How to Calculate Gallons from Inches

This guide shows you how to convert a measurement in inches (usually the depth of water in a container) into gallons. You’ll need the container’s dimensions and a simple formula. By the end, you’ll be able to calculate the volume of water in a tank, pool, or any rectangular or cylindrical container using just a tape measure and a calculator.

Formula

The basic formula to convert inches to gallons depends on the shape of your container. For a rectangular tank or pool:

Gallons = (Length × Width × Depth in feet) × 7.48052

Where:

• Length (L) – the longest side of the container, measured in feet.
• Width (W) – the shorter side, also in feet.
• Depth (D) – the water depth, measured in inches (convert to feet by dividing by 12).
• 7.48052 – the number of gallons in one cubic foot.

For a cylindrical container (like a drum or round pool), use:

Gallons = (π × (radius in feet)² × depth in feet) × 7.48052

Here, radius is half the diameter. If you measure the diameter in inches, convert to feet first (divide by 12).

Worked Example

Let’s use the calculator defaults: a rectangular tank with length = 4 ft, width = 2 ft, and water depth = 6 inches.

1. Convert depth from inches to feet: 6 inches ÷ 12 = 0.5 ft.
2. Calculate the volume in cubic feet: 4 ft × 2 ft × 0.5 ft = 4 ft³.
3. Convert cubic feet to gallons: 4 ft³ × 7.48052 = 29.92208 gallons.

Rounded to one decimal place, the calculator shows 29.9 gallons.

When to Use This Calculation

You’ll need this conversion when filling a fish tank, treating water in a pool, determining fertilizer or chemical doses for a pond, or figuring out how much water a container holds. It’s also useful for rainwater harvesting – you can measure the depth of rain in a rain gauge and estimate how many gallons fell on your roof.

Common Mistakes

• Mixing units: The most frequent error is forgetting to convert inches to feet. If you input length and width in feet but depth in inches without dividing by 12, your result will be 12 times too large.
• Using the wrong conversion factor: 7.48052 is for US gallons. If you need imperial gallons (used in the UK), the factor is 6.22884. Always check which gallon you’re using.
• Measuring the outside of the container: For accurate volume, measure the internal dimensions. Wall thickness can add a few inches and throw off your calculation.
• Forgetting to halve the diameter: In a cylindrical container, radius is half the diameter. Using the full diameter in the formula will quadruple the volume.
• Rounding too early: If you round intermediate results, your final answer can be off by several gallons. Keep at least 4 decimal places until the end.

Tips

• Use a stiff tape measure for large containers – a flexible cloth tape can sag and give inaccurate readings.
• Measure depth at several points and average them if the bottom is uneven.
• If you’re measuring a pool with a sloped bottom, divide it into sections and calculate each separately, then add them.
• For a quick estimate, remember that 1 cubic foot ≈ 7.5 gallons. This is close enough for many practical purposes.
• Always double-check your unit conversions. Write down each step to catch errors.

Troubleshooting

Result seems too high or too low. Re-check your measurements. Make sure length and width are in feet, depth in inches. If you measured in centimeters, convert: 1 inch = 2.54 cm. If you measured in meters, convert to feet (1 m = 3.28084 ft).

Calculator shows an error. Ensure all inputs are positive numbers. If you’re using the cylindrical formula, radius must be positive. If you see a negative result, you likely entered a negative depth – check your numbers.

You get a different result using an online calculator. Some calculators use imperial gallons, others use US. Confirm which one you’re using. Also, some round intermediate steps – our calculator does not.

Your container is not a perfect rectangle or cylinder. For irregular shapes, use the water displacement method: fill a known volume and measure the depth change, or use a flow meter. Alternatively, approximate with a trapezoid or ellipse formula.

Frequently Asked Questions

How many gallons are in a cubic foot?

There are 7.48052 US gallons in one cubic foot. For imperial gallons, it’s 6.22884.

Can I use this formula for any container?

No, this formula works for rectangular and cylindrical containers. For other shapes, you need a different volume formula. For example, a sphere uses (4/3)πr³.

What if I only know the diameter in inches?

Convert the diameter to feet by dividing by 12, then halve it to get the radius. Then use the cylindrical formula: π × (radius in ft)² × depth in ft × 7.48052.

How do I convert gallons to liters?

One US gallon equals 3.78541 liters. Multiply your gallons by that number.

Why is the conversion factor 7.48052?

It comes from the definition of a gallon: 1 US gallon = 231 cubic inches. Since 1 cubic foot = 1728 cubic inches, 1728/231 = 7.48052.

Is there a difference between US and imperial gallons?

Yes. A US gallon is about 3.785 liters, while an imperial gallon is about 4.546 liters. The conversion factor changes accordingly.

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in l | Length | ft | 4 | 0 | 1000
in w | Width | ft | 2 | 0 | 1000
in h | Depth (inches) | in | 6 | 0 | 120
out gal | Gallons | gal | (l * w * (h / 12)) * 7.48052 | 1
out ft3 | Cubic Feet | ft³ | l * w * (h / 12) | 2