Short answers to the questions that come up most. Each section links to the calculator or guide that goes deeper.
By Aquarium Volumes · · 1 min read
Short answers to the questions that come up most often about aquarium volume: how it is calculated, how to measure, which shape calculator to use, gallons versus litres, water weight and displacement. Each answer links to the calculator or guide that goes deeper.
Open any question below. If yours is not covered, send it to us and we will add it.
Frequently asked questions
The amount of water a tank holds, usually quoted in US gallons or litres. The useful figure is the water actually in the tank, which is less than the geometric volume of the glass box.
Labels are rounded nominal sizes from exterior dimensions. Glass, the fill gap and anything in the tank reduce real water volume by 10 to 20 percent.
Medication, fertiliser and reef supplements are dosed per gallon or litre. Filters, heaters and lights are rated by volume. Stocking guidelines assume real water. A 20 percent error in volume is a 20 percent error in all of them.
Inside, always. Exterior figures include the glass thickness.
From the inside floor of the tank to the rim for full volume, or to the water surface for the fill height option.
Halve it for the radius. The cylinder calculators take the radius.
Bow front if the back and sides are flat, half cylinder if the curve is a full semicircle, corner bow front or corner cylinder if the tank sits in a corner.
That is a flat back hexagon and has its own calculator. The regular hexagon formula does not apply.
Yes, same shape and the same calculator.
Floor area times height. For a box that is length × width × height. In inches divide by 231 for gallons; in centimetres divide by 1,000 for litres.
A US gallon is defined as 231 cubic inches.
For tanks with vertical walls, proportionally. Water at 14 inches in a 16-inch-tall tank is 14 ÷ 16 of the full volume. Bowls are the exception and use sphere geometry.
3.785 litres in a US gallon, 4.546 in an imperial gallon.
US gallons for North American tanks. UK sources occasionally use imperial gallons; most of the world uses litres.
8.34 pounds per US gallon, or 1 kilogram per litre. Reef water at specific gravity 1.025 is about 8.55 pounds per gallon.
About 460 pounds of water plus 60 to 80 pounds of glass and whatever substrate and rock you add, so 600 pounds is a sensible planning figure.
About 62 percent of the bed volume, because 38 percent of a gravel bed is water-filled voids. Two inches of gravel in a 48 × 18 tank displaces roughly 4.6 gallons.
Internal filters, heaters and powerheads displace a fraction of a gallon each. The displacement calculator includes typical values.