Pool volume by the average-depth method — rectangular or circular, in cubic metres, litres and US gallons — plus fill time from your hose's real flow and the mass of all that water. This tool computes volume only; chemical dosing must follow product instructions and professional guidance.
Measuring a Pool the Average-Depth Way
Measure the inside dimensions — wall to wall at the water line, not coping to coping. For length and width on a rectangle, take the widest inside spans. For a circular pool, measure the diameter across several places (round pools are rarely perfectly round after years of ground pressure), average it, and halve it for the radius.
Depth is where the average-depth method earns its name. Measure the depth at the shallow end and at the deep end — floor to the water line, not to the coping — and average them: (shallow + deep) ÷ 2. A pool that runs 1.0 m to 2.0 m has an average depth of 1.5 m. The method assumes the floor slopes evenly between the two ends, which describes most family pools built with a straight hopper or wedge profile.
It does not describe everything. Pools with a distinct shallow sport area, a deep diving well and a flat middle should be split into two or three boxes: compute each section's volume with its own dimensions and average depth, then add them. Freeform pools (kidney shapes and worse) can be approximated as a rectangle of average length and width, or split into a rectangle plus a circle — expect the estimate to land within about 10%, which is close enough for planning fills and pump runs.
Fill time depends on a number nobody knows until they measure it: the real flow at the end of your hose. Garden hoses vary widely — commonly somewhere in the 10–20 L/min range depending on tap pressure, hose length and diameter — so run the bucket test: time how many seconds the hose takes to fill a 10 L bucket, and enter 10 ÷ seconds × 60 as your L/min. On a 48,000-litre pool, the difference between 20 L/min and 15 L/min is 13 extra hours.
What the Results Mean
- Volume / Litres / US Gallons: The same water in three units. Metric suppliers, pump spec sheets and US product labels each speak one of them.
- Surface Area: Length × width (or π r²). The figure for covers, evaporation and heat-loss reasoning.
- Fill Time: Pool litres ÷ your measured flow, in hours. Plan the fill around it — a big pool is a two-day hose job.
- Water Mass: Volume ≈ tonnes, because a cubic metre of water has a mass of about 1,000 kg. Useful for reasoning about structure, not for dosing.
Pro Tip
Start the fill in the morning, not at dusk. A two-day fill left unattended overnight is how gardens flood and wells run dry — set alarms for periodic checks, keep the hose end below the water line once there's cover, and stop to bucket-test the flow again if you add fittings or a second hose.
The Math Behind the Calculator
Surface area times average depth, then unit conversions:
volume = length × width × average_depth (rectangle)
volume = π × radius² × average_depth (circle)
litres = volume × 1000
us_gallons = volume × 264.172
fill_hours = litres ÷ fill_flow_L_per_min ÷ 60
mass_tonnes ≈ volume (water ≈ 1000 kg/m³)
The conversions are exact definitions for volume: 1 m³ = 1,000 L = 264.172 US gallons (a US gallon is 3.785 L). The mass approximation treats water as 1,000 kg/m³, which is accurate for fresh water at everyday temperatures — pool water with salt systems runs slightly denser, but not enough to matter for planning.
Why Average Depth Works
For a floor that slopes in a straight line from shallow to deep, the average of the two end depths is the mean depth of the whole floor, so surface area × average depth is the true volume. The moment the profile is not a straight slope — a hopper, a sport bottom, a safety ledge — the average drifts and you should section the pool instead. When in doubt, section: two small boxes are always more accurate than one averaged one.
Assumptions & Limitations
Every number above rests on these constants. If your project differs, change the matching input — or read the linked guide for the full reasoning.
- Average-depth method: (shallow + deep) ÷ 2, accurate for uniform straight slopes
- Volume to the water line; steps, benches and fittings not deducted
- Fill flow constant at your measured value — flows drift with pressure and fittings
- Water mass taken as 1,000 kg/m³ — fresh water at everyday temperatures
- No chemical dosing guidance — treatment products and professionals govern that
- Quantities only — this site deliberately does not price labour, delivery or materials
Worked Example: 8 × 4 m Family Pool
A rectangular family pool, 8 m × 4 m (26 × 13 ft) inside the walls, sloping evenly from a 1.0 m shallow end to a 2.0 m deep end, filled from a hose that bucket-tests at 20 L/min. These are the calculator's defaults, so this is what loads on the page:
- Average depth: (1.0 + 2.0) ÷ 2 = 1.5 m.
- Volume: 8 × 4 × 1.5 = 48 m³.
- Litres: 48 × 1,000 = 48,000 L.
- US gallons: 48 × 264.172 ≈ 12,680 US gal.
- Surface area: 8 × 4 = 32 m².
- Fill time: 48,000 ÷ 20 = 2,400 min ÷ 60 = 40 hours.
- Water mass: ≈ 48 tonnes sitting on that slab.
The round-pool version: a 3 m (10 ft) radius pool — 6 m across — at 1.4 m average depth holds π × 3² × 1.4 = 39.6 m³, about 39,600 L or 10,457 US gallons, and takes 33 hours to fill at the same 20 L/min. Enter 3 as the radius, set shape to circle, and check the maths against the calculator.
Reminder
Volume numbers feed planning — fills, pump run times, covers — not water treatment. Dosing decisions belong to the product label and a pool professional.
Safety: Chemicals, Structure and Supervision
Reminder
Pool chemical dosing must follow the product's instructions and professional guidance — this tool computes volume only. Never scale a dose from an estimated volume, never mix different treatment chemicals, and store them locked, cool and dry, away from each other. If your volume estimate feeds any treatment decision, verify the pool's true volume with the builder's drawings or a professional measurement first.
Two more numbers here deserve respect. First, the mass: 48 tonnes of water is a serious structural load, and pools — in-ground or above-ground — are engineered assemblies, not watertight boxes on any convenient slab. Second, the fill itself: an unattended hose over many hours can overflow a pool, waterlog the surrounding ground and undermine an above-ground wall base. Check the fill periodically, and never leave children unsupervised around a pool at any water level.