A swimming pool is the largest body of water most households will ever manage, and nearly every decision about it runs through one number: how many litres it holds. Heater output, pump run times, fill schedules, winter covers, and above all chemical treatment — all of them are worked out per cubic metre of pool volume.
The geometry is honest: a pool is a hole with a measurable surface area and a measurable depth, and the maths from there is multiplication. This guide covers the average-depth method for sloped floors, the formulas for rectangular, circular and freeform pools, the conversions between litres, US gallons and UK gallons, and how to turn volume into a realistic fill time.
Pool volume = surface area × average depth, and for a sloped floor average depth = (shallow end + deep end) ÷ 2. A 10 × 5 m pool running from 1.0 m to 2.0 m deep holds 10 × 5 × 1.5 = 75 m3 = 75,000 L ≈ 19,800 US gallons. Filling is slower than people expect: at a typical 15–25 L/min garden tap, 40 m3 takes roughly a day and a half. The Pool Volume Calculator runs all of this, in litres and both gallons.
Why the Volume Number Drives Everything
Pool equipment is specified against volume, not length or waterline area. A heater is sized by how many litres it must warm: double the volume and you need double the kilowatts, or double the patience, to reach swimming temperature. A pump is rated by how fast it circulates the whole body of water — the trade's turnover concept; residential pools commonly target one full turnover every 4–8 hours of pumping, though practice varies by region and setup. A 75 m3 pool turned over in 6 hours needs a flow of 75 ÷ 6 = 12.5 m3/h, roughly 208 L/min.
Chemical treatment is where the volume number earns its keep, because dosing is directly proportional to it: most pool products specify an amount per 10,000 L or per cubic metre, so a volume estimate that is 20 percent off feeds a dose that is 20 percent off.
Pool dosing must follow the product manufacturer's instructions and professional guidance — never guesswork. The volume figures in this guide are planning estimates from simple geometry; real floors, steps and fittings all shift the number slightly, so confirm your pool's working volume with a pool professional before treating water. Pool chemicals are concentrated substances, and wrongly treated water is a health issue, not just a cloudy one.
The Average-Depth Method for Sloped Floors
Few pools have flat floors. The standard residential profile slopes from a shallow end to a deep end, and the standard approximation treats that slope as even: average depth = (shallow-end depth + deep-end depth) ÷ 2. A pool running from 1.0 m to 2.0 m has an average depth of (1.0 + 2.0) ÷ 2 = 1.5 m.
It is an approximation. Real slopes are often not even — flat shallow sections, steeper mid-slopes, hopper-shaped deep ends — so the method lands close for a simple two-depth pool but overestimates one with a large shallow shelf and a small deep basin. Where the floor shape is genuinely irregular, use the zone method below and treat each part honestly.
Rectangular Pools
For a rectangular or square pool: volume (m3) = length × width × average depth. Worked through for a 10 × 5 m pool with a 1.0 m shallow end and a 2.0 m deep end:
- Average depth: (1.0 + 2.0) ÷ 2 = 1.5 m.
- Volume: 10 × 5 × 1.5 = 75 m3.
- Litres: 75 × 1,000 = 75,000 L.
- Gallons: ≈ 19,800 US gal or ≈ 16,500 UK gal.
A 75-tonne body of water in a hole 10 m long — that is what a modest family pool actually is, and why the fill-time section later in this guide is worth reading before you open a tap.
Circular Pools
Round pools swap the rectangle's length-and-width for a radius: volume (m3) = π × radius2 × average depth. The radius is centre-to-edge, half the diameter. Worked through for an 8 m round pool (radius 4 m) sloping from 1.0 m to 1.4 m:
- Average depth: (1.0 + 1.4) ÷ 2 = 1.2 m.
- Surface area: π × 42 = 3.1416 × 16 ≈ 50.3 m2.
- Volume: 50.3 × 1.2 ≈ 60.3 m3 = 60,300 L (≈ 15,900 US gal).
The classic error here is using the diameter instead of the radius — an 8 m pool entered as r = 8 instead of r = 4 overstates the volume fourfold. Half first, then square.
Freeform Pools: Break It Into Zones
Kidney shapes and custom freeforms have no single formula, but they decompose into shapes that do. Divide the pool into rectangles, half-circles and triangles, measure each zone's surface area and average depth, compute each zone's volume, and add them up. For a curvy outline, two field tricks work: measure the width at regular intervals along the length, average the widths and multiply by the length for surface area; or sketch the pool to scale on graph paper and count squares. Neither is surveyor-precise; both land within a few percent, which is plenty for planning.
Litres, US Gallons and UK Gallons
Pool volumes are quoted in cubic metres or litres in metric countries, US gallons in North America, and UK (imperial) gallons in Britain — and the two gallons are not the same unit: a UK gallon is about 20 percent larger. Product instructions may be written for either, so convert once, carefully, before doing anything with a dose.
| From | To litres | To US gallons | To UK gallons |
|---|---|---|---|
| 1 m3 | 1,000 L | ≈ 264 US gal | ≈ 220 UK gal |
| 1 US gallon | 3.785 L | 1 | ≈ 0.833 UK gal |
| 1 UK gallon | 4.546 L | ≈ 1.201 US gal | 1 |
| 1 tonne of water | 1,000 L | 1 m3 of water has a mass of about 1 t | |
How Long Will It Take to Fill?
Fill time is pure division: time = volume ÷ flow rate. The volume side you now have; the flow side needs measuring, because pressure, pipe size, hose length and any restrictors all matter. A typical garden tap through a hose delivers roughly 15–25 L/min — a typical band, not a promise. Measure yours: time how long the hose takes to fill a 10 L bucket. A bucket that fills in 28 seconds is about 10 ÷ 28 × 60 ≈ 21 L/min.
The worked example, using a 40 m3 pool (a mid-sized family pool, 40,000 L) at 20 L/min:
- Minutes: 40,000 ÷ 20 = 2,000 minutes.
- Hours: 2,000 ÷ 60 ≈ 33 hours.
Roughly a day and a half of running water:
| Flow rate | Time to fill 40 m3 (40,000 L) |
|---|---|
| 15 L/min (slow tap, long hose) | ≈ 44 hours |
| 20 L/min (typical garden tap) | ≈ 33 hours |
| 25 L/min (good pressure, short hose) | ≈ 27 hours |
Practical notes: check on the fill regularly rather than leaving it fully unattended; plan chemicals for once the pool is full, not partway; and on a private well, watch the well — a pool fill is a serious draw. Where supply is restricted or time matters, tanker delivery turns a two-day fill into an afternoon.
Evaporation, Covers and Top-Ups
Evaporation typically removes a few millimetres of depth per day — commonly quoted around 3–6 mm in warm, sunny, breezy weather, far less in cool, still conditions; wind moves the number more than temperature does. The arithmetic is the same rule as rain gauge maths: 1 mm of depth over 1 m2 is 1 litre, so a 50 m2 pool losing 4 mm a day is losing about 200 L — on the order of 1,400 L a week.
A well-fitted cover changes the equation: covers are typically credited with stopping the large majority of evaporation loss — commonly quoted around 90 percent — while keeping debris out and retaining heat overnight. For an outdoor pool it is the single most effective water-saving accessory there is.
What remains is top-up water — the hose, run weekly or as needed. Top-ups add fresh water that dilutes the balance, so follow product and professional guidance on rebalancing after a significant one. Rain does its share: a 20 mm storm on a 50 m2 pool drops a free 1,000 L in.
Plan It With the Pool Volume Calculator
The Pool Volume Calculator takes length, width or radius plus shallow and deep depths, applies the average-depth method, and returns cubic metres, litres, US gallons and UK gallons — along with fill time at your measured tap flow. For garden ponds, the same geometry runs in the Pond Volume Calculator.
Open the Pool Volume CalculatorFrequently Asked Questions
How do I calculate the volume of a pool with a sloped floor?
Use the average-depth method: (shallow-end depth + deep-end depth) ÷ 2, then multiply by the surface area. A 10 × 5 m pool running from 1.0 m to 2.0 m has an average depth of 1.5 m and holds 75 m3 = 75,000 L. The method assumes a roughly even slope; for unusual floor shapes, split the pool into zones and calculate each separately.
How long does it take to fill a pool with a garden hose?
Time = volume ÷ flow. A typical garden tap through a hose delivers roughly 15–25 L/min — measure yours with a 10 L bucket and a watch (28 seconds to fill is about 21 L/min). At 20 L/min, a 40 m3 pool takes 40,000 ÷ 20 = 2,000 minutes ≈ 33 hours.
How do I convert cubic metres to litres and gallons?
1 m3 = 1,000 L ≈ 264 US gal ≈ 220 UK gal. The two gallons differ by about 20 percent (a US gallon is 3.785 L, a UK gallon 4.546 L), so check which one a product's instructions use before converting a dose.
How much water does a pool lose to evaporation?
Typically a few millimetres per day — often quoted around 3–6 mm in warm, sunny, breezy weather, and far less when cool and still. Because 1 mm over 1 m2 is 1 litre, a 50 m2 pool losing 4 mm a day loses about 200 L daily. A well-fitted cover typically stops the large majority of that loss.
Why does knowing my pool's exact volume matter?
Heater sizing, pump turnover and chemical dosing are all calculated per litre, so a volume estimate that is 20 percent off propagates into everything. Dosing in particular must follow product instructions and professional guidance — never guesswork. Treat calculated volumes as planning figures and confirm with a pool professional.
Related MyHouseLogic Guides
Volume is the first number in the Pools & Water set; the others build on it.
- Pond Volume Calculator — the same geometry for garden ponds, where volume drives pump and filter choices.
- Pond Liner Calculator — sizing a flexible liner from the deepest point, with overlap.
- The Pool project hub — the fill-and-treat workflow in order, with the calculators each step needs.
- All Pools & Water resources — volume, liners and rainwater tools in one place.
The bottom line: pool volume is surface area × average depth, with average depth the mean of shallow and deep ends. Convert once, carefully, between litres and whichever gallon your instructions use. Measure your tap with a bucket before believing any fill-time promise, and let product labels and professionals — not guesswork — set every chemical dose.