The useful volume of a tank comes from its internal dimensions. Outside width, wall thickness and a nominal product label can describe a different quantity from the water space you want to calculate.
The Whatson Water Tank Capacity Calculator supports a rectangular tank, a vertical cylinder and a horizontal cylinder. It reports litres and both Imperial and US gallons.
Choose the shape before measuring
For a rectangle, enter internal length, width and height. For a vertical cylinder, use internal diameter and vertical height. For a horizontal cylinder, use internal diameter and the length along its axis.
The selected measurement unit applies to those dimensions. The water level input is a percentage of internal height, not a direct reading in litres.
Shapes with rounded ends, baffles, taper or other internal obstructions are outside these simple models. Use manufacturer capacity data or a suitable calibration for those tanks.
What the rectangular example holds
An internal tank measuring 8 × 5 × 5 ft has a full geometric volume of 200 cft. The checked capacity is 5,663.37 litres.
At an 80% water height setting, this rectangular tank contains 4,530.70 litres. That water is about 996.61 Imperial gallons or 1,196.88 US gallons.
Those gallon figures are different descriptions of the same water volume. The unit must be specified when comparing a supplier’s capacity claim. NIST’s unit table distinguishes the two gallon definitions.
A horizontal tank behaves differently
For a horizontal cylinder, a quarter of the height is not a quarter of the volume. The water cross section is a circular segment, so the calculator uses the segment geometry at the selected height.
At half height, a simple horizontal cylinder is half full by volume. Below or above that level, do not multiply capacity by the height percentage as though the relationship were linear.
Geometric capacity is not usable storage
An outlet above the base, overflow position, freeboard and sediment can reduce the amount you can actually draw. The calculator does not deduct those operating details.
It also does not size the tank for household demand or check whether a roof or support can carry it. Use the calculated litres to compare volumes, then obtain the relevant installation and structural requirements separately.
Before saving a result, note the shape and whether the measurements were internal. That information is more useful than a capacity number without its measurement basis.