Aquarium Volume Calculator
Turn tank dimensions into US gallons and liters, plus the filled water weight your stand and floor must hold.
Filled weight: about 166 lb (75 kg)
Glass, substrate, rock, and decor add weight beyond the water itself — budget extra capacity on your stand and floor for a fully aquascaped tank.
How it works
Enter the inside dimensions of your tank, not the outside frame. Glass and plastic trim add a bit of thickness on every side, so measuring the water space rather than the cabinet gives a more honest number. The calculator converts your length, width, and height into cubic volume, then reports that volume in both US gallons and liters, plus the approximate weight of the water once the tank is full.
Say you have a standard 20-gallon-long footprint: 30 inches long, 12 inches wide, and 12 inches tall. That works out to roughly 18.7 gallons (about 70.8 liters) of actual water capacity, which is a bit under the "20-gallon" label on the box. Tank sizes are marketed in round numbers, but the real figure matters for stocking decisions and for dosing water conditioner accurately.
The weight estimate assumes plain water at roughly 8.34 lb per US gallon. It does not include the tank glass itself, the stand, substrate, rock, or driftwood, all of which can add real weight once you start aquascaping. If you are placing a tank on a piece of furniture that was not built to hold it, check the furniture's weight rating against the fully loaded total, not just the water.
FAQ
Why doesn't my tank hold as much as the label says?
Manufacturers round to a nominal size and measure outside dimensions, while the glass itself takes up interior space. A "55-gallon" tank typically holds closer to 48 to 50 gallons once you account for glass thickness, and you never fill a tank to the brim anyway, so usable volume is a bit lower still.
Does substrate change the volume I should plan for?
Yes. A couple of inches of gravel or aquasoil displaces several gallons in a larger tank. If you want a precise number for dosing medication or fertilizer, subtract 5 to 10 percent from the calculated volume once your substrate and hardscape are in, and use that adjusted figure going forward.
How much does a full tank actually weigh?
Water alone runs about 8.34 lb per gallon, so a full 55-gallon tank is around 460 lb of water before you add the glass, stand, substrate, and rockwork. See what size aquarium a beginner should get for how tank size interacts with floor and stand strength.
What should I set up before filling the tank?
Level the stand, confirm the floor can handle the loaded weight, and gather your gear before you add a drop of water. Our guide to setting up a first freshwater aquarium walks through the order of operations, and essential aquarium equipment for beginners covers what to buy first.