How this calculator works
The water volume is the floor area multiplied by the water height. A rectangular tank uses length times width. A cube uses the side squared. A full cylinder uses pi times the radius squared, and a half or quarter cylinder uses the matching fraction. A bow front tank is treated as a rectangular body plus a half-elliptical bulge, which adds pi times the bow depth divided by four to the average width. Fresh water has a density of about 1,000 kilograms per cubic metre, so one litre weighs one kilogram. The result is the water alone: gravel, glass, stand and equipment add more weight.
Formula
rectangular: volume = length x width x height. cube: volume = side^2 x water height. cylinder: volume = pi x (diameter / 2)^2 x height, with half and quarter cylinders using 1/2 and 1/4 of that. bow front: volume = length x height x (width + pi x bow depth / 4). water mass = volume in cubic metres x 1,000 kg/m3.
Worked example
A 60 cm by 30 cm tank filled to 40 cm holds 60 x 30 x 40 = 72,000 cm3, which is 72 litres and 72 kilograms of water. A cylinder 40 cm across and filled to 40 cm holds pi x 20^2 x 40 = 50,265 cm3, or about 50.3 litres. The same cylinder cut in half holds about 25.1 litres.
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Frequently asked questions
Should I use inside or outside dimensions?
Use inside dimensions, because the glass thickness is not water. On a small tank the difference is a few percent; on a large tank with thick glass it matters more. Measure the clear space inside the panes and the height from the bottom pane to the water line.
How much does saltwater weigh?
Seawater at a typical reef salinity of 35 parts per thousand has a density of about 1,023 kg/m3 at 25 C, roughly 2.3 percent more than fresh water. That is why a litre of saltwater weighs about 1.023 kilograms rather than 1 kilogram.
Does the water weight include gravel and rock?
No. The answer is the water alone. Gravel, sand, rock and equipment each add weight, and wet substrate is much heavier than its dry weight. Check the total against the floor or stand rating, not just this figure.
Why does the bow front calculation use a quarter of pi?
A bow front bulges outward in a half-elliptical arc across the full length. The area of a half ellipse is half of pi times the two semi-axes, and with a length semi-axis of length / 2 and a depth semi-axis of bow depth, the added area simplifies to length x bow depth x pi / 4.