Aquarium Volume Calculator
Enter your tank dimensions to get the exact volume in gallons and liters, the estimated filled weight, and the right filter and heater size for that volume. Works for rectangular, cube, bowfront, cylinder, and hexagon tanks.
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US gallons
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liters
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UK gallons
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lb filled (est.)
Real water volume: about once you subtract substrate, rock, and the gap at the top (roughly 90% of the empty-tank figure). Always dose medication and fertilizer to this number.
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How to Calculate Aquarium Volume
The volume of a standard rectangular aquarium is simply length times width times height. Measure in inches, multiply the three numbers to get cubic inches, then divide by 231 because one US gallon equals 231 cubic inches. To convert to liters, multiply gallons by 3.785. Other shapes use the same idea with a different area formula: a cylinder uses the circle area (pi times radius squared) times height, and a bowfront adds roughly ten percent to the rectangular figure to account for the curved front.
Tank Volume Is Not the Same as Water Volume
The number printed on the box is the empty interior volume. Real water volume is always lower. Substrate displaces water, rock and hardscape displace water, equipment takes up space, and you never fill right to the rim. As a practical rule, plan on real water volume being about 85 to 90 percent of the calculated figure. This matters most when dosing: medications and fertilizers should always be measured against the true water volume, since overdosing is a leading cause of fish and invertebrate loss.
Why Volume Drives Every Other Decision
Once you know your gallons, almost everything else follows. Filtration is sized to turn the water over four to ten times per hour. Heaters are sized at roughly three to five watts per gallon. Stocking, water-change volume, dosing, and even the weight your floor must carry all scale directly with volume. That is why this is the first calculator to run for any new build, and why the tool above suggests filter and heater sizes the moment you enter your dimensions.
Keep going: size the rest of your build.
Frequently Asked Questions
How do I calculate aquarium volume in gallons?
Multiply the tank length by width by height in inches to get cubic inches, then divide by 231, since one US gallon is 231 cubic inches. For example, a 36 x 18 x 18 inch tank is 11,664 cubic inches, divided by 231 is about 50 gallons. This calculator does the math for rectangular, cube, bowfront, cylinder, and hexagon tanks, and also gives you liters and the filled weight.
Why is my tank a few gallons less than the calculation?
The glass thickness, the trim, the substrate, rocks, and the fact that you never fill to the very top all reduce the real water volume. A good rule is that actual water volume is roughly 85 to 90 percent of the empty box calculation. That difference matters for dosing medications and fertilizers, so always dose to the real water volume, not the labeled tank size.
Should I dose medication to the tank size or the real volume?
Always dose to the true water volume, which is lower than the tank label once you subtract substrate, rock, and the gap at the top. A 55-gallon tank often holds closer to 45 to 50 gallons of actual water. Overdosing medication is a common way to harm or kill fish and invertebrates, so estimate conservatively and, when in doubt, dose to a slightly smaller volume.
How much does a filled aquarium weigh?
Water alone weighs about 8.34 pounds per gallon, so a 40-gallon tank holds roughly 334 pounds of water. Once you add the glass, substrate, rock, and equipment, a filled freshwater tank works out to roughly 10 pounds per gallon as a planning figure. A 75-gallon setup can easily exceed 850 pounds, which is why stand and floor support matter. Use our weight calculator for a full breakdown.
What size filter and heater do I need for my tank?
Aim for a filter rated to turn over your water 4 to 10 times per hour, so a 40-gallon tank wants roughly 160 to 400 gallons per hour of filtration. For heating, plan on about 3 to 5 watts per gallon in a normal room, so a 40-gallon tank typically uses a 150-watt heater. This calculator suggests sizes automatically once you enter your dimensions.
Is bigger always better for a beginner aquarium?
Larger tanks are actually more forgiving for beginners because more water dilutes mistakes and keeps temperature and chemistry stable. A common piece of advice is to buy the biggest tank you have room and budget for. A 20 to 40 gallon tank is far more stable and beginner-friendly than a tiny 5-gallon nano, which swings quickly when something goes wrong.