Pets FIELD NOTES · 4 MIN READ
Tank Math: Nominal Gallons, Real Litres and the Weight of Water
Why a '20-gallon' tank is really 18.7, how US and imperial gallons differ by 20%, and why the water alone weighs a kilogram per litre.
Every aquarist eventually collides with the same surprise: the "20-gallon" tank on the box is not 20 gallons, the water does not fill to the rim, and the whole assembly weighs as much as an adult person. Tank volume is the simplest geometry on the site — and the most commonly misread numbers in the hobby.
The geometry: length × width × height
A rectangular tank is a cuboid, so the brimful volume is simply the three inside dimensions multiplied together. Work in centimetres and divide by 1,000 to get litres (a litre is 1,000 cubic centimetres), or work in inches and divide cubic inches by 231 to get US gallons. A 60 × 30 × 30 cm tank is 54,000 cm³ — 54 litres, 14.3 US gallons. The one mistake that matters most is measuring outside the glass: glass and trim eat a centimetre or two per dimension, and across three axes that quietly removes 5–10% of the real volume. Measure the water space itself, inside face to inside face.
Why nominal tank names don't match real volume
Tank names are marketing categories, not measurements. The classic "20-gallon long" measures 30 × 12 × 12 inches — 4,320 cubic inches, which works out to 18.7 US gallons. The nominal "10-gallon" (20 × 10 × 12 in) is really 10.4, and so on up the line: manufacturers round generously, and the label always wins the argument with the tape measure. This matters more than it seems, because stocking advice, filter ratings and medication dosing all quote gallons — and everything downstream inherits the label's optimism. Calculating from real dimensions puts you back in charge; the Aquarium Volume Calculator does this arithmetic in both unit systems.
US gallons, imperial gallons, litres
The gallon is the trap for anyone reading advice across borders. The US gallon is 3.785 litres; the imperial (UK) gallon is 4.546 litres — about 20% more. A 54-litre tank is 14.3 US gallons but only 11.9 imperial gallons. US tank labels mean US gallons, and most online stocking tables mean US gallons too, but UK-published advice usually means imperial. When a filter says "up to 20 gallons", check which side of the Atlantic it was made on before trusting the flow rate. Litres cut through all of it, which is why serious dosing instructions (fertiliser, dechlorinator, medication) increasingly quote per-litre rates.
Water weighs one kilogram per litre
Volume is only half the answer; weight is the part your floor cares about. Water is almost exactly 1 kg per litre (2.2 lb), so a filled 54-litre tank holds 54 kg of water — about 119 lb. A 240-litre display is 240 kg of water before the glass, gravel, stand or filter enter the picture. Wet substrate is nearly as dense as water per litre displaced, and glass adds roughly a tenth again the water weight. This is why the first question any experienced fishkeeper asks about a new tank placement is "what's under it?" — a modest aquarium on a weak bookshelf is not a hobby problem, it is a structural one. The calculator reports the filled water weight in both kilograms and pounds so the stand can be checked against reality.
Fill level, substrate and displacement
Real tanks run below the brim: the waterline sits a couple of centimetres under the frame, and everything solid in the tank — substrate, rocks, driftwood, equipment — displaces real water. A heavily aquascaped 100-litre tank might hold 85 litres of actual water. Displacement barely changes the weight story (the rock replaces water of similar heft), but it changes the volume story: less water means stocking and dosing should follow the working volume, not the geometry. Setting the fill percentage honestly gets the estimate close; measuring the actual water level when doing a water change closes the gap entirely.
Try it yourself
Open the Aquarium Volume Calculator and enter your own tank's inside dimensions in centimetres or inches — then check the result against the sticker on the box and see who was rounding. Set the fill level to where your waterline actually sits and watch the volume and weight drop together. Compare a "20 long" in US and imperial gallons to feel the 20% gap between the two gallons. When you know the real water volume, the running costs of keeping it warm and lit can be estimated with the Electricity Cost Calculator, the heater's wattage questions live in the Energy Converter, and the long-term budget for fish, food and filters is a job for the Pet Annual Cost Calculator.