Gallons of Water Calculator: Volume in US or Imperial

This calculator works out how many gallons of water a container holds from its dimensions (length, width, and height) or from a known volume in liters, cubic feet, or cubic meters. You enter the shape and measurements, and it returns the volume in US gallons (and, if you like, imperial gallons and liters). The first result is the US gallon figure, which is the standard for most water storage, aquariums, and fuel in the United States.

Formula

For a rectangular container (like a fish tank or a box), the volume in cubic feet is:

Volume (ft³) = Length × Width × Height

Then convert cubic feet to US gallons by multiplying by 7.48052 (the number of US gallons in one cubic foot).

For a cylindrical container (like a water tank or a barrel), the volume is:

Volume (ft³) = π × (Diameter/2)² × Height

For a sphere (like a round storage tank), the volume is:

Volume (ft³) = (4/3) × π × (Diameter/2)³

In all cases, the final step is the same: multiply the cubic feet by 7.48052 to get US gallons. If you measure in inches, divide each dimension by 12 first to get feet, or use the direct conversion factor 0.004329 (gallons per cubic inch).

Worked Example

Using the default values from the calculator above (rectangular shape, length = 2 ft, width = 1 ft, height = 1 ft):

1. Calculate cubic feet: 2 × 1 × 1 = 2 ft³
2. Convert to US gallons: 2 × 7.48052 = 14.96104 gallons
3. Rounded to two decimals: 14.96 US gallons

The calculator shows 14.96 gallons, 56.63 liters, and 12.45 imperial gallons for these defaults.

Why the Type of Gallon Matters

There are two common definitions of a gallon, and they are not the same size. The US gallon equals 231 cubic inches (about 3.785 liters), while the imperial gallon (used in the UK, Canada, and some other countries) equals 277.42 cubic inches (about 4.546 liters). That is a 20% difference. If you fill a container marked “5 gallons� with an imperial gallon jug, you will be short by about 1.2 US gallons. Always confirm which gallon your pump, tank, or recipe uses before relying on the number.

Most online calculators and US product labels use US gallons. If you are in a country that uses imperial, multiply the US gallon result by 0.832674 to get imperial gallons, or use the calculator’s third output line.

How to Measure a Container Accurately

For the calculator to give a correct answer, the dimensions must be measured correctly:

• Use the inside dimensions of the container, not the outside. Wall thickness can add several percent to the volume if ignored.
• Measure to the water level, not the top edge, if the container will not be filled to the brim. For aquariums, subtract the gravel depth and the space left for air.
• For cylinders, measure the diameter at the widest point, not the base if it is tapered.
• If the container is not a perfect rectangle, cylinder, or sphere, break it into simpler shapes and add the volumes.

For irregular shapes (like a pond with sloped sides), a better method is to measure the water added with a known-volume bucket or a flow meter.

Common Mistakes and How to Avoid Them

The most frequent error is mixing units. If you measure length in feet and width in inches, the formula will be off by a factor of 12 (or 144). Always convert every dimension to the same unit before calculating.

Another mistake is using the outside dimensions of a tank. A typical glass aquarium has walls that are 0.25 to 0.5 inches thick, which can reduce the true water volume by 2–5%.

Also, remember that the calculator gives the total volume when filled to the top. In practice, you rarely fill a tank completely. For a fish tank, you usually leave an inch or two of air at the top, so the actual water volume is less than the calculated volume. Subtract the empty space (height × length × width of the air gap) from the total.

Finally, be careful with the difference between US and imperial gallons, as described above. A 10-gallon aquarium in the US holds about 37.85 liters, while a 10-gallon imperial tank holds about 45.46 liters.

When to Use This Calculator

This calculator is useful for:

• Determining the water volume of a fish tank or aquarium to dose medications or choose a filter.
• Estimating the capacity of a rain barrel, storage tank, or pool.
• Calculating how much water a container can hold for camping, emergency storage, or gardening.
• Converting a volume in liters or cubic feet to gallons for a recipe or a project.

For water weight, note that one US gallon of water weighs about 8.34 pounds (3.78 kg) at room temperature. So a 50-gallon tank weighs about 417 pounds just for the water, plus the weight of the tank itself. This is critical for floor support in a home aquarium.

Limitations of the Formula

The formulas assume perfect geometric shapes. Real containers often have rounded corners, sloped sides, or internal structures (like aquarium braces or tank baffles) that reduce the actual volume. The calculated value is an overestimate in those cases.

Also, the conversion factor 7.48052 is exact for US gallons (231 cubic inches per gallon), but the volume of water changes slightly with temperature. At 4°C (39°F), water is densest, so a gallon of cold water weighs slightly more than a gallon of warm water. For most practical purposes, the difference is negligible (less than 0.5%).

If you need the volume of an irregular container, the best approach is to fill it with a known volume of water and measure how much you added. That method is always more accurate than measuring dimensions.

Frequently Asked Questions

How many gallons is a 2x1x1 foot tank?

Using the rectangular formula: 2 × 1 × 1 = 2 cubic feet, which is 14.96 US gallons. That is the volume if filled to the brim. With an inch of air space at the top, the actual water volume is about 13.8 gallons.

How do I convert liters to gallons?

Divide the number of liters by 3.78541 to get US gallons, or by 4.54609 to get imperial gallons. For example, 10 liters is about 2.64 US gallons or 2.20 imperial gallons.

What is the difference between a US gallon and an imperial gallon?

A US gallon is 231 cubic inches (3.785 liters), while an imperial gallon is 277.42 cubic inches (4.546 liters). The imperial gallon is about 20% larger. The US gallon is used in the United States and some Latin American countries; the imperial gallon is used in the UK, Canada, and several Commonwealth nations.

How much does a gallon of water weigh?

At room temperature (about 20°C/68°F), one US gallon of water weighs approximately 8.34 pounds (3.78 kg). The weight changes slightly with temperature and dissolved minerals, but this is a good estimate for most purposes.

Can I use this calculator for a round pool?

Yes, if the pool is a perfect cylinder. Measure the diameter and the water depth (not the pool height). The calculator will give the volume in gallons. For an oval or kidney-shaped pool, use the rectangular formula with the average length and width, or better, use a pool-specific calculator that accounts for the shape.

Why does my tank hold less water than the calculated volume?

Because the calculation assumes a perfect shape and filling to the very top. In reality, you leave air space, the glass has thickness, and there may be decorations, gravel, or equipment inside. Subtract the volume of any substrate (gravel) and decorations, and leave at least an inch of air space for gas exchange in a fish tank.

Related

• Height in Feet to cm Converter & Formula
• Inches to Feet Conversion Calculator & Formula
• Liters to Gallons Converter & Formula

in shape | Shape | Rectangular=1, Cylindrical=2, Spherical=3
in length | Length (or diameter) | ft | 2 | 0
in width | Width (or height) | ft | 1 | 0
in height | Height (or depth) | ft | 1 | 0
out gallons | Volume (US gallons) | gal | shape == 1 ? length * width * height * 7.48052 : shape == 2 ? 3.14159 * (length/2)^2 * width * 7.48052 : (4/3) * 3.14159 * (length/2)^3 * 7.48052 | 2
out liters | Volume (liters) | L | gallons * 3.78541 | 2
out impgal | Volume (imperial gallons) | gal | gallons * 0.832674 | 2