Free aquarium volume calculator: enter tank length, width, and height in inches to get US gallons, litres, and filled water weight.
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Measure the inside length, width, and height of the tank in inches, from one inner glass face to the opposite face. The calculator multiplies them for cubic inches, divides by 231 for US gallons, since one US gallon is 231 cubic inches, and converts to litres and filled water weight. For the most accurate dosing and stocking figure, use the intended water-fill height rather than the total glass height, because tanks are rarely filled to the brim.
A rectangular tank is a box, so its volume is length times width times height. In inches that gives cubic inches, and dividing by 231 gives US gallons, because one US gallon is defined as 231 cubic inches. In centimetres the same product gives cubic centimetres, and dividing by 1,000 gives litres. A 24 inch by 12 inch by 16 inch tank is 4,608 cubic inches, which is 19.95 US gallons and 75.51 litres. A 36 inch by 18 inch by 18 inch tank is 11,664 cubic inches, which is 50.49 US gallons and 191.14 litres. Water weighs 8.34 pounds per US gallon and 1 kilogram per litre, before glass, substrate, and stand weight.
Measure inside dimensions, not the outside frame, and use the same unit for all three sides. Enter length, width, and height in inches, then read cubic inches, US gallons, litres, and water weight together. If your tape cannot fit inside, measure outside and subtract glass thickness on both sides, usually about a quarter inch per side. For a partially filled tank, enter the water depth as the height to get the water actually in the tank, not the rated glass capacity.
Rectangular tanks use length times width times height. Cylindrical tanks use pi times radius squared times height, where radius is diameter divided by 2: a 12 inch diameter cylinder 18 inches high is 2,035 cubic inches, or 8.8 US gallons. Bow-front tanks are close to a rectangular tank plus a curved section, and hexagonal tanks can be split into rectangles and triangles, or measured by filling with a measuring bucket and counting buckets. When in doubt, the bucket method is the most accurate for any odd shape.
Standard aquarium sizes help you check your result: a 10 gallon tank is about 20 by 10 by 12 inches, a 20 gallon high is 24 by 12 by 16 inches, a 29 gallon is 30 by 12 by 18 inches, a 40 gallon breeder is 36 by 18 by 16 inches, a 55 gallon is 48 by 13 by 21 inches, a 75 gallon is 48 by 18 by 21 inches, and a 125 gallon is 72 by 18 by 21 inches. Nominal sizes are rounded, so a tank sold as 20 gallons may calculate to 19.95 gallons, which is normal and not a measurement error.
The rated glass volume is not the usable water volume. Substrate, rocks, driftwood, decorations, and equipment typically displace 10 to 15 percent, so a 100 gallon tank holds about 85 to 90 gallons of water in practice. Use the usable figure for medication dosing, salt dosing, and stocking decisions. The old 1 inch of fish per gallon rule is only a rough starting point for young fish under 2 inches: heavy-bodied fish like goldfish and plecos need far more space per inch, and every fish should be sized for its adult size, not its size at purchase.
Filled water weight adds up fast: a 20 gallon tank holds about 166 pounds of water, a 55 gallon tank about 460 pounds, and a 125 gallon tank about 1,040 pounds, before glass, substrate, and stand. Add 10 to 15 percent for substrate, rock, and glass when checking a stand or floor. Place large tanks across floor joists, near a load-bearing wall, and on a level stand made for the tank footprint. Never trust a table or shelf that was not built for aquarium load.
Mixing inches and centimetres in one calculation, measuring outside dimensions without subtracting glass, and using total glass height instead of water depth are the three common errors. Another is confusing US gallons with UK gallons: one UK gallon is 4.546 litres, about 20 percent larger than a US gallon, so a 55 US gallon tank is only about 45.8 UK gallons. Finally, do not dose medication on the rated size when displacement is large; dose on the usable water volume and recheck with a test kit.
Multiply length times width times height in inches, then divide by 231 for US gallons. For centimetres, divide the product by 1,000 for litres. A 24 by 12 by 16 inch tank is 4,608 cubic inches, or 19.95 gallons.
A 55 US gallon tank holds about 208 litres, since one US gallon is 3.78541 litres. A UK gallon is larger at 4.546 litres, so the same tank is about 45.8 UK gallons.
Substrate, rocks, driftwood, decorations, and equipment displace water, typically 10 to 15 percent. A 100 gallon rated tank therefore holds about 85 to 90 gallons of usable water.
Fresh water weighs 8.34 pounds per US gallon and 1 kilogram per litre. A 75 litre tank holds about 75 kilograms of water alone, before glass, substrate, and stand, and saltwater is about 2.5 percent heavier.
It means 1 inch of young fish body length per 1 gallon of water, and it works only as a rough guide for young fish under 2 inches. Heavy-bodied fish need more space, and fish should be stocked for adult size.
Use pi times radius squared times height, with radius equal to diameter divided by 2, then divide cubic inches by 231 for US gallons. A 12 inch diameter tank 18 inches high is 8.8 gallons.
Yes. Fill a measuring bucket, count how many buckets fill the tank, and multiply by the bucket volume. It is very accurate and works for any shape, including hexagons and bow-front tanks.
Always size for adult size. Five 2 inch juveniles may fit a 10 gallon tank at first, but at 3 to 4 inches each they need a 15 to 20 gallon tank to grow, stay healthy, and breed reliably.