Dimensions to Gallons Calculator: How to Convert

This calculator converts the dimensions of a rectangular or square container (length, width, and height) into its volume in US gallons. You enter the three dimensions in inches, feet, or centimeters, and the calculator returns the equivalent volume in gallons, along with liters and cubic feet for reference.

Formula

The core formula is straightforward: multiply the three dimensions to get the volume in cubic inches, then divide by 231 to convert to US gallons.

V (gallons) = (L × W × H) / 231

• L = length in inches
• W = width in inches
• H = height in inches
• 231 = number of cubic inches in one US gallon

If you measure in feet, multiply the result by 7.48052 (cubic feet per gallon). If you measure in centimeters, multiply the cubic centimeters by 0.000264172.

Worked Example

Using the default values from the calculator: a box 12 inches long, 12 inches wide, and 12 inches high.

1. Volume in cubic inches: 12 × 12 × 12 = 1,728 in³
2. Convert to gallons: 1,728 ÷ 231 = 7.48 gallons
3. Convert to liters: 1,728 × 0.0163871 = 28.32 liters
4. Convert to cubic feet: 1,728 ÷ 1,728 = 1.00 ft³

The calculator shows 7.48 US gallons, which matches the result above.

When to Use This Calculator

This tool is useful for a variety of practical situations:

• Determining the water capacity of a fish tank or aquarium
• Estimating the volume of a storage container, cooler, or planter box
• Calculating how much liquid a rectangular container can hold for shipping or mixing
• Converting dimensions of a pool or pond into gallons for chemical dosing

Remember that this calculator assumes a perfectly rectangular shape with straight sides. If your container is cylindrical, spherical, or has tapered sides, use a shape-specific formula instead.

How to Read the Results

The first result is the volume in US gallons. The second is the same volume in liters (1 US gallon = 3.78541 liters). The third is in cubic feet (1 cubic foot = 7.48052 gallons). These are useful if you need to compare with other measurements or if you are working in metric units.

Note that US gallons differ from imperial gallons (used in the UK and some Commonwealth countries). 1 US gallon = 0.832674 imperial gallons. This calculator gives US gallons only.

Common Mistakes and How to Avoid Them

• Mixing units: Always use the same unit for all three dimensions. If you enter length in inches but width in feet, the result will be wildly wrong. Convert everything to inches before using the formula.
• Forgetting the conversion factor: The 231 in the formula is for cubic inches. If you measure in centimeters, you must multiply by 0.000264172, not divide by 231.
• Rounding too early: If you round the volume in cubic inches before dividing, you may lose accuracy. Keep full precision until the final step.
• Using outside dimensions: For a container with thick walls, the actual capacity is based on the inside dimensions, not the outside. Measure the interior space.

Limitations of the Formula

This formula assumes a perfect rectangular prism with flat, perpendicular sides. Real containers often have rounded corners, sloped sides, or irregular shapes. For those, the calculated volume will be slightly higher than the actual capacity. If you need extreme precision, fill the container with water and measure the volume directly, or use a 3D model.

Also, the formula gives the total volume. If you are filling with a liquid, remember that you should not fill to the very brim if the liquid expands with temperature or if you need headspace for mixing.

Frequently Asked Questions

How do I convert dimensions in feet to gallons?

First multiply length × width × height in feet to get cubic feet. Then multiply by 7.48052 to get US gallons. For example, a 2 ft × 2 ft × 2 ft cube is 8 cubic feet, which is 8 × 7.48052 = 59.84 gallons.

How many gallons is a 10-inch by 10-inch by 10-inch cube?

10 × 10 × 10 = 1,000 cubic inches. Divide by 231 to get 4.33 US gallons. That is about 16.4 liters.

What is the difference between US gallons and imperial gallons?

A US gallon is 231 cubic inches (about 3.785 liters). An imperial gallon is about 277.42 cubic inches (about 4.546 liters). The imperial gallon is about 20% larger. This calculator uses US gallons.

Can I use this for a cylindrical tank?

No, this calculator is only for rectangular boxes. For a cylinder, use the formula V = π × r² × h, where r is the radius and h is the height. Convert the result from cubic inches to gallons by dividing by 231.

Why is the gallon number 231?

The US gallon was historically defined as the volume of a cylinder 7 inches in diameter and 6 inches tall, which works out to about 231 cubic inches. It is now defined as exactly 231 cubic inches by law.

Does the material of the container affect the volume?

No, the volume is purely a geometric property. However, the wall thickness reduces the inside dimensions, so measure the interior. Also, if the container is flexible, the volume can change with pressure or shape.

Related

• Cubic Foot Calculator Circle: Volume of a Round Container
• Cubic Foot Calculator with Inches
• Excavation Calculator per Cubic Yard

in len | Length | in | 12 | 0
in wid | Width | in | 12 | 0
in hei | Height | in | 12 | 0
out gal | Volume | US gallons | (len * wid * hei) / 231 | 2
out lit | Volume | liters | (len * wid * hei) * 0.0163871 | 2
out cft | Volume | cubic feet | (len * wid * hei) / 1728 | 2