Gallons of Water: Calculation and Conversion Guide

Calculate gallons of water by multiplying the container’s length, width, and water depth, then converting its volume to gallons. Enter the three measurements in feet below to get US gallons and cubic feet; the result assumes a rectangular container filled to the stated depth.

Formula

For a rectangular space, volume in cubic feet equals length × width × water depth. Multiply that volume by 7.48051948 to convert cubic feet to US liquid gallons:

Gallons = length (ft) × width (ft) × depth (ft) × 7.48051948

• Length (l): inside length of the water-filled space, in feet.
• Width (w): inside width, in feet.
• Depth (d): actual water depth, in feet—not necessarily the container’s full height.
• 7.48051948: the number of US liquid gallons in one cubic foot.

The calculator’s second output is the intermediate volume in cubic feet. For dimensions measured in inches, first convert each one to feet by dividing by 12, or use the direct conversion: cubic inches ÷ 231 = US gallons. For metric measurements, one cubic metre contains about 1,000 litres, and one US gallon is about 3.785 litres.

Worked Example

Using the calculator’s default dimensions—4 ft long, 3 ft wide, and 2 ft deep—work out the volume in two steps:

1. Multiply the dimensions: 4 × 3 × 2 = 24 cubic feet.
2. Convert cubic feet to gallons: 24 × 7.48051948 = 179.53246752 US gallons.

Rounded to two decimal places, the calculator returns 179.53 US gallons and 24.00 cubic feet. In metric terms, that is about 679.7 litres. Actual fill may be lower if the container is not filled to the measured depth.

Which Measurements to Use

Measure the space that water occupies, not automatically the outside dimensions. For a tank or box with thick walls, use inside length and width. Measure the water depth from the inside bottom to the water surface. If a pool has a sloped bottom or a changing depth, a single depth cannot describe its full volume precisely; use the average depth as an estimate or split the shape into simpler sections.

Keep all three measurements in the same unit before multiplying. The calculator takes feet. Convert inches to feet by dividing by 12; convert centimetres to metres by dividing by 100 if using a metric calculation. Multiplying a length in feet by a width in inches and a depth in centimetres produces a meaningless result unless each measurement is converted to a common unit first.

For a rectangular container with a sloped floor, a useful estimate is to average the shallow and deep water depths, then multiply by the inside length and width. This works when the bottom changes evenly from one end to the other. Irregular shapes, rounded corners, steps, and internal equipment reduce or complicate the usable volume, so treat a simple rectangular estimate accordingly.

When the Calculation Is Useful

Volume estimates help with filling a rectangular tank, estimating water in a simple pool section, or checking how much water a storage space could hold. They can also support other calculations, such as estimating the mass of water. Fresh water has a mass close to 1 kilogram per litre under ordinary conditions, so 679.7 litres has a mass of roughly 680 kg, excluding the container. This is a conversion, not a recommendation about whether a floor or support can carry that load.

For multiple rectangular sections, calculate each section’s gallons separately and add the results. For example, a main pool area and a rectangular step area can be estimated separately if their dimensions and depths are known. Do not count overlapping sections twice. If the measured sections do not match the actual shape, the sum will not make the estimate more accurate.

US Gallons, Imperial Gallons, and Litres

“Gallon” can refer to more than one unit. This calculator uses the US liquid gallon: one US gallon is exactly 231 cubic inches and approximately 3.78541 litres. The imperial gallon, used in some contexts outside the United States, is larger: approximately 4.54609 litres. Confusing the two changes the result by about 20 percent.

Useful conversions include:

• 1 cubic foot ≈ 7.48052 US gallons.
• 1 US gallon ≈ 3.78541 litres.
• 1 litre ≈ 0.264172 US gallons.
• 1 cubic metre = 1,000 litres ≈ 264.172 US gallons.

Check which gallon a container label, specification, or reference uses before comparing figures. If a capacity is given in litres, divide litres by 3.78541 for US gallons. If it is in imperial gallons, convert through litres or use the imperial-gallon factor instead of the calculator’s US-gallon result.

Common Errors and Limits

• Using the full container height: enter the water depth, which may be less than the inside height.
• Mixing units: convert all dimensions to feet for this calculator, or use a consistent metric formula.
• Using outside dimensions: wall thickness can make the inside capacity noticeably smaller.
• Rounding too early: keep several decimal places through the calculation and round the final displayed answer.
• Assuming every shape is rectangular: cylinders, cones, and irregular tanks need shape-specific formulas.

The displayed result is a mathematical estimate, not necessarily the amount a real container can hold. Curved sides, rounded corners, fittings, pipes, and a required gap at the top affect usable capacity. Water level can also change during filling, and a label may state nominal capacity rather than the volume available at a particular fill line. Use a suitable shape-specific method or manufacturer-provided capacity information when those details matter.

Frequently Asked Questions

How do I calculate gallons of water in a rectangular tank?
Multiply the tank’s inside length, inside width, and water depth in feet. Multiply the resulting cubic feet by 7.48051948 to get US gallons.

How many gallons are in a cubic foot?
One cubic foot is approximately 7.48052 US liquid gallons. The conversion is different if you mean imperial gallons.

How do I calculate gallons from inches?
For a rectangular space, multiply its length, width, and depth in inches to get cubic inches, then divide by 231 for US gallons. Use inside measurements and the actual water depth.

How do I convert litres to US gallons?
Divide the number of litres by approximately 3.78541. For example, 100 litres is about 26.42 US gallons.

Does this calculation work for a round tank?
No. A cylindrical tank needs a different formula: gallons equal π × radius squared × water depth in cubic feet × 7.48051948. Measure the inside radius and water depth in feet.

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in l | Length | ft | 4 | 0 | 100000
in w | Width | ft | 3 | 0 | 100000
in d | Water depth | ft | 2 | 0 | 100000
out gal | Water volume | US gal | l * w * d * 7.48051948 | 2
out ft3 | Volume | ft³ | l * w * d | 2