Aquarium Volume Calculator

For a rectangular tank, multiply length by width by height in inches and divide by 231, the number of cubic inches in a US gallon. A 48 × 18 × 21 inch tank gives 78.5 gallons to the brim. In centimetres, multiply the three and divide by 1,000 to get litres.

For a cylinder, use π × (diameter ÷ 2)² × height, then divide by 231 for gallons. A 24 inch wide, 30 inch tall cylinder holds 58.8 gallons to the brim.

That is the volume of the whole box, though, not the water in it. The calculator above adjusts for the three things that make real volume smaller.

• Glass thickness. Tanks are sold by their outside size. The glass on each side and the bottom takes a bite out of the inside, and thicker glass on big tanks takes more.
• The water line. Tanks are normally filled about an inch below the rim, which on a short tank is a noticeable share of the height.
• Rock, sand and decor. Anything solid pushes water out of the tank.

The water-line column assumes ¼ inch glass and water an inch below the rim. Larger tanks often use thicker glass, so your 75 gallon may hold a little less than the 71.1 gallons shown.

Rock displaces its solid volume only. Water fills the pores, and that water still counts as water in your system. So the calculator divides the dry weight by the density of the material: about 2.8 g/cm³ for aragonite and limestone rock, 2.9 for aragonite sand and 2.65 for silica sand. Twenty pounds of rock then displaces about 0.86 gallons.

Rules of thumb that give a bigger number often count the rock’s pores as displaced. If you prefer a different density, open “Material densities” in the calculator to change it.

Dosing, medication and water changes are all worked out against the water in the whole system, so add a sump, refugium and plumbing to the display tank. A 75 gallon display with a 20 gallon sump is a 95 gallon system, but the sump usually runs partly full, so measure what it holds at its working level and enter that.