Tank Volume Calculator
Gallons in a rectangular or cylindrical tank from inside dimensions.
Input sheet
DIY at your own risk. Calcora's calculators and guides are general estimates and information only — not professional, engineering, legal, or safety advice. Always verify local building codes and permit requirements, and hire a licensed pro for electrical, gas, plumbing, structural, or any work you're not fully comfortable doing yourself.
Aquariums, stock tanks, drums, and water storage all come down to the same question: how many gallons fit inside these dimensions? This calculator handles the two shapes that cover nearly every tank — rectangular and cylindrical — from inside measurements in inches.
How it works
Rectangular: length × width × height in inches. Cylindrical: π × radius² × height. Divide cubic inches by 231 for US gallons.
The conversion constant is 231: a US gallon is exactly 231 cubic inches. Rectangular tanks multiply length × width × height; cylinders use π × radius² × height with the diameter entered in the length field. Divide by 231 and you have gallons.
Measure inside dimensions when precision matters — glass, plastic walls, and substrate all displace water. A '120 gallon' aquarium with 2 inches of gravel and rock holds noticeably less actual water, which is what chemical dosing and stocking rules care about.
Rectangular: length × width × height (in) ÷ 231. Cylindrical: π × (diameter ÷ 2)² × height ÷ 231.
Worked examples
A 48 × 24 × 24 in aquarium. → About 119.7 gallons — the standard '120 gallon' tank.
48 × 24 × 24 = 27,648 cu in ÷ 231 ≈ 119.7 gallons.
A cylindrical rain barrel 24 in across and 30 in tall. → About 58.8 gallons.
π × 12² × 30 ≈ 13,572 cu in ÷ 231 ≈ 58.8 gallons.
Tips & gotchas
- Water weighs about 8.34 lb per gallon — a filled 120 gallon aquarium tops 1,000 lb before the glass and stand, so check what the floor under it can carry.
- Fill to the real waterline, not the rim; most tanks run an inch or two low, which trims a few percent off the nominal volume.
- For fish stocking and dosing, subtract the displacement of substrate, rock, and decor — 10–15% is a common allowance in a decorated aquarium.
FAQ
How much does a filled tank weigh?
Water is ~8.34 lb per gallon — a 120 gallon aquarium holds ~1,000 lb of water before glass and rock. Check the floor.
Why divide by 231?
The US gallon is defined as 231 cubic inches, so any volume in cubic inches converts to gallons by that single division. (An imperial gallon is larger — 277.4 cu in.)
How do I measure an oddly shaped tank?
Break it into rectangles and cylinders, calculate each, and add them — or fill it with a container of known volume and count. Bow-front aquariums are usually within a few percent of the rectangle that bounds them.
Related calculators
Unit Price Calculator · Dog Age Calculator · Age Calculator · Date Difference Calculator · Fuel Cost Calculator