hosepipe flow: the figure on the label is a maximum, and the real flow is lower.
Tools & materials for this project
Affiliate links As an Amazon Associate we earn from qualifying purchases. These links do not change what you pay.
Frequently asked questions
How many litres are in a swimming pool?
Multiply the surface area by the average depth in metres and by 1,000. A 6 × 3 m pool with an average depth of 1.30 m holds 23,400 litres (23.4 m³).
How many litres is a 4 m round pool?
At a flat water depth of 1.2 m, about 15,080 litres. Enter your own depth in the calculator for an exact figure.
How do I work out the volume of an oval pool?
Multiply length × width × 0.7854 to get the surface area, then the average depth and 1,000. A 6 × 3 m oval at 1.2 m deep holds 16,965 litres, about 79% of a rectangle of the same size (21,600 litres).
How much does the water in a pool weigh?
About 1 kg a litre, so 1 tonne for every cubic metre. This pool holds 23.4 m³, so the water weighs about 23.4 tonnes.
Can I fill a pool with a hosepipe during a hosepipe ban?
Usually not. Water company notices ban filling or topping up a domestic swimming or paddling pool with a hosepipe while the ban lasts, with a limited exception for the first fill of a newly built fixed pool. Water from a bucket, a water butt or a private borehole is not covered. Check your water company's notice.
Add this calculator to your website
Running a blog, a contractor site or a class page? You can put this calculator on it for free. Copy the code below, and please leave the link underneath the calculator in place. The small script just resizes the frame to fit the results; if your site strips scripts, the calculator still works inside a scrolling frame.
Related calculators & guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.