PRACTICAL METHOD
Aquarium gallons, water changes, and water-only weight
A short reference for reading the estimate and checking its assumptions.
For a rectangular aquarium, multiply inside length × width × water depth. Convert cubic inches to U.S. gallons by dividing by 231, or cubic centimeters to liters by dividing by 1,000. The calculator uses the same geometry in metric units and converts liters to U.S. gallons. Cylinder volume uses π × radius² × depth. Bow-front capacity uses a documented half-circle approximation and varies with the actual curve.
Use the waterline depth rather than the tank’s outside height. If your dimensions already reflect water displaced by substrate and decor, leave displacement at zero. U.S. Geological Survey reference values put one gallon of water at about 8.33 pounds and one liter near one kilogram; temperature and salinity change density slightly. Water weight excludes glass, substrate, equipment, and stand.