What is an Underground Water Cistern?
An underground water tank (or subterranean cistern) is a buried storage structure built from reinforced concrete or heavy-duty plastic designed to store drinking water, rainwater runoff, or fire suppression water below frost lines.
Learn more about underground and surface cisterns in the US EPA Rainwater Harvesting Practices guide.
Underground Water Cistern Formulas
Rectangular Concrete Sump Formula:
Volume (V) = Length × Width × Depth
Cylindrical Buried Poly Tank Formula:
Volume (V) = π × r² × Depth = π × (D / 2)² × Depth
How to Calculate Underground Water Storage
- Measure Internal Dimensions: Measure length, width, and depth (or internal diameter and height).
- Convert to Metric or Imperial Units: Standardize measurements into meters or feet.
- Calculate Volume: Apply the rectangular or cylindrical volume formula.
- Convert to Litres / Gallons: Multiply cubic meters by 1,000 to get Litres, or cubic feet by 7.48052 for US Gallons.
Step-by-Step Numerical Example
For an underground concrete water sump measuring 10 ft long, 8 ft wide, and 6 ft deep:
V = 10 × 8 × 6 = 480 cubic feet (ft³)Total Capacity in US Gallons = 480 × 7.48052 ≈ 3,590.65 US GallonsTotal Capacity in Litres = 3,590.65 × 3.78541 ≈ 13,592.15 Litres
Underground Cistern Sizing Reference Chart
| Cistern Sizing | Dimensions (L × W × D) | Capacity (Litres) | Capacity (US Gal) |
|---|---|---|---|
| Small Residential Sump | 2.0m × 1.5m × 1.5m (6.6ft × 4.9ft × 4.9ft) | 4,500 L | 1,188 gal |
| Medium Home Vault | 3.0m × 2.0m × 1.8m (9.8ft × 6.6ft × 5.9ft) | 10,800 L | 2,853 gal |
| Large Domestic Cistern | 4.0m × 2.5m × 2.0m (13.1ft × 8.2ft × 6.6ft) | 20,000 L | 5,283 gal |
| Commercial Sump Tank | 5.0m × 3.0m × 2.5m (16.4ft × 9.8ft × 8.2ft) | 37,500 L | 9,906 gal |
Underground Installation & Structural Safety
Buried water tanks experience heavy lateral soil pressure and ground water buoyancy forces. Ensure subterranean walls are adequately reinforced and empty plastic tanks are anchored to a concrete hold-down pad to prevent floating during heavy rains.
Underground Water Cistern Sizing & Filtration
Underground water tanks and rainwater cisterns store large volumes of potable or non-potable water while protecting it from thermal fluctuations and algae growth. Integrated sediment traps and leaf diverters prevent debris accumulation inside the tank.