Tank Gallons Calculator: How to Figure Volume

This calculator works out the volume of a rectangular or square tank in US gallons from its length, width, and height (or depth). You enter the three dimensions in feet or inches, and it returns the gallon capacity instantly, plus the volume in cubic feet for reference. It is meant for fish tanks, fuel tanks, water storage, and similar rectangular containers—not cylindrical tanks, which need a different formula.

Formula

The volume of a rectangular tank is simply the product of its three internal dimensions:

V (ft³) = L × W × H

Then convert cubic feet to US gallons:

Gallons = V × 7.48052

Where:

• L = internal length of the tank in feet (ft)
• W = internal width in feet (ft)
• H = internal height (or water depth) in feet (ft)
• 7.48052 = number of US gallons in one cubic foot

If you measure in inches, first convert to feet by dividing each dimension by 12, or use the formula: Gallons = (L × W × H in inches) ÷ 231, since one US gallon equals 231 cubic inches.

Worked Example

Using the default values: L = 4 ft, W = 2 ft, H = 1.5 ft.

1. Calculate cubic feet: 4 × 2 × 1.5 = 12 ft³
2. Convert to gallons: 12 × 7.48052 = 89.76624 gallons
3. Rounded to one decimal: 89.8 US gallons

The calculator shows 89.8 gallons and 12.00 ft³, matching the steps above.

When to Use This Calculator

Use it whenever you need the capacity of a rectangular tank or container: aquarium sizing, water storage tanks, fuel tanks, sump basins, or even a shipping box if you need to know how much liquid it can hold. The result tells you the maximum volume when filled to the brim. For real-world water or fuel storage, leave a few inches of air space at the top for expansion and to prevent overflow.

If your tank is cylindrical (like many propane or oil tanks), this calculator will not work—use a cylinder volume formula instead (π × r² × h).

Common Mistakes

• Mixing units: Entering length in feet but width in inches gives a wildly wrong answer. Always use the same unit for all three dimensions.
• Using outside dimensions: Tank walls take up space. For accurate capacity, measure the inside dimensions, not the outside.
• Forgetting to subtract internal fittings: In a fish tank, gravel, decorations, and equipment displace water. The calculator gives total volume, not the actual water volume.
• Confusing US and UK gallons: This calculator uses US gallons (7.48052 per ft³). Imperial gallons are larger—one imperial gallon is about 1.2 US gallons.
• Ignoring the meniscus: Water curves at the surface; filling to the very top is rarely practical.

Tips

• Measure in inches for small tanks, then divide each by 12 before entering, or use the cubic-inch formula (÷231) for a more precise result.
• For a partially filled tank, enter the water depth instead of the full height to get the current volume.
• If your tank has a sloped bottom or irregular shape, break it into rectangular sections, calculate each, and add them together.
• Round up when ordering materials like liner or paint, but round down when determining safe water volume for fish—overcrowding is harmful.
• For aquariums, a common rule is 1 inch of fish per gallon, but that is a rough guide; research your specific species.

Troubleshooting

Result seems too large or too small: Recheck your units. If you entered inches as feet, the volume will be 1/1728 of the correct value. Convert everything to feet first.

Calculator shows an error or negative number: Ensure all dimensions are positive numbers. The calculator rejects zero or negative inputs—if you see an error, check that you have not left a field empty or typed a minus sign.

You need liters instead of gallons: Multiply the gallon result by 3.78541 to get liters, or by 4.54609 for imperial gallons.

Tank is not a perfect rectangle: This calculator assumes straight sides and a flat bottom. If your tank is tapered or has rounded corners, the actual volume will be slightly less. For critical applications, measure water added with a known container to verify.

Frequently Asked Questions

Q: How do I calculate gallons in a cylindrical tank?
A: Use the formula V = π × r² × h, where r is the radius and h is the height, both in feet. Then multiply by 7.48052 to get US gallons. For example, a tank 2 ft in diameter (r=1) and 3 ft tall holds π × 1² × 3 × 7.48052 ≈ 70.5 gallons.

Q: What is the difference between US and imperial gallons?
A: A US gallon is 231 cubic inches (about 3.785 liters). An imperial gallon is 277.42 cubic inches (about 4.546 liters). This calculator uses US gallons. To convert US to imperial, multiply by 0.83267.

Q: How many gallons does a 4×2×2 ft tank hold?
A: 4 × 2 × 2 = 16 ft³, times 7.48052 = 119.7 US gallons. That is a common large aquarium size.

Q: Can I use this for a fuel tank in my truck?
A: Yes, if the tank is rectangular and you can measure its interior dimensions. Keep in mind that fuel tanks often have irregular shapes and internal baffles, so the actual capacity may differ. Check the manufacturer’s spec when possible.

Q: How do I measure a tank that is already full?
A: You cannot measure internal dimensions directly. Instead, measure the outside dimensions and subtract wall thickness (usually 0.25–0.5 inches for plastic, more for concrete). Alternatively, use a flow meter or a dipstick with known volume markings.

Q: What if my tank has a tapered bottom?
A: Treat it as a trapezoidal prism. Calculate the average width (top width + bottom width) ÷ 2, then use that as the width in the formula. For example, if the bottom is 1 ft wide and the top is 2 ft, use 1.5 ft.

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in l | Tank Length | ft | 4 | 0 | 100
in w | Tank Width | ft | 2 | 0 | 100
in h | Tank Height | ft | 1.5 | 0 | 100
out gal | Volume | US gallons | l * w * h * 7.48052 | 1
out ft3 | Volume | ft³ | l * w * h | 2