How much water is in your pool?
Pool volume is the number everything else hangs on: the chlorine dose, the filter run time, the cost of filling and the amount you lose to evaporation every week. It is also easy to get wrong, because pools are rarely the simple shapes their brochures suggest. Measure the surface, work out the average depth, then apply the formula for the shape.
Surface area first
- Rectangle: length × width. A 40 ft × 20 ft pool is 800 ft² (74 m²).
- Circle: π × radius². A 24 ft round pool is about 452 ft² (42 m²).
- Oval: π × half the long axis × half the short axis.
- Kidney or freeform: maximum length × average width × a shape factor of 0.55–0.75, depending on how much of the rectangle the pool actually fills.
Then the average depth
Add the shallow-end and deep-end depths and divide by two. Using the deep end alone overstates the volume badly, because the shallow half of most pools is genuinely shallow. For a pool with a spa shelf, a sun ledge or a sloping beach entry, subtract that area and calculate it separately.
The formulas in one place
| Shape | Formula |
|---|---|
| Rectangle | L × W × average depth |
| Round | π × r² × average depth |
| Oval | π × (L ÷ 2) × (W ÷ 2) × average depth |
| Kidney / freeform | L × average W × depth × 0.65 |
One cubic metre is 1,000 litres, 264.2 US gallons and about one tonne of water — a useful sanity check, because a 48 m³ pool really does weigh 48 tonnes, which is why the shell has to be engineered for it.
Filling, topping up and running costs
Filling a pool is a one-off cost, but topping it up is not: an uncovered pool loses roughly 4 mm of depth a day to evaporation in mild weather, more in heat and wind. On a 32 m² surface that is about 128 litres a day — 3,800 litres a month, or one full pool every year. A cover cuts that by 85–95%, which usually pays for itself within a couple of seasons. Running costs then come from the pump: a residential pool is normally turned over completely every 6–8 hours, so a 48 m³ pool needs a pump moving about 8 m³/h (35 GPM).
Draining: read this before you empty the pool
An empty in-ground pool can float or crack when groundwater pushes against the shell, and a vinyl liner can shrink once it dries. Never drain a pool completely without checking the manufacturer's procedure, and never drain it into a storm drain or waterway — most areas require the water to be dechlorinated and discharged to a sanitary sewer or a landscaped area. If you only need to reduce the mineral level, a partial drain of 10–20% is enough.
Frequently asked questions
How many gallons is my pool?
Multiply the surface area by the average depth, then convert: 1 cubic metre is 264.2 US gallons. A 20 ft × 40 ft pool at an average of 5 ft is 4,000 ft³, which is about 29,900 gallons (113 m³) — the classic "30,000 gallon" pool.
What is the average depth of a pool?
Add the shallow end and the deep end and divide by two. A pool that runs from 3 ft to 8 ft has an average depth of 5.5 ft. Using 8 ft instead overstates the volume by nearly 50%, which then throws off every chemical dose you calculate from it.
How do I calculate the volume of a round or oval pool?
For a round pool, π × radius² × depth. A 24 ft round pool at 4 ft deep is 3.14 × 144 × 4 = 1,809 ft³, or about 13,500 gallons. For an oval, use π × half the long axis × half the short axis × depth.
How much water does a pool lose to evaporation?
An uncovered pool typically loses about 4 mm of depth a day in mild weather, and 7 mm or more when it is hot and windy — that is roughly 4 litres per square metre per day. On a 32 m² pool, about 128 litres a day, or 3,800 litres a month. A cover cuts evaporation by 85–95%.
What size pump does my pool need?
Size the pump so the whole volume turns over in 6–8 hours: divide the volume by 6 for the flow in cubic metres per hour. A 48 m³ pool needs about 8 m³/h (35 GPM). Check the pump curve at your actual plumbing resistance rather than the headline rating.
Can I drain my pool completely?
Not without checking with the manufacturer. An empty in-ground pool can float or crack under groundwater pressure, and a vinyl liner can shrink when it dries. Partial drains of 10–20% are used to reduce stabiliser or mineral levels; full drains should follow a documented procedure, and the water usually has to be dechlorinated before disposal.