Aquarium Volume Calculator

An aquarium volume calculator helps you determine exactly how many gallons or liters of water your fish tank holds based on its shape and dimensions, accounting for displacement from rock, sand, and equipment.

Rectangle Dimensions

Standard rectangular aquarium

Length (in)

Width (in)

Height (in)

Displacement Deductions

Account for volume displaced by rock, sand, and equipment. Leave blank to skip.

Live Rock (lbs)

~0.025 gal/lb displacement

Sand Depth (in)

Uses tank footprint area

Equipment (gallons)

Overflow, heater, pumps, etc.

Enter your tank dimensions above to calculate water volume.

Volume Calculation Tips

✓Measure inside dimensions. Use the interior length, width, and height - not the outer frame measurements. Glass thickness and trim can reduce volume by 5-10%.
✓Account for displacement. Live rock, sand, and equipment can reduce actual water volume by 15-20% compared to the rated tank size.
✓Knowing net volume matters for dosing. Supplement and medication doses are based on actual water volume, not the display tank size. Overdosing from inaccurate volume estimates can harm livestock.
✓Include sump volume separately. If you run a sump, add its water volume to your net total for a complete system volume.
✓Corner and hexagonal tanks are approximate. These shapes use geometric approximations. For precise volume, you can fill the tank with a measured amount of water.

Frequently Asked Questions

How do I calculate aquarium volume?

Measure the inside dimensions of your tank (length, width, and height) in inches or centimeters. For a rectangular tank, multiply length x width x height, then divide by 231 to get US gallons or by 61.024 to get liters. For non-rectangular shapes like bow fronts or cylinders, use the shape-specific calculator above.

How much water does a 55 gallon tank actually hold?

A standard 55-gallon tank holds about 55 gallons when completely filled. After adding a 2-inch sand bed, 40-50 pounds of live rock, and equipment, the actual water volume drops to 42-46 gallons - roughly 15-20% less than the rated size. Use the displacement section above for an accurate estimate.

How do I account for live rock displacement?

Porous live rock displaces approximately 0.025 gallons per pound. For a reef tank with 50 pounds of live rock, that is about 1.25 gallons of displaced water. Enter the total weight of your rock in the displacement section for a precise calculation.

What tank shape holds the most water?

For a given footprint, a cylinder holds the most water because a circle maximizes area for a given perimeter. Among practical aquarium shapes, a standard rectangle is the most volume-efficient. Bow front, hexagonal, and corner pentagon tanks hold less water than a rectangle with the same maximum dimensions.

How do I convert aquarium volume between gallons and liters?

Multiply US gallons by 3.785 to get liters, or divide liters by 3.785 to get US gallons. For example, a 75-gallon tank holds approximately 284 liters. This calculator shows both units automatically.

Track Your Tank Parameters and More

Track your tank parameters and more with My Reef Log - a free reef keeping journal. Log water tests, visualize trends, manage livestock, and never miss a maintenance task.

Start Tracking Free

No credit card required