Find the capacity and the water in a rectangular, vertical cylindrical or horizontal cylindrical tank, full or partly filled, in litres, m³ and gallons, or work out the tank size you need from daily demand.
| Tank shape | Full capacity | Partly filled (water depth h) |
|---|---|---|
| Rectangular | V = L × W × H | V = L × W × h |
| Vertical cylinder | V = π × r² × H | V = π × r² × h |
| Horizontal cylinder | V = π × r² × L | V = L × [r² acos((r − h)/r) − (r − h)√(2rh − h²)] |
r = diameter ÷ 2. Convert m³ to litres by multiplying by 1,000, to US gallons by 264.17 and to imperial gallons by 219.97.
Litres held per metre of water depth (vertical tank) or per metre of length (horizontal tank, full).
| Diameter (m) | Litres per metre | US gal per foot | Example: 2 m high (L) |
|---|---|---|---|
| 0.5 | 196 | 15.8 | 393 |
| 0.6 | 283 | 22.8 | 565 |
| 0.8 | 503 | 40.5 | 1,005 |
| 1.0 | 785 | 63.2 | 1,571 |
| 1.2 | 1,131 | 91.1 | 2,262 |
| 1.5 | 1,767 | 142.3 | 3,534 |
| 1.8 | 2,545 | 204.9 | 5,089 |
| 2.0 | 3,142 | 253.0 | 6,283 |
| 2.5 | 4,909 | 395.3 | 9,817 |
| 3.0 | 7,069 | 569.2 | 14,137 |
| 3.5 | 9,621 | 774.7 | 19,242 |
| 4.0 | 12,566 | 1,011.8 | 25,133 |
| Size L × W × H (m) | Litres | US gallons | Imperial gallons |
|---|---|---|---|
| 1 × 1 × 1 | 1,000 | 264 | 220 |
| 1.5 × 1 × 1 | 1,500 | 396 | 330 |
| 2 × 1 × 1 | 2,000 | 528 | 440 |
| 2 × 1.5 × 1 | 3,000 | 793 | 660 |
| 2 × 1.5 × 1.5 | 4,500 | 1,189 | 990 |
| 2 × 2 × 1.5 | 6,000 | 1,585 | 1,320 |
| 3 × 2 × 1.5 | 9,000 | 2,378 | 1,980 |
| 3 × 2 × 2 | 12,000 | 3,170 | 2,640 |
| 4 × 2 × 2 | 16,000 | 4,227 | 3,520 |
| 4 × 3 × 2 | 24,000 | 6,340 | 5,279 |
| 5 × 4 × 2 | 40,000 | 10,567 | 8,799 |
| 6 × 4 × 2.5 | 60,000 | 15,850 | 13,198 |
Sizing a fire water reserve instead? Use the Fire Water Tank Calculator and see Fire Water Tank Sizing. For the pump that draws from the tank, use the Pump Head Calculator.
Multiply the internal dimensions to get the volume, then convert to litres. Rectangular tank: length × width × height. Vertical cylinder: π × r² × height. 1 m³ = 1,000 litres, so a 2 × 1.5 × 1.5 m tank holds 4.5 m³ = 4,500 litres.
1,000 litres (1 m³), which is about 264 US gallons or 220 imperial gallons.
Use the circular segment area: A = r² × acos((r − h)/r) − (r − h) × √(2rh − h²), where r is the radius and h the water depth, then multiply by the tank length. A horizontal tank that is half full by depth holds exactly half its capacity, but at one quarter depth it holds only about 20%.
Multiply the number of people by the daily use per person and the number of days of storage. For 6 people at 200 litres per day with 1 day of storage, you need 1,200 litres of usable water; allow extra height for freeboard and the dead volume below the outlet.
1,000 litres = 264.2 US gallons = 220.0 imperial gallons.
Water below the outlet connection cannot be drawn, and the top of the tank above the overflow is never filled. Usable volume is normally 80–90% of the nominal capacity.
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