Calculate litres, US gallons and UK gallons from your tank dimensions.
Open live calculator →A 90 × 45 × 45 cm tank has a gross capacity of 182.3 litres, 48.2 US gallons or 40.1 UK gallons.
Measure a rectangular aquarium in centimetres or inches and convert its geometric capacity into three common aquarium volume systems.
The live tool updates immediately and shows the result in the units commonly used by aquarium keepers around the world.
Measure the internal water space where possible.
Use water height—not the outside glass height.
Calculate net volume separately after adding substrate and décor.
Measure the inside of the aquarium if you can reach it, because glass thickness reduces the real water space—on a large tank the panels alone can remove several litres. Use the length and width of the base, then measure the height only up to the level where the water will actually sit, which is usually a few centimetres below the trim. If you can only measure the outside, subtract roughly twice the glass thickness from each dimension for a closer estimate.
The number printed on a tank box is geometric capacity: length times width times height with nothing inside. A running aquarium never holds that much water. Substrate, rocks, driftwood, internal filters and the air gap at the top typically remove 10–20% of the advertised capacity. That difference matters when you dose medication or fertiliser, because dosing for 200 litres in a tank that really holds 165 litres overdoses the livestock by more than 20%.
Aquarium equipment is sold globally, and the three volume systems do not line up. A US gallon is 3.785 litres while an Imperial (UK) gallon is 4.546 litres, so a "40 gallon" figure differs by almost 30 litres depending on which gallon the writer meant. American care sheets and products almost always use US gallons; older British literature uses Imperial gallons. Keeping litres visible is the safest way to compare heaters, filters and stocking advice from different countries.
If your result looks surprising, compare it with standard sizes: a 60 × 30 × 30 cm starter tank is about 54 litres, a 90 × 45 × 45 cm tank is about 182 litres, and a 120 × 50 × 50 cm tank is about 300 litres. Results far outside those patterns usually mean a unit mix-up—entering inches while the calculator is set to centimetres is the most common cause, and it inflates the result by a factor of sixteen.
Use the actual water height. Most aquariums run with the water line 2–5 cm below the top rim, and substrate raises the effective bottom. Using full tank height overstates the real volume.
No—it calculates geometric volume from the dimensions you enter. If you measured outside dimensions, subtract roughly twice the glass thickness from each measurement, or measure internally for the best estimate.
Divide litres by 3.785. For example, 180 litres is about 47.6 US gallons or 39.6 UK gallons. The calculator shows all three systems automatically so nothing needs converting by hand.
This tool assumes a rectangular footprint. For other shapes use our dedicated bow front, cylinder, hexagon and corner tank calculators, which apply the correct geometry for each shape.