Aquarium Tank Volume Calculator

Why actual water volume matters:

The number printed on your tank (e.g., “55 gallon”) is the total capacity when filled to the brim with nothing inside.
Your actual water volume is always less. Substrate, rocks, driftwood, equipment and the inch you leave below the rim between them take out 10 to 25%.

Accurate water volume is critical for:

• Medication dosing (overdose kills fish)
• Salt and chemical treatments
• Heater and filter sizing
• Stocking calculations

Rectangular tank formula:

Volume (gallons) = Length × Width × Height (all in inches) ÷ 231

Volume (liters) = Length × Width × Height (all in cm) ÷ 1,000

Displacement adjustment:

Enter one combined figure. The percentages below add up, so a planted tank with rockwork is usually 12 to 18% rather than any single line on its own.

• Substrate: roughly 3% per inch of bed depth in a normal 18 to 22 inch tank. A 1-inch fish-only bed is about 3%, a 3-inch planted bed about 9%.
• Rocks and driftwood: 5–10%, and this is the one worth measuring rather than guessing. Drop the hardscape in a bucket, mark the rise, and you have it exactly.
• Equipment (filter intake, heater, internal pump): usually 2–3%

A point most tank-volume guides get wrong: substrate displaces far less than its own bulk, because water fills the gaps between the grains. Aquarium gravel is only about 57% solid by volume and sand about 60%, so an inch of gravel that occupies 5% of the tank’s height removes closer to 3% of the water. Sand packs tighter and takes out marginally more per inch than gravel does, which is the opposite of what most people assume from how heavy a bag of sand feels.

Worked example:

A 48 × 13 × 21 inch tank (the standard 55 gallon):

• Brim-full volume = 48 × 13 × 21 ÷ 231 = 56.7 gallons
• Nobody fills to the brim, so enter the water height, 20 inches: 48 × 13 × 20 ÷ 231 = 54.0 gallons
• Displacement, added up: 1 inch of gravel (3%) + moderate rockwork (7%) + equipment (2%) = 12%
• 54.0 × 0.88 = 47.5 gallons actual

That “55 gallon” tank really holds about 47 gallons of water, an eighth less than the number on the box. On a medication with a narrow safe window that eighth is the difference between a treatment and an overdose.

Cylinder tanks: Volume = π × Radius² × Height (measure in inches, then divide by 231 for gallons)

Bow-front tanks: Measure as rectangular and add approximately 10–15% for the curved front section.

Common stocking rule: 1 inch of adult fish per gallon of actual water volume (freshwater community fish).
Treat that as a ceiling rather than a target. It works for slim-bodied community fish and falls apart on anything with bulk: a 10-inch oscar is not ten neon tetras, and goldfish produce waste at a rate the inch rule never accounted for.