Gallons in a Rectangle Calculator

This calculator works out the volume of a rectangular container (like a fish tank, pool, or fuel tank) in gallons. You enter the length, width, and depth in feet (or inches), and it returns the volume in US gallons and cubic feet. It uses the standard formula: volume = length × width × depth, then converts cubic feet to gallons.

Formula

The volume of a rectangular prism (a box) is:

V = L × W × D

Where:

• V = volume in cubic units
• L = length of the rectangle (ft)
• W = width of the rectangle (ft)
• D = depth or height (ft)

To get gallons, you multiply the cubic feet by 7.48052 (the number of US gallons in one cubic foot). If you measure depth in inches, first convert to feet by dividing by 12. The calculator above does that conversion automatically.

Worked Example

Using the default values: length = 40 ft, width = 25 ft, depth = 3 in.

1. Convert depth to feet: 3 in ÷ 12 = 0.25 ft
2. Calculate volume in cubic feet: 40 × 25 × 0.25 = 250 ft³
3. Convert to gallons: 250 × 7.48052 = 1,870.13 US gallons

The calculator shows 1,870.13 gallons (rounded to two decimals). That is the same result you get by hand.

Why Use This Calculator?

This tool is useful for anyone filling a rectangular container with liquid. Common uses include:

• Fish tanks and aquariums – to know how much water to add or treat
• Swimming pools – to estimate water volume for chemical dosing
• Fuel or water storage tanks – to check capacity before filling
• Garden ponds – to size a pump or liner
• Shipping or storage – to estimate how much a rectangular space holds

Knowing the volume in gallons helps you buy the right amount of water treatment, paint, or other liquid products.

How to Measure a Rectangle

To get an accurate result, measure the inside dimensions of the container, not the outside. For a tank with walls, the internal length, width, and depth determine the actual volume. Use a tape measure and record measurements in the same unit – feet or inches. If you mix units, convert first. For example, if length is in feet and depth is in inches, divide depth by 12 before multiplying.

For a container with sloped sides (like many ponds), this calculator is not accurate. It assumes straight vertical walls and a flat bottom. For a tapered shape, use a different formula or break it into sections.

Common Mistakes and Misconceptions

One frequent error is confusing US gallons with imperial gallons. The calculator uses US gallons (7.48052 per cubic foot). Imperial gallons are larger (about 1.2 US gallons). If you need imperial, multiply the US result by 0.83267.

Another mistake is forgetting to convert depth from inches to feet. Many people multiply length × width × depth in inches and get a huge number. Always convert to the same unit before multiplying.

Also, the calculator gives the volume of the entire space. If the container is not filled to the brim, you need to measure the actual water depth, not the container height. For example, a 40 ft × 25 ft pool with 3 inches of water holds 1,870 gallons, but if you only fill it 2 inches deep, it holds about 1,247 gallons.

US vs. Metric Units

The calculator works in feet and inches, but the same formula works in meters and centimeters. One cubic meter equals 1,000 liters, and one liter equals 0.264172 US gallons. So if you measure in meters, the volume in cubic meters × 1,000 gives liters, then × 0.264172 gives US gallons.

For example, a rectangle 12 m × 5 m × 0.1 m (about 4 inches deep) has a volume of 6 m³, which is 6,000 liters, or about 1,585 US gallons. The calculator above does not accept meters, but you can convert your measurements to feet first (1 m = 3.28084 ft).

Limitations of the Formula

The formula assumes a perfect rectangular prism with flat, parallel sides. Real-world containers often have rounded corners, sloped bottoms, or irregular shapes. For those, this calculator will overestimate or underestimate the true volume. For a fish tank with a framed base, the actual water volume is slightly less than the calculated volume because the glass takes up space.

Also, the calculator does not account for displacement from objects inside the container (like rocks, decorations, or fish). If you need the actual water volume in an aquarium with substrate and decorations, subtract the volume those objects occupy. A rough way is to measure the water you add when filling – that is always the most accurate.

Frequently Asked Questions

How many gallons are in a 40×25×3 inch rectangle?

Using the formula: 40 ft × 25 ft × 0.25 ft = 250 ft³, which is 1,870.13 US gallons. That is the default example in the calculator above.

Can I use this for a round pool?

No. This calculator is only for rectangles. For a round pool, use the formula for a cylinder: π × radius² × depth. The result will be in cubic feet, then multiply by 7.48052 for gallons.

What is the difference between US and imperial gallons?

A US gallon is 231 cubic inches, while an imperial gallon is about 277.42 cubic inches. One US gallon equals 0.83267 imperial gallons. This calculator gives US gallons; multiply by 0.83267 to get imperial.

How do I convert cubic feet to gallons?

Multiply cubic feet by 7.48052. For example, 10 ft³ × 7.48052 = 74.8 US gallons. The calculator does this for you.

What if my depth is in centimeters?

Convert centimeters to feet first: divide by 30.48. Or convert length and width to centimeters, compute volume in cm³, then divide by 3,785.41 (cubic centimeters per US gallon).

Does the calculator account for the thickness of the tank walls?

No. It uses the dimensions you enter. To get the true water volume, measure the inside dimensions. If you measure outside, subtract the wall thickness from each dimension.

Related

• Block Calculation for Wall: How to Estimate Blocks
• Besser Block Wall Calculator: How to Estimate
• Block Wall Grout Calculator: Mix & Fill

in l | Lawn length | ft | 40 | 0 | 100000
in w | Lawn width | ft | 25 | 0 | 100000
in d | Depth | in | 3 | 0 | 120
out yd | Volume | yd³ | l * w * d / 324 | 2
out ft3 | Volume | ft³ | l * w * d / 12 | 1