Find out how much water your aquarium really holds — not just the number on the box. The calculator handles rectangular, cube, bow-front, hexagon, corner, cylinder, and half-cylinder tanks, then subtracts the glass, the substrate bed, and the gap below the rim to show the actual water volume you should dose medications and conditioners for. It also estimates the filled weight and suggests heater wattage and filter flow.
Working on something bigger? The pool volume calculator handles swimming pools and the pond liner calculator sizes outdoor ponds. For any other conversion between gallons, liters, and cubic inches, use the unit converter. Plumbing a sump or canister filter? The pipe volume calculator tells you how much water the tubing holds.
Frequently asked questions
How do I calculate how many gallons my fish tank holds?
For a rectangular tank, multiply length × width × height in inches and divide by 231, the number of cubic inches in a US gallon. A standard 20-gallon long tank is 30 × 12 × 12 inches: 4,320 ÷ 231 = 18.7 gallons. In centimeters, multiply the three sides and divide by 1,000 to get liters. Other shapes use their own area formula — π × radius² for a cylinder, for example — multiplied by the height.
Why is my tank's real water volume less than its label?
Tanks are sold by their outside dimensions, and even those are rounded. The glass takes up space on every side and the bottom, the water line sits an inch or so below the rim, and a 2-inch bed of gravel or sand displaces more. Together these usually cut the real water volume by 10–20 %. A '55 gallon' tank typically holds around 45–50 gallons of actual water once it is set up. Use the actual water figure for medication and water-conditioner doses.
How do I measure a bow-front, hexagon, or corner tank?
For a bow-front, measure the straight back panel for length, the depth at the ends, and how far the center of the bow sticks out beyond the ends. For a hexagon, measure one of the six panels — the calculator assumes all six are equal. For a corner (pentagon) tank, measure one of the two back panels that sit against the walls and the short side panels that meet them at a right angle. Always measure the outside edges and let the glass thickness setting take care of the rest.
How much does a filled aquarium weigh?
Fresh water weighs about 8.34 pounds per gallon and saltwater about 8.55, so water alone is the bulk of it. Add the glass — this calculator estimates it from the panel area and thickness at the density of float glass — plus substrate at roughly 1.5 kg per liter of gravel or sand. A set-up 75-gallon tank can easily top 800 pounds. Put large tanks on a stand rated for the weight, over floor joists or near a load-bearing wall.
What size heater and filter do I need?
The usual heater rule of thumb is 3 to 5 watts per gallon of actual water, using the high end if the room is cold or the tank needs to be much warmer than the room. For filters, aim for a flow rate that turns the whole tank over 4 to 6 times an hour, so a 40-gallon tank wants a filter rated around 160–240 gallons per hour. Heavily stocked tanks, goldfish, and reef systems usually need more flow than that.