How to use it
- Choose the Pool shape: Rectangle or Round.
- Type the size of the water surface: Length and Width for a rectangle, or Diameter for a round pool.
- Type the Depth (m). For a rectangle with a sloped floor, also type the Depth at the deep end (m); leave it empty for a flat floor.
- Read Water volume (m³) and Water volume (litres). How it is worked out shows the steps with your numbers.
How the volume is calculated
A rectangular pool with a flat floor is a box: length times width times depth.
When the floor slopes in a straight line from a shallow end to a deep end, the water's cross-section along the length is a trapezoid. Its two parallel sides are the two depths, so the average depth is their mean, and the volume is the surface times that average.
A round pool of even depth is a cylinder: pi times the radius squared, times the depth.
Rectangle: V = L x W x (d₁ + d₂) / 2, with a flat floor d₂ = d₁. Round: V = π x (D / 2)² x dWhat the tool does not include
- The water volume only. Steps, benches, a skimmer box and other fittings take up space, so the real water is slightly less than the figure.
- Pools that are not a rectangle or a circle, such as kidney or free-form shapes, and floors that curve or have several slopes. For those, a survey gives a better figure.
- Water lost to splashing, evaporation or backwash, and the water you add each week.
- Chemical doses. The tool gives the volume only. Use the dosing instructions on the product label for your pool.
Worked example: a rectangular pool with a sloped floor
A rectangular pool is 8 m long and 4 m wide. The floor runs from 1.0 m deep at the shallow end to 2.0 m at the deep end, so the average depth is (1.0 + 2.0) / 2 = 1.5 m.
The volume is 8 x 4 x 1.5 = 48 m³, which is 48,000 litres.
| Result | Value |
|---|---|
| Water volume | 48.00 m³ |
| Water volume | 48,000 L |
| Average depth | 1.50 m |
Frequently asked questions
How do I measure the depth of a sloped floor?
Measure from the water surface down to the floor at the shallowest end and again at the deepest end, with a rule or a long measuring tape. Type those two values, and the tool works out the average.
Why is the average of the two depths used?
The floor descends in a straight line, so the water's cross-section is a trapezoid. The area of a trapezoid is half the sum of its parallel sides times its height, which here gives the mean of the two depths times the length.
Does the tool allow for steps or a bench in the pool?
No. The tool gives the volume of the basic shape. Steps and benches take up space, so the water in a pool with them is a little less than the figure.
How many litres are in a cubic metre?
One cubic metre holds 1,000 litres. A litre is one cubic decimetre, as the NIST table of units accepted for use with the SI states.
Which chemical dose should I use for the volume?
The tool does not give doses. Use the dosing instructions on the label of the product you buy, and compare them with the volume shown here.
Is the result exact?
It is exact for the shape you enter, but the shape itself is an estimate. A pool that is not a rectangle or a circle, or whose floor has several slopes, needs a survey for a more accurate figure.
Sources
- Wolfram MathWorld: Trapezoid (area one half of the sum of the parallel sides times the height)
- Wolfram MathWorld: Cylinder (volume pi r^2 h)
- NIST Special Publication 330, section 4, Table 8: 1 L = 1 dm³ = 10⁻³ m³
Figures and text last checked on Tuesday, November 10, 2026.