Aquarium Volume Calculator

How to Use the Aquarium Volume Calculator

Select your tank shape and enter the interior dimensions in inches. For rectangular and bow-front tanks, enter length, width, and height. For cylinders, enter the diameter and height. The calculator converts cubic inches to both US gallons and liters using the exact conversion factors (1 US gallon = 231 cubic inches).

Volume Formulas by Tank Shape

• Rectangular: Volume (in³) = Length × Width × Height
• Cylinder: Volume (in³) = π × (diameter/2)² × Height
• Bow-front: Approximated as rectangular with a small volume correction for the front bow curve (typically 90–95% of rectangular volume)

To convert cubic inches to US gallons: divide by 231. To convert to liters: multiply by 0.016387.

Why Actual Volume Differs from the Label

A "20-gallon" tank is a nominal label based on gross exterior dimensions. The actual water volume is always less because: glass thickness (typically 3/8" on each side for a standard 20-gallon) reduces interior dimensions, gravel substrate displaces 1–2 gallons depending on depth, decorations and rocks displace additional water, and you should leave an air gap of 1–2 inches at the top. A typical "20-gallon" tank holds 15–18 gallons of actual water at normal fill levels. This matters when dosing medications, dechlorinators, and fertilizers — always base doses on actual water volume, not the nominal tank size.

Practical Implications of Volume

Knowing accurate volume is essential for: dosing water conditioner (usually 1 tsp per 10 gallons), calculating medication doses (many are per 10 gallons, and overdosing harms fish), determining filter flow rate (aim for 5–10× tank volume per hour), planning water changes (a 25% change on a 15-gallon fill is 3.75 gallons, not 5 gallons), and estimating weight for floor/stand load calculations (water weighs 8.34 lbs per gallon — a true 55-gallon tank weighs over 500 lbs fully set up).

Common Uses

• Calculating correct water treatment and medication doses
• Determining accurate fish stocking capacity (use with our Fish Stocking Calculator)
• Planning water change volumes for weekly maintenance
• Sizing filtration equipment by required flow rate