Knowing your pool's volume isn't optional — it's essential for every chemical treatment, pump selection, and water change calculation. Add too little chlorine and your pool goes green; add too much and you're swimming in an irritant. This guide shows you how to calculate volume for any pool shape, and what to do with that number.
Why Pool Volume Matters
- Chemical dosing: All pool chemicals are dosed per 10,000 litres. Without knowing your volume, you're guessing.
- Pump and filter sizing: The pump must circulate the full pool volume every 4–8 hours (the "turnover rate").
- Water change cost: Knowing your volume tells you exactly how much water (and chemical re-dosing) is needed for a full or partial change.
- Heating cost: Pool heating is priced per kWh needed to raise the volume by 1°C.
Formulas by Pool Shape
Rectangular: Volume (m³) = Length × Width × Average depth
Circular: Volume (m³) = π × radius² × Average depth
Oval: Volume (m³) = π × (Length/2) × (Width/2) × Average depth
Convert m³ to litres: × 1,000 | To UK gallons: × 219.97
Calculating Average Depth for Sloped Pools
Most pools have a shallow end and a deep end with a slope in between. Average depth = (Shallow end depth + Deep end depth) ÷ 2. For a pool with 1.0m shallow and 1.8m deep: average = (1.0 + 1.8) ÷ 2 = 1.4m.
Worked Examples
| Pool Type | Dimensions | Avg Depth | Volume (m³) | Volume (litres) |
|---|---|---|---|---|
| Standard rectangular | 8m × 4m | 1.4m | 44.8 | 44,800 L |
| Small rectangular | 6m × 3m | 1.2m | 21.6 | 21,600 L |
| Above ground round | 4.6m diameter | 1.2m | 20.0 | 20,000 L |
| Kidney / irregular | Use 80% of bounding rect. | 1.5m | ~34.5 | ~34,500 L |
Chemical Dosing From Your Volume
| Chemical | Purpose | Typical dose | For 40,000L pool |
|---|---|---|---|
| Chlorine (granular) | Sanitiser | 2g per 1,000L | 80g (initial shock) |
| pH+ (sodium carbonate) | Raise pH | 15g per 10,000L per 0.1 pH rise | 60g to raise 0.1 |
| pH- (dry acid) | Lower pH | 15g per 10,000L per 0.1 pH drop | 60g to drop 0.1 |
| Algaecide | Prevent algae | 125ml per 40,000L | 125ml weekly |
| Stabiliser (cyanuric acid) | Protect chlorine | 2g per 10,000L | 8g |
Pump Sizing from Pool Volume
A pool pump should circulate the full pool volume every 4–8 hours. For a 40,000-litre pool with a 6-hour turnover target:
- Required flow rate: 40,000 L ÷ 6 hours = 6,667 L/hour
- Converted to m³/hr: 6.67 m³/hr
- Choose a pump rated at ≥7 m³/hr at your total head (pipe length + filter resistance)