Aquarium Volume Calculator
Calculate the water volume of a rectangular fish tank in US gallons and litres from its length, width, and height.
How to use this tool
- Measure the internal length, width, and height of your tank.
- Enter the water fill height (not the tank height).
- The volume is shown in both litres and US gallons.
Calculate the exact water volume of your fish tank in litres and US gallons. Knowing your tank volume is essential for dosing medications, conditioners, and calculating stocking levels.
⚠ This tool provides general estimates for education only and is not financial, tax or legal advice. Figures may not reflect your situation — verify with a qualified professional.
Formula
Volume (L) = length (cm) × width (cm) × height (cm) ÷ 1000
Volume (US gal) = Volume (L) ÷ 3.78541
How it works
This calculator multiplies the three interior dimensions of a rectangular tank to get the total cubic centimetre capacity, then divides by 1,000 to convert to litres and by a further 3.78541 to convert to US gallons.
Results are rounded to one decimal place. The calculation assumes a perfect rectangular prism filled to the stated water height; it does not account for substrate, decorations, or glass thickness reducing usable water volume.
Worked example
- Tank dimensions: 90 cm long × 40 cm wide, filled to 45 cm water height.
- Volume in cm³ = 90 × 40 × 45 = 162,000 cm³.
- Convert to litres: 162,000 ÷ 1,000 = 162.0 L.
- Convert to US gallons: 162.0 ÷ 3.78541 ≈ 42.8 US gal.
Volume = 162.0 L (42.8 US gal)
Common mistakes to avoid
- Measuring the outside of the tank instead of the inside water column -- glass thickness (typically 6-12 mm per side) causes a meaningful overestimate on smaller tanks.
- Using the full height of the tank rather than the actual water fill level -- most tanks are filled 2-5 cm below the rim, so the real volume is lower than the geometric calculation.
- Forgetting that substrate, rocks, and decorations displace water -- a heavily aquascaped tank may hold 15-20% less water than the calculated volume, which affects chemical dosing.
Key terms
- Water height
- The depth of water inside the tank, measured from the substrate surface (or tank floor) to the waterline — not the full tank height.
- US gallon
- A unit of liquid volume equal to 3.78541 litres, commonly used to describe aquarium sizes in North America.
- Litre (L)
- The SI unit of volume equal to 1,000 cubic centimetres, widely used for aquarium volume outside North America.
- Displacement volume
- The volume displaced by substrate, rocks, and decorations; subtracting this from the calculated tank volume gives actual water volume.
Frequently asked questions
- Why does the volume differ from what's printed on my tank?
- Manufacturers typically quote gross (external) volume. Internal dimensions and the actual water fill level are smaller, giving a lower real volume. Always use your own measurements for accurate dosing.
- How do I convert litres to US gallons?
- Divide litres by 3.785 to get US gallons (1 US gallon = 3.785 L).