Aquarium Volume Converter

Aquarium volume from dimensions.

Formula:

• Volume (gallons) = Length x Width x Height / 231 (all measurements in inches; 231 cubic inches = 1 US gallon)
• Volume (liters) = Length x Width x Height / 1000 (all measurements in centimeters)

Common standard tank sizes

The first column is what the tank is sold as. The third is what the outside dimensions actually work out to, filled to the brim, which is what this calculator returns when you type those dimensions in. They are not the same number and it is worth knowing why before you wonder which one is wrong.

Two things pull the numbers apart. The dimensions above are outside measurements, so the glass itself eats a bit of the box, and nobody fills to the rim. The 40 breeder is the worst offender in the list, marketed at 40 and geometrically 44.9, because it is a wide shallow tank whose name was rounded down hard. Most of the rest sit within a gallon or two either way.

Weight of water:

• 1 US gallon of fresh water = 8.34 lbs (3.78 kg)
• 1 US gallon of saltwater at reef salinity = 8.55 lbs, about 2.5% heavier
• The water alone in a 55-gallon tank is about 460 lbs (209 kg). Add the glass, stand, substrate and rock and the whole setup passes 600 lbs.

That water-weight line at the end is the one that actually matters before you buy. Water is heavy, about 8.3 pounds a gallon, and a tank concentrates it onto a small footprint. A 55-gallon setup with rock and substrate pushes past 600 pounds, which is more than most furniture is built for, so it needs a proper aquarium stand and, for anything over about 40 gallons, a floor that can take the point load. People who skip this learn it the expensive way.

There’s also a gap between the number you calculate and the water actually in the tank. Substrate, rocks, and decorations displace volume, and you never fill to the very brim, so real water volume usually runs 10 to 15% below the geometric figure. That matters whenever you dose medication or water conditioner, because those are measured per actual gallon: dose by the full calculated volume and you’ll overdose a tank that’s really holding less. For stocking and chemicals, estimate the net water volume, not the box’s nominal size.