Water Tank Capacity
Calculate water tank volume and capacity
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What the defaults hold
A tank measuring 2 m by 1.5 m by 1 m holds 3 cubic metres, which is 3,000 litres — about 793 US gallons.
Use internal dimensions. Wall thickness on a concrete tank can be 100–200 mm a side, which is a 5–10% error on the volume before you have even filled it.
How tank capacity is calculated
For a rectangular tank, capacity is simply volume, converted to litres:
Capacity = Length × Width × Height, and 1 m³ = 1,000 litres
A 2 m × 1.5 m × 1 m tank: 2 × 1.5 × 1 = 3 m³ = 3,000 litres.
| Length | Width | Height | Volume | Litres |
|---|---|---|---|---|
| 1.0 m | 1.0 m | 1.0 m | 1.0 m³ | 1,000 L |
| 1.5 m | 1.0 m | 1.0 m | 1.5 m³ | 1,500 L |
| 2.0 m | 1.5 m | 1.0 m | 3.0 m³ | 3,000 L |
| 2.0 m | 2.0 m | 1.5 m | 6.0 m³ | 6,000 L |
| 3.0 m | 2.0 m | 2.0 m | 12.0 m³ | 12,000 L |
Leave headroom below the top so the tank can overflow safely. Related: pipe volume and construction calculators.
The formula
Volume = length × width × height, then multiply by 1,000 to convert cubic metres to litres.
2 × 1.5 × 1 = 3 m³. 3 × 1,000 = 3,000 L.
Worked example
Length 2.0 m
Width 1.5 m
Height 1.0 m
Volume = 3.000 m³
Capacity = 3,000 L ≈ 793 US gal ≈ 660 imperial gal
What 3,000 litres means in use
A typical household uses 300–500 litres a day, so this tank is roughly a week of storage. For fire reserve or irrigation the same figure applies — 3,000 L is 3,000 L whatever it is for.
Why the usable capacity is lower
- Freeboard. You never fill to the brim; an overflow and a lid clearance cost 5–10%.
- Outlet height. Water below the outlet cannot be drawn, which on a flat-bottomed tank is another few percent.
- Shape. A cylindrical tank uses πr²h, not length × width, and the difference is substantial at the same footprint.
- Structural displacement. Internal baffles or a sump reduce the stored volume.
Weight, which matters more than volume
Water is 1 kg per litre, so 3,000 L weighs 3 tonnes, plus the tank itself. That load has to be carried by the floor or roof structure — a filled tank on an upper floor is a structural question, not a plumbing one.
Frequently Asked Questions
How many litres is a 2 × 1.5 × 1 m tank?
Volume is 3 cubic metres, and each cubic metre is 1,000 litres — so 3,000 litres, or about 793 US gallons.
Should I measure inside or outside?
Inside. Wall thickness on a concrete or brick tank can be 100–200 mm, which is a significant fraction of the volume on a small tank.
How much does 3,000 litres weigh?
About 3 tonnes, since water is 1 kg per litre. Add the tank’s own weight and any platform before assessing what the structure can carry.
How long does 3,000 litres last a household?
Roughly a week at 300–500 litres a day. A single person might stretch it to two weeks; a large family with a garden could use it in three days.
How do I calculate a cylindrical tank?
Use π × radius² × height. A 1 m diameter, 2 m tall cylinder holds π × 0.5² × 2 = 1.57 m³, or 1,570 litres.
How many litres per metre of height in this tank?
The footprint is 2 × 1.5 = 3 m², so every metre of depth adds 3,000 litres. That is a useful way to check a partially buried tank: measure the water depth, multiply by 3,000 and you have the volume without knowing the total height.
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