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Aquarium Calculator
Use this free aquarium calculator to work out the size of any fish tank. Pick the tank shape, enter the dimensions in inches, and it gives the volume in gallons and liters, the recommended glass thickness, the weight of the water, and how many inches of fish the tank can hold.
How it works
Tank volume comes from the inside dimensions in cubic inches, divided by 231, because one gallon holds exactly 231 cubic inches. A rectangular tank is the simplest case: length times width times height, divided by 231. A cylinder uses pi times the radius squared times the height, a hexagonal tank uses the area of a regular hexagon (about 0.866 times the width across the flats, squared) times the height, and a bowfront tank adds a half-cylinder bulge to the rectangular volume, using the bow depth you enter.
Glass thickness follows the water pressure, which grows with both volume and height. The calculator uses the common rule of thumb: 1/4-inch glass up to 55 gallons, 3/8-inch glass up to 125 gallons, and 1/2-inch glass above that. Tall tanks put more pressure on the lower panes than short tanks of the same volume, so if your tank is unusually tall, confirm the thickness with the tank maker rather than relying on volume alone.
Water weight matters as much as volume, because a full tank is far heavier than most people expect. Each gallon of water weighs 8.34 pounds, so a 50-gallon tank holds over 420 pounds of water before the glass, stand, gravel, and rocks are counted. Always check that the stand and the floor underneath can carry the total — a tank on an upper floor should sit across floor joists, not parallel to them.
Fish capacity uses the classic one-inch-per-gallon rule: a 50-gallon tank can hold about 50 inches of adult fish in total, so ten 5-inch fish or twenty-five 2-inch fish. It is a starting guide, not a hard limit — messy fish like goldfish need more room per inch, while small schooling fish can be stocked a little denser. The volume the gravel and decorations take up also reduces the real water volume slightly, so treat the result as a planning figure.
Worked example
A standard rectangular tank measures 36 inches long, 18 inches wide, and 18 inches tall.
Step 1: Volume in cubic inches — 36 × 18 × 18 = 11,664 cu in.
Step 2: Gallons — 11,664 ÷ 231 = 50.5 gallons.
Step 3: Liters — 50.5 × 3.785 = 191.1 liters.
Step 4: Water weight — 50.5 × 8.34 = 421.1 lb.
Step 5: Glass and fish — 50.5 gallons is under 55, so 1/4-inch glass is enough, and the tank holds about 50 inches of fish.
The tank holds 50.5 gallons (191.1 liters) of water weighing 421.1 lb, needs 1/4-inch glass, and can hold about 50 inches of adult fish — for example, ten 5-inch tetras and barbs with room to spare.
FAQ
Why divide by 231 to get gallons?
One US gallon is defined as 231 cubic inches. So any volume measured in cubic inches, divided by 231, gives gallons. The same rule works for any tank shape once you have its volume in cubic inches.
How thick should aquarium glass be?
As a rule of thumb, use 1/4-inch glass up to 55 gallons, 3/8-inch glass up to 125 gallons, and 1/2-inch glass for larger tanks. Very tall tanks put extra pressure on the bottom panes, so confirm the thickness with the manufacturer for tall or custom builds.
How much does a full aquarium weigh?
Water weighs 8.34 pounds per gallon, so a 50-gallon tank holds about 421 lb of water, plus the glass, stand, gravel, and decorations. A 125-gallon setup can pass 1,400 lb in total, so always check the stand and floor load.
What is the one-inch-per-gallon fish rule?
It means a tank can hold about one inch of adult fish per gallon of water — a 50-gallon tank holds roughly 50 inches of fish in total. Count adult size, not the size at purchase, and stock messy fish such as goldfish more lightly.
How do you measure a bowfront tank?
Measure the flat back length, the side width, the height, and the bow depth — how far the curved front bulges past the flat sides. The calculator treats the bow as a half-cylinder bulge added to the rectangular volume, which is a close estimate for standard bowfront tanks.