📖 How To Use
How to Use This Underground Tank Volume Calculator
Calculating your buried cistern or underground tank capacity takes under a minute:
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Choose your tank shape
Select Rectangular, Cylindrical (Vertical), or Cylindrical (Horizontal). The input fields update automatically to match your selection. Most pre-cast concrete and polyethylene underground tanks are rectangular or horizontal cylinders.
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Measure internal dimensions
Use a measuring tape inside the tank or refer to the manufacturer's data sheet. For rectangular tanks enter Length, Width, and Depth. For cylindrical tanks enter the internal Diameter and Depth (or Length if horizontal). Always use internal dimensions — wall thickness can be 50–150 mm on concrete tanks.
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Select your units
Each input field has its own unit selector. You can mix units freely — the calculator converts everything internally before computing.
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Set the fill level (optional)
Drag the slider to see volume at a partial fill — useful if you want to know how much is currently stored, or if you want to calculate usable storage leaving a safety headspace.
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Hit Calculate
Results appear instantly — litres, US and UK gallons, cubic metres, cubic feet, and water weight. Use Copy or Print to save your results.
Underground-specific tip: Most underground tanks have a small unusable sump at the bottom (typically 5–10 cm) where sediment accumulates. Subtract that layer from the depth for true usable capacity. Also, always leave 10–15% headspace to handle groundwater pressure fluctuations and prevent overflow.
📐 The Formula
Underground Water Tank Volume Formula
The formula depends on your tank's shape. All are based on the same principle — cross-sectional area multiplied by depth (or length).
Rectangular Underground Tank
Volume = Length × Width × Depth
Litres (cm inputs): V (L) = (L × W × D) ÷ 1,000
Cubic metres (m inputs): V (m³) = L × W × D
Vertical Cylindrical Tank
Volume = π × (Diameter ÷ 2)² × Depth
Litres (cm inputs): V (L) = π × r² × D ÷ 1,000
Where r = Diameter ÷ 2 (radius)
Horizontal Cylindrical Tank
Volume = π × (Diameter ÷ 2)² × Length
Litres (cm inputs): V (L) = π × r² × L ÷ 1,000
Where r = Diameter ÷ 2 (internal radius)
Unit Conversion Reference
| Unit | Equals Litres | From Litres |
|---|
| 1 Litre (L) | 1 L | × 1 |
| 1 Cubic Metre (m³) | 1,000 L | ÷ 1,000 |
| 1 US Gallon | 3.785 L | ÷ 3.785 |
| 1 UK Gallon | 4.546 L | ÷ 4.546 |
| 1 Cubic Foot (ft³) | 28.317 L | ÷ 28.317 |
| 1 Cubic Inch (in³) | 0.01639 L | × 61.024 |
📏 Reference Data
Common Underground Water Tank Sizes
Typical manufactured underground tank sizes and their approximate volumes:
| Capacity | Shape | L × W × D (cm) | Litres | US Gal |
|---|
| 1,000 L | Rectangular | 120 × 100 × 85 | 1,020 | 269 |
| 2,500 L | Rectangular | 180 × 120 × 118 | 2,548 | 673 |
| 5,000 L | Horizontal Cyl. | Ø 160 × 250 | 5,027 | 1,328 |
| 10,000 L | Rectangular | 250 × 200 × 205 | 10,250 | 2,707 |
| 20,000 L | Rectangular | 350 × 250 × 233 | 20,388 | 5,385 |
| 50,000 L | Rectangular | 500 × 350 × 292 | 51,100 | 13,500 |
Note: Actual manufacturer dimensions vary. Always verify against the supplier's data sheet before ordering excavation works.
❓ FAQ
Frequently Asked Questions
How do I calculate the volume of an underground water tank?
For a rectangular underground tank, multiply the internal Length × Width × Depth. If dimensions are in centimetres, divide by 1,000 to get litres. For a cylindrical tank use π × radius² × depth (or length). This calculator handles all shapes and unit conversions automatically — just enter your measurements and hit Calculate.
Should I use internal or external dimensions for my underground tank?
Always use internal dimensions. Concrete underground tanks have walls that are 100–200 mm thick, so external measurements can overstate actual capacity by 15–20% on a typical residential cistern. Use internal dimensions from the manufacturer's data sheet, or measure inside the tank through the access hatch.
What is the typical size underground water tank for a family home?
For a household of 4 people using roughly 150 litres per person per day, a 3-day emergency supply needs 1,800 litres. For rainwater harvesting that covers most irrigation needs, 5,000–10,000 litres is common. Whole-house potable storage (replacing mains supply for weeks) typically requires 20,000–50,000 litres depending on region and usage patterns.
How deep should an underground water tank be buried?
Most residential underground tanks are installed so the top of the tank is at least 300–600 mm below finished ground level. This protects against frost in colder climates and allows for traffic loading if the area will be driven over. The access riser (manhole) should be flush with or slightly above grade. Always follow the manufacturer's installation guidelines for minimum cover depth — overloading with fill or vehicle traffic can crack unrated tanks.
How heavy is a full underground water tank?
Water weighs 1 kg per litre. A 5,000 L underground tank contains 5,000 kg of water. Add the tank shell weight (a concrete tank at that size typically weighs 1,500–3,000 kg) and you're looking at 6,500–8,000 kg total. This is why proper bedding and backfill per the manufacturer's spec is critical — inadequate support causes uneven settlement and structural cracking.
What is the difference between a cistern and an underground water tank?
The terms are often used interchangeably. Technically, a cistern refers to a sealed vessel for storing water, often used for rainwater collection; an underground tank is simply any tank installed below grade. Both are calculated the same way using this underground water tank volume calculator. The material — concrete, polyethylene, fibreglass — affects installation method but not the volume formula.
How do I calculate the usable storage volume of an underground tank?
Usable volume is less than total volume for several reasons: (1) a sump zone at the bottom (typically 5–10 cm) collects sediment and cannot be pumped without stirring up contamination; (2) a headspace of 10–15% should remain empty to handle fluctuations and prevent overflow through the vent; (3) the pump inlet is usually 50–100 mm above the floor. Use the fill-level slider in this calculator to model the effective usable portion — typically 80–90% of total volume.
Can I use this calculator for septic tanks or fuel storage tanks?
Yes — the volume calculation is shape-dependent, not liquid-dependent. The formula for a rectangular or cylindrical underground tank is identical regardless of contents. However, the water weight estimates in this calculator assume a density of 1 kg/L (fresh water). Diesel is roughly 0.85 kg/L, petrol about 0.74 kg/L, and sewage approximately 1.02 kg/L. For fuel or wastewater tanks, use the volume result and multiply by the correct liquid density for accurate weight.