Square Foot Calculator Tank: Volume in Gallons

This calculator works out the volume of a rectangular aquarium in gallons and liters from its length, width, and height in feet (or inches). Enter the tank’s dimensions and the calculator returns the total water volume in US gallons, plus the volume in liters and cubic feet.

Formula

The volume of a rectangular tank is simply the product of its three internal dimensions:

V = L × W × H

Where:

• V = volume in cubic feet (ft³)
• L = length in feet (ft)
• W = width in feet (ft)
• H = height in feet (ft)

To convert cubic feet to US gallons, multiply by 7.48052 (the number of US gallons in one cubic foot). To convert cubic feet to liters, multiply by 28.3168 (the number of liters in one cubic foot).

Worked Example

Take a standard 4 ft × 1.5 ft × 1.5 ft aquarium (the default values above).

1. Calculate the volume in cubic feet: 4 × 1.5 × 1.5 = 9 ft³
2. Convert to US gallons: 9 × 7.48052 = 67.32468 gal → rounds to 67.32 gal
3. Convert to liters: 9 × 28.3168 = 254.8512 L → rounds to 254.9 L

The calculator shows 67.32 gallons, 254.9 liters, and 9.00 cubic feet for these defaults.

Why Use Square Feet for a Tank?

Aquarium dimensions are often given in feet (e.g., a “4-foot tank”) or in inches (e.g., 48″ × 18″ × 18″). The term “square foot calculator” is a common search phrase, but a tank is a three-dimensional object, so you actually need cubic feet (volume) rather than square feet (area). This calculator treats the “square foot” input as the base area (length × width) and then multiplies by height to get volume. If you only know the floor area and the height, you can still compute volume: multiply the area by the height, then convert to gallons.

How to Choose the Right Tank Size

When selecting an aquarium, the volume determines how many fish it can safely hold. A common rule of thumb is “1 inch of fish per gallon,” but that is a rough guide and does not account for fish species, filtration, or aquascaping. Always check the specific needs of the fish you plan to keep. Also consider that the actual water volume is less than the tank’s total volume because of substrate, decorations, and the fact that you leave some air space at the top. A typical filled tank holds about 90–95% of its calculated volume.

For example, a 75-gallon tank (48″ × 18″ × 21″) actually holds about 70 gallons of water when filled to within an inch of the top. Use the calculator with your tank’s outside dimensions, then subtract 5–10% for a realistic water volume.

Common Mistakes and How to Avoid Them

• Mixing units: If you measure in inches, convert to feet first (divide by 12) or use the calculator with inches and then convert. Do not mix feet and inches in the same calculation.
• Using outside dimensions: Glass thickness reduces the internal volume. For a standard glass tank, subtract about 0.5 inches from each dimension for a more accurate internal volume.
• Forgetting the conversion factor: 1 cubic foot = 7.48052 US gallons, not 7.5. The difference matters for large tanks.
• Rounding too early: Round only at the final step to avoid small errors that compound.

Limits of the Formula

The formula assumes a perfectly rectangular, flat-bottomed tank. It does not account for:

• Curved or bow-front tanks (volume is less than a rectangle of the same max dimensions)
• Hexagonal or cylindrical tanks (use different formulas)
• Thickness of glass or acrylic (reduces internal volume)
• Displacement by substrate, rocks, and decorations (reduces water volume)
• Air gap at the top (you never fill to the brim)

For a bow-front tank, a good approximation is to multiply the rectangular volume by 0.85–0.90. For a cylinder, use V = π × r² × h, where r is the radius and h is the height.

US Gallons vs. Imperial Gallons

There are two different “gallons” in use. The US gallon is 231 cubic inches (3.785 liters), while the Imperial gallon (used in the UK and some Commonwealth countries) is 277.42 cubic inches (4.546 liters). This calculator uses US gallons, which is the standard for aquarium measurements in North America. If you need Imperial gallons, multiply the US gallon result by 0.83267.

For example, a 100 US gallon tank is about 83.3 Imperial gallons. Always check which gallon your fish store or equipment manufacturer uses.

Frequently Asked Questions

How many gallons is a 4×2×2 tank?

A 4 ft × 2 ft × 2 ft tank has a volume of 16 cubic feet (4×2×2). Multiply by 7.48052 to get 119.7 US gallons. This is often sold as a “120-gallon” tank in the aquarium trade.

How do I convert inches to feet for the calculator?

Divide the number of inches by 12. For example, 48 inches ÷ 12 = 4 feet. If you have a mixed measurement like 4 ft 6 in, convert the inches to feet (6 ÷ 12 = 0.5) and add it to the feet (4 + 0.5 = 4.5 ft).

Does the calculator work for round or hex tanks?

No, this calculator is only for rectangular tanks. For a cylindrical tank, use V = π × r² × h (in feet) and then multiply by 7.48052 for gallons. For a hexagon, break it into triangles or use a specialized aquarium volume calculator.

Why is my tank’s actual water volume less than the calculated volume?

Because you don’t fill the tank to the rim, and because substrate, rocks, and decorations displace water. Also, the glass walls take up space. A good rule is to subtract 5–10% for the air gap and another 5–10% for decorations, depending on how heavily aquascaped the tank is.

What is the difference between a 10-gallon and a 20-gallon long tank?

A 10-gallon is typically 20″ × 10″ × 12″ (L×W×H), while a 20-gallon long is 30″ × 12″ × 12″. The 20-long has more surface area for gas exchange, which is better for certain fish, but it has the same height as the 10-gallon, so it holds twice the volume.

Can I use this calculator for a sump or refugium?

Yes, as long as the sump is rectangular. Measure the internal dimensions of the sump chamber (length × width × height) and enter them. Remember that the sump is not filled to the top; the actual water volume is typically 50–75% of the total volume, depending on the water level.

Related

• Land Cent to Square Feet Calculator
• Horizontal Cylindrical Tank Volume Calculator
• Horizontal Dish End Tank Volume Calculator

in length | Length | ft | 4 | 0
in width | Width | ft | 1.5 | 0
in height | Height | ft | 1.5 | 0
out gallons | Volume (US gal) | gal | length * width * height * 7.48052 | 2
out liters | Volume (L) | L | length * width * height * 28.3168 | 1
out cuft | Volume (cu ft) | ft³ | length * width * height | 2