Calculate rectangular pool water volume
Use average water depth for a straight slope from shallow to deep.
Back to calculator ↑Measure water surface and depths
Measure internal length and width at the water surface, then water depth at shallow and deep ends. Do not use structural depth above the water line.
Measure depths from the water surface to the floor, not from the pool rim. If water sits 0.15 m below the rim, rim-based depths can materially overstate volume. Take several depth readings if you are unsure the floor slope is straight, then split the pool into segments.
Average a straight slope
For a straight floor slope and vertical walls, average depth is (shallow + deep) ÷ 2. Multiply by length and width, then convert m³ to litres.
The endpoint average works for a uniform straight slope. If a long shallow shelf drops suddenly to a deep zone, averaging only the two ends can misstate volume. Divide the pool into simpler sections and add each section’s volume.
Worked default example
An 8 × 4 m pool with 1 m shallow and 2 m deep ends has 1.5 m average depth, giving 48 m³ or 48,000 L. Fill cost remains unknown until a water price per m³ is entered.
At an entered fill-water rate of THB 20/m³, the arithmetic cost is THB 960 before fixed fees or tiered pricing. In the same 32 m² surface footprint, lowering water level by 0.1 m changes volume by roughly 3.2 m³.
Check shape and maintenance needs
Steps, benches, multiple slopes and freeform pools change actual volume. This result does not prescribe chemical dosing or filtration turnover.
Use this volume as a starting point for comparing fill bills or discussing the pool with a maintainer. Do not dose chemicals from volume alone; test water and follow product directions. Pump and filter selection also depends on plumbing, fittings and system requirements, not just litres.
Entering THB 0 for fill water should mean that price is genuinely zero; blank means unknown and leaves cost unknown. If the water bill uses tiers, multiplying all 48 m³ by one rate is only a scenario. Obtain a quote or estimate from the applicable schedule before a large fill.
If filling by water delivery truck, charges may be per trip with a minimum delivery size, rather than a simple price per m³. Compare this arithmetic fill cost with a quote for the chosen method.
Common questions
Why use average depth?
For a straight slope, volume equals surface area times average depth.
What if depth is uniform?
Enter the same value for shallow and deep.
Can I use this to dose chlorine?
Do not rely on volume alone; follow product directions and test the water.
Sources and calculation guidance
Check these against your chosen product’s label and instructions.