📖 How To Use

How to Use This Underground Tank Volume Calculator

Calculating your buried cistern or underground tank capacity takes under a minute:

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.
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 (2–6 in) on concrete tanks.
Select your units Each input field has its own unit selector. You can mix units freely — the calculator converts everything internally before computing.
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 to work out the usable storage between the pump intake and the overflow. The percentage is of the volume; in a horizontal cylinder it is not the same as the percentage of depth (the note under the results explains).
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 layer at the bottom (typically 5–10 cm, 2–4 in) below the pump intake where sediment settles, and the top stops at the overflow or inlet — measure the depth between those two levels for the usable capacity. And keep in mind that an empty buried tank in high groundwater can float out of the ground; follow the maker's anchoring and backfill instructions, and avoid pumping it dry when the water table is high.

📐 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
US gallons (ft inputs): V (ft³) × 7.48

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)

For a horizontal tank that is only partly full, the volume is not proportional to the depth — use the horizontal tank calculator with a dipstick reading. Tanks with domed ends hold a little more than the flat-ended formula shows.

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	2,549	673
5,000 L	Horizontal Cyl.	Ø 160 × 250	5,027	1,328
10,000 L	Rectangular	250 × 200 × 205	10,250	2,708
20,000 L	Rectangular	350 × 250 × 233	20,388	5,386
50,000 L	Rectangular	500 × 350 × 292	51,100	13,499

Note: Actual manufacturer dimensions vary. Always verify against the supplier's data sheet before ordering excavation works. In the US, buried cisterns are commonly sold in sizes such as 500, 1,000, 1,500 and 2,500 gallons (about 1,900, 3,800, 5,700 and 9,500 L).

🏗️ Use Cases

When to Use an Underground Water Tank Volume Calculator

🏠

Residential Rainwater Harvesting

Size a buried cistern to capture roof runoff for garden irrigation or toilet flushing, optimising storage vs excavation cost.

🌾

Agricultural Irrigation Storage

Calculate buried storage to hold water during off-peak pump times, ensuring sufficient supply for crop irrigation schedules.

🏭

Commercial & Industrial Sites

Check that existing or planned underground reservoirs hold the volume set for fire water (by the local fire code or fire authority), process water or cooling water.

🆘

Emergency Water Storage

Confirm buried tanks provide enough supply for multi-day outages in regions with unreliable grid water or for off-grid properties.

🏘️

Community Water Systems

Determine whether an underground communal cistern has sufficient capacity for the population it serves based on daily usage estimates.

🔄

Tank Replacement Planning

When upgrading an ageing concrete cistern, calculate how much capacity a replacement needs to maintain continuous supply.

🧮 Worked Example

Worked example: a buried concrete cistern in feet

A rectangular concrete cistern measures 8 ft long, 5 ft wide and 5 ft deep inside. The pump intake sits 4 in above the floor and the overflow 6 in below the lid.

Total volume: 8 × 5 × 5 = 200 ft³ × 7.48 = 1,496 US gallons (5,663 L).
Usable depth: 5 ft − 4 in − 6 in = 4 ft 2 in (4.17 ft).
Usable volume: 8 × 5 × 4.17 = 166.7 ft³ × 7.48 = 1,247 gallons (4,720 L) — about 83% of the total.
Per inch of depth: 8 × 5 ÷ 12 = 3.33 ft³ ≈ 25 gallons, handy for reading the level with a dipstick.

Using the outside size instead — say 8 ft 8 in × 5 ft 8 in × 5 ft 8 in with 4 in walls, floor and lid — would give about 2,080 gallons, nearly 40% too much.

❓ 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. With feet, multiply the cubic feet by 7.48 for US gallons; with 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 often have walls, floor and lid 100–200 mm (4–8 in) thick, so external measurements can overstate the capacity of a small residential cistern by a third or more. 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 (40 US gallons) per person per day, a 3-day supply needs 1,800 litres (about 475 gallons). 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?

It depends on the tank and the climate. Many residential tanks need at least 300–600 mm (1–2 ft) of soil cover, and many plastic tanks also have a maximum burial depth. In cold climates the water should sit below the local frost line, which can be 1.2 m (4 ft) or more in the northern US, Canada and northern Europe. Tanks under driveways need a traffic-rated design. 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 about 1 kg per litre, or 8.34 lb per US gallon. A 5,000 L (1,320-gallon) underground tank contains about 5,000 kg (11,000 lb) of water. A precast concrete tank of that size can weigh several tonnes on its own — often as much as the water — while a polyethylene one weighs a few hundred kilograms. 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) the top stops at the overflow or inlet pipe, usually some way below the lid; (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 close to water at about 1.0 kg/L. Fuel tanks are regulated storage in most countries (for example under US EPA underground storage tank rules), so follow the local rules as well as the maths. For fuel or wastewater tanks, use the volume result and multiply by the correct liquid density for accurate weight.

✅ Sources & Review

Sources and review

The formulas are standard geometry; the conversion factors and figures on this page come from these references:

NIST Special Publication 811, Guide for the Use of the SI — US gallon (exactly 231 in³ = 3.785411784 L), cubic foot and cubic inch conversion factors.
UK Weights and Measures Act 1985, Schedule 1 — the imperial gallon is exactly 4.54609 litres.
USGS Water Science School — Water Density: about 1 kg per litre (62.4 lb/ft³, 8.34 lb per US gallon).

Reviewed by John Mac, PE, Water Systems Specialist

Texas-licensed Professional Engineer with 13+ years in residential water storage and 180+ tank projects. John writes and reviews every calculator on this site.

Last updated October 9, 2026

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