The capacity of a rectangular tank is its internal volume. Multiply the three internal dimensions in metres to get cubic metres, then convert to litres — one cubic metre of water is 1000 litres.
Volume (m³) = L × B × D
L = internal length, B = internal breadth, D = water depth, all in metres. Use the depth to the overflow, not the full tank height.
Capacity (litres) = Volume (m³) × 1000
Worked example
A tank 2 m long, 1.5 m wide, filled to a depth of 1.2 m:
2 × 1.5 × 1.2 = 3.6 m³ = 3600 litres
Measure the inside of the tank: on a cast or masonry tank, a capacity worked out from outside dimensions includes the walls and overstates what the tank holds.
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Other units
1 m³ is 1000 litres, 264.2 US gallons or 219.97 imperial gallons. The free unit converter switches between them.