Aquarium Volume Calculator

Every shape here starts from the same idea: multiply the tank's interior dimensions together to get a volume, then convert that volume into gallons or liters. For a rectangle, that's simply length × width × height. For a cylinder, it's the area of the circular base (using the radius, half the diameter) times the height. A bow front is trickier because the front glass curves outward, so this calculator treats it as a rectangle with the same footprint and adds 5% to account for the extra water the curve holds — a reasonable stand-in for the gentle bows used on most commercially made tanks.

If you can, measure the interior dimensions rather than the outside of the glass, since wall thickness eats into usable space more than people expect, especially on larger tanks with thicker panes. If you only have the exterior measurements or the dimensions from a listing, that's still close enough for planning purposes like stand sizing or comparing tank options — just don't expect it to match a manufacturer's spec sheet to the decimal point.

Tank names like "20 gallon" or "55 gallon" describe roughly how much water the tank holds in practice, which is close to — but not exactly the same as — its raw interior volume. Run the actual inside dimensions of a real 55 gallon tank through the geometry and you'll typically land somewhere in the high 50s gross, a bit above the labeled size. That gap is normal and comes from how these standard sizes were named decades ago, plus the glass and trim eating into the interior space.

On top of that gap, this calculator also reports a net figure at roughly 90% of gross, because no one fills a tank to the literal rim, and substrate, rock, driftwood, and hardware all displace water once the tank is actually set up and running. If you're sizing a filter, dosing medication, or calculating a water change, the net number is the more honest one to plan around — gross volume is really just the tank's theoretical upper limit.

Freshwater weighs about 8.345 lb per gallon, or almost exactly 1 kg per liter, so once you know the net water volume the weight calculation is simple multiplication. Saltwater runs a little heavier, around 8.5-8.6 lb per gallon, because of the dissolved salt — this tool assumes freshwater, so add a small margin if you're planning a reef or marine tank.

Water weight alone, though, understates what a stand or floor actually has to hold. A widely used shortcut for a fully loaded aquarium — water, glass, substrate, rock, and the stand itself — is about 10 lb per gallon of rated tank size, noticeably more than the roughly 8.3 lb per gallon of water by itself. For a 55 gallon tank that's the difference between around 426 lb of water and something closer to 550 lb all-in. That difference is exactly why it's worth checking your stand's rated capacity and, for anything above about 40-55 gallons, thinking about where the load sits relative to floor joists — particularly on an upper floor of a house rather than a slab foundation.

These are standard rectangular tank dimensions in inches, with the gross (interior) and net (~90% full) gallon figures this calculator would produce for each. Real-world tanks can vary slightly by manufacturer, so use these as a sanity check rather than an exact match for your own tank.

Notice the 10 gallon tank computes to 10.4 gross gallons — very close to its name — while larger tanks like the 55 gallon (56.7 gross) and 75 gallon (78.5 gross) drift further above their labeled size. That's a normal side effect of how these standard sizes were named against usable capacity rather than raw interior volume, not an error in either the tank or the math.