Aquarium Volume Calculator – Gallons & Liters for Any Shape

To calculate aquarium volume, measure the interior dimensions (or exterior minus glass thickness) in inches or centimeters, then apply the shape-specific formula. For a rectangular tank: multiply length × width × height to get cubic inches, then divide by 231 for US gallons (or multiply by 0.00433). For liters, multiply cubic inches by 0.01639. Our calculator handles all common tank shapes automatically — just select your shape and enter the measurements.

A US gallon equals 231 cubic inches (approximately 3.785 liters), while a UK (Imperial) gallon equals 277.42 cubic inches (approximately 4.546 liters). A UK gallon is about 20% larger than a US gallon. Most aquarium products in North America use US gallons, while UK and some Commonwealth countries use Imperial gallons. Our calculator displays both so you never have to guess.

The theoretical volume is based on empty tank dimensions, but actual water volume is typically 10-15% less due to substrate (gravel/sand), rocks, driftwood, plants, filter equipment, and the fact that tanks aren't filled to the very brim. Use the slider in our calculator to estimate actual water volume. For a typical freshwater setup, 90% is a good starting estimate; for heavily decorated tanks, 80-85% may be more accurate.

For a bow front aquarium, you need three measurements: length (side to side), total depth (from back glass to the furthest point of the curved front), and bow depth (how far the curved glass protrudes beyond where a flat front would be). The bow depth is typically 1-4 inches for smaller tanks and 3-6 inches for larger ones. Our calculator uses an arc-area approximation to account for the curved front.

Water weight: 1 US gallon of water weighs approximately 8.34 pounds (3.78 kg). A 55-gallon tank holds about 459 lbs of water alone! Add the weight of the glass tank itself (typically 40-80 lbs for medium sizes), substrate (1-2 lbs per gallon of substrate volume), and any stand or canopy. Always ensure your floor and stand can support the total weight. Our calculator displays estimated water weight in both pounds and kilograms.

Conversion factors: 1 US gallon = 3.78541 liters; 1 UK gallon = 4.54609 liters; 1 liter = 0.26417 US gallons. To quickly convert: multiply US gallons by 3.8 to get liters (approximate), or divide liters by 3.8 to get US gallons. Our calculator performs all conversions precisely, showing US gallons, UK gallons, and liters simultaneously.

Common rectangular sizes: 10 gallon (20"×10"×12"), 20 gallon high (24"×12"×16"), 20 gallon long (30"×12"×12"), 40 gallon breeder (36"×18"×16"), 55 gallon (48"×13"×20"), 75 gallon (48"×18"×21"), 90 gallon (48"×18"×24"), 125 gallon (72"×18"×21"), 180 gallon (72"×24"×24"). Use our rectangular preset buttons for quick calculations with these standard sizes.

The old "one inch of fish per gallon" rule is overly simplistic. Better guidelines consider adult fish size, activity level, bioload, and territorial needs. For a rough estimate: allow 1 gallon per inch of small, slender fish (tetras, danios) and 5+ gallons per inch for larger or messier fish (cichlids, goldfish). Always research each species' specific requirements and use a quality filter rated for at least 2× your tank volume. AqAdvisor.com is an excellent resource for stocking calculations.

Yes, slightly. Standard aquarium glass ranges from 1/4" (6mm) for small tanks to 1/2" (12mm) or more for large tanks. If you measure external dimensions, the actual internal volume is 2-5% less than calculated. For precise dosing of medications or water treatments, measure internal dimensions when possible. For most hobbyist purposes (stocking, filtration sizing), external measurements provide a sufficiently accurate estimate.

For regular hexagonal tanks: area = (3√3 / 2) × side² ≈ 2.598 × side², then multiply by height for volume. For regular octagonal tanks: area = 2(1 + √2) × side² ≈ 4.828 × side². If you know the panel-to-panel width (distance between opposite flat sides) instead of side length, our calculator can handle both measurement types — simply toggle the measurement option. These shapes are popular for their panoramic viewing angles and space-efficient corner placement.