Tools · Fishkeeping
Aquarium Tank Volume Calculator (Gallons & Liters)
Knowing your tank’s real water volume is the first step in dosing any medication correctly, almost every treatment is measured “per gallon.” Enter your tank’s dimensions below and this calculator gives you the volume in gallons and liters, plus a realistic estimate of the actual water volume once substrate and decorations are accounted for.
How to use it: pick your tank shape, enter the inside dimensions (length, width, height, and diameter for cylinders), and choose your units. You will get the full volume and a “true water volume” estimate. Use the true water volume when dosing medication, because gravel, rocks, and the gap below the rim mean your tank holds less water than its outer size suggests.
Estimates only. Measure the inside dimensions where possible. The true-water-volume figure is a practical approximation, when dosing medication, follow the product label and your veterinarian’s direction.
Why true water volume matters for dosing
Medication doses are calculated on the water the fish actually swim in, not the size printed on the box. A “20-gallon” tank rarely holds 20 gallons of water: gravel or sand, rocks, driftwood, equipment, and the inch or two of air below the rim all take up space. Dosing to the full nominal volume can mean giving more medication than the water needs. Using the true water volume gives a more accurate dose.
How the calculation works
For a rectangular tank, volume is length × width × height. For a cylinder, it’s π × radius² × height.[1] We convert cubic inches or centimeters to gallons and liters, then estimate true water volume at roughly 90% of the full figure[2]. A common rule of thumb for a moderately decorated tank. A heavily planted or rocky tank may hold even less; a bare tank holds closer to the full volume.
Frequently asked questions about tank volume
Why is my real water volume less than my tank size?
Gravel, sand, rocks, driftwood, equipment, and the air gap below the rim all displace water. A typical decorated tank holds roughly 90% of its nominal volume; a heavily planted or rocky one holds less.
Which volume do I use to dose medication?
Use the true water volume, not the tank’s printed size. Dosing to the full nominal volume can overdose the actual water. Always follow the product label and your veterinarian’s direction.
Should I measure inside or outside dimensions?
Measure the inside of the glass where you can. Outside measurements include the glass thickness and slightly overstate the volume.
How do I convert gallons and liters?
One US gallon is about 3.785 liters. One UK (imperial) gallon is about 4.546 liters. This calculator shows all three so you can match whatever your medication label uses.
How this tool is reviewed
This calculator and its accompanying guidance have been reviewed for accuracy by Dr. Julia Bauer, our fish and aquarium veterinarian. See our Veterinary Review Board for her credentials.
Sources & References
- [1] Volume formulas: rectangular prism (length x width x height) and cylinder (pi x radius squared x height); unit conversions use 231 cubic inches per US gallon and 1000 cubic centimeters per liter (standard references).
- [2] The ~90% true-water-volume rule of thumb reflects displacement by substrate, hardscape, equipment, and the below-rim air gap in a moderately decorated aquarium; actual figures vary by setup.
For educational use. PrepperPetSupply is a licensed, veterinarian-staffed pharmacy; prescriptions are dispensed only after review by a licensed veterinarian and are not a substitute for in-person veterinary care. Not for human use.