How aquarium volume calculator works
Tank volume is simple geometry — length × width × height — but the labels lie: a "20-gallon" tank is a nominal product size, and the actual brimful volume of a 30 × 12 × 12 inch tank works out to about 18.7 US gallons. The calculator works from the real dimensions, in centimetres or inches, and reports the exact volume in litres, US gallons and imperial gallons (the UK gallon is 4.546 litres, about 20% larger than the US 3.785-litre gallon).
Real tanks are rarely filled to the brim — the waterline sits below the frame, and substrate, rocks and decorations displace real volume. Set the fill percentage to match your actual waterline and the calculator scales both volume and weight. The filled water weight matters as much as the volume: water is almost exactly 1 kg per litre (2.2 lb), so a full 240-litre tank is about 240 kg of water before you count the glass, stand, filter and gravel.
Measure the inside of the glass, not the outside — glass and trim eat 1–2 cm per dimension and it compounds across three axes. For stocking rules of thumb like "one inch of fish per gallon", use the filled figure, not the nominal label. Everything runs in your browser; the calculation is pure geometry with nothing sent anywhere.