Round Tank Volume Calculator: Gallons & Liters

This calculator works out the volume of a vertical or horizontal cylindrical tank from its diameter and length, giving you the result in gallons, liters, and cubic feet. You enter the tank’s inside diameter and its straight-side length (or height), and it returns the total volume, plus the volume per inch of depth for vertical tanks. It uses the standard cylinder formula and assumes the tank is a perfect cylinder with flat ends, so it is best for simple round tanks, not those with domed or dished heads.

Formula

The volume of a round tank is the same as the volume of a cylinder:

V = π × r² × L

• V = volume (cubic feet, gallons, or liters)
• π ≈ 3.14159
• r = inside radius of the tank = diameter ÷ 2 (feet or meters)
• L = length of the tank’s straight side, or its height for a vertical tank (same unit as diameter)

To convert cubic feet to US gallons, multiply by 7.48052. To convert cubic feet to liters, multiply by 28.3168. If you measure in meters, 1 m³ = 1,000 liters and 1 m³ ≈ 264.172 US gallons.

Worked Example

Using the default values from the calculator: diameter = 4 ft, length = 10 ft.

1. Radius = 4 ÷ 2 = 2 ft
2. Radius squared = 2² = 4 ft²
3. Area of the circular end = π × 4 ≈ 12.566 ft²
4. Volume = area × length = 12.566 × 10 = 125.66 ft³
5. In US gallons: 125.66 × 7.48052 ≈ 940.0 gal
6. In liters: 125.66 × 28.3168 ≈ 3,558 L

The calculator shows 125.66 ft³, 940.0 gal, and 3,558 L for these inputs.

How to Use the Round Tank Volume Calculator

Measure the inside diameter of the tank, not the outside, because wall thickness reduces the usable volume. For a vertical tank, measure the height from the inside bottom to the inside top. For a horizontal tank, measure the straight-side length from weld seam to weld seam, excluding any domed or dished ends.

Enter both measurements in the same unit (feet, meters, inches, etc.). The calculator assumes you enter feet, so if you measure in inches, convert to feet first (divide by 12) or use a unit-aware tool.

The result is the total volume when the tank is full to the brim. In practice, you never fill a tank completely—leave headroom for expansion, especially for liquids that expand with temperature.

Common Mistakes and How to Avoid Them

• Using outside dimensions – Always use the inside diameter and length. A ¼-inch wall on a 4-ft tank reduces volume by roughly 2–3%.
• Mixing units – If diameter is in feet and length is in inches, the result is wrong. Convert everything to one unit first.
• Forgetting the ends – Many real tanks have elliptical or hemispherical heads. This calculator only covers the cylindrical section. Add the head volume separately if your tank has them.
• Confusing radius and diameter – The formula uses radius, not diameter. A common error is plugging in the diameter as the radius, which overestimates volume by 4×.
• Rounding too early – If you round π to 3.14, you lose about 0.05% accuracy. Use 3.14159 or the calculator’s built-in π.

When to Use This Calculator vs. Other Tank Shapes

This calculator is for cylindrical tanks—vertical or horizontal. It works for:

• Water storage tanks
• Fuel and oil tanks
• Chemical storage drums
• Propane tanks (but those have curved ends, so add head volume)

If your tank is rectangular, use a length × width × height formula. If it’s spherical, use V = (4/3)πr³. For horizontal tanks with dished heads, you need a more complex formula that accounts for the head shape and the liquid level.

Safety and Practical Notes

Always leave at least 5–10% empty space at the top of a liquid tank to allow for thermal expansion. For pressurized vessels, never fill beyond the manufacturer’s rated capacity. When measuring a tank that has contained fuel or chemicals, follow proper safety procedures: vent the tank, test the atmosphere, and use non-sparking tools. The volume calculated is the geometric volume; the actual usable volume may be less due to internal fittings, baffles, or sediment.

For horizontal tanks, the volume at partial fill is not linear—a tank half full by height holds about 50% of its volume, but at 25% height it holds far less than 25% of the volume. Use a dedicated partial-fill calculator for that.

Frequently Asked Questions

How do I calculate the volume of a round tank in gallons?

Measure the inside diameter and length (or height) in feet. Use the formula V = π × (d/2)² × L to get cubic feet, then multiply by 7.48052 to get US gallons. For example, a 4-ft diameter, 10-ft long tank holds about 940 gallons.

What is the difference between a vertical and horizontal round tank?

The formula is identical; you just measure the length differently. For a vertical tank, the length is the height. For a horizontal tank, it’s the straight-side length. The orientation only affects how you measure, not the volume.

Does the calculator include the volume of the tank heads (ends)?

No. It only calculates the cylindrical section. If your tank has elliptical or hemispherical heads, you must add their volume separately. For a hemispherical head, each head adds (2/3)πr³. For an elliptical head, the volume is roughly (π/6) × d² × head depth.

Why is my tank’s actual capacity less than the calculated volume?

Several reasons: wall thickness reduces inside dimensions, internal fittings (baffles, dip tubes) take up space, and you can’t fill to the absolute top. Also, if the tank is not a perfect cylinder (e.g., has a slight taper), the formula overestimates.

Can I use this for a propane tank?

You can get the cylindrical portion, but propane tanks have curved (usually elliptical) ends. The total volume will be somewhat higher than the cylinder-only calculation. For safety-critical applications like propane, always refer to the tank’s stamped water capacity.

How do I convert cubic feet to liters?

Multiply the cubic feet by 28.3168. For example, 125.66 ft³ × 28.3168 = 3,558 liters. If you measured in meters, 1 m³ = 1,000 liters.

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in d | Tank diameter | ft | 4 | 0
in L | Tank length/height | ft | 10 | 0
out vol_ft | Volume (cubic ft) | ft³ | pi * (d/2)^2 * L | 2
out vol_gal | Volume (US gallons) | gal | pi * (d/2)^2 * L * 7.48052 | 1
out vol_l | Volume (liters) | L | pi * (d/2)^2 * L * 28.3168 | 1