How it's calculated
volume = L × W × H, US gal = litres ÷ 3.78541
L, W, H = tank length, width, height; a cylinder uses π × (D/2)² × H. Volume converts to litres then US gallons (÷ 3.78541). Water weighs about 8.345 lb per US gallon.
Common questions
How heavy is a full fish tank?
Water alone weighs about 8.3 lb per US gallon, so a 55 gallon tank holds roughly 460 lb of water before adding gravel, rock, and the tank itself.
Does tank shape change the math?
Yes. A cylindrical or bow-front tank uses a different volume formula than a plain rectangle, so the tool adjusts for the shape you pick.