Circle Tank Volume Calculator: Gallons & Liters

This circle tank volume calculator works out the capacity of a cylindrical water tank (a circle viewed from above) in gallons, liters, and cubic feet. You enter the tank diameter and height (or water depth), and it returns the total volume and the volume for a given fill level. The calculation uses the standard cylinder formula, so it works for any upright or horizontal round tank as long as you measure the inside dimensions.

Formula

The volume of a cylinder is the area of the circular base times the height (or depth) of the water. In plain terms:

Volume = π × (radius)² × height

Because you usually measure the diameter, not the radius, the formula becomes:

V = (π/4) × d² × h

Where:

• V = volume in cubic feet (or any cubic unit)
• d = inside diameter of the tank, in feet
• h = depth of water (or height of tank), in feet
• Ï€ ≈ 3.14159

To convert cubic feet to US gallons, multiply by 7.48052. To convert to liters, multiply by 28.3168. If you measure in inches, convert to feet first by dividing by 12.

Worked Example

Using the default values from the calculator: diameter = 4 ft, water depth = 2 ft.

1. Square the diameter: 4 × 4 = 16 ft²
2. Multiply by π/4: 16 × 0.7854 = 12.566 ft² (this is the circular area)
3. Multiply by the depth: 12.566 × 2 = 25.13 ft³
4. Convert to US gallons: 25.13 × 7.48052 = 188.0 gallons
5. Convert to liters: 25.13 × 28.3168 = 711.6 liters

The calculator shows 25.13 ft³, 188.0 gallons, and 711.6 liters – exactly matching these steps.

When to Use This Calculator

This tool is for cylindrical tanks that sit upright (like most vertical water tanks, barrels, or cisterns). It is also correct for horizontal cylindrical tanks when the water depth is the full diameter – but if the tank is horizontal and partially filled, the volume is not a simple cylinder; it requires a segment-area calculation. Use this calculator when:

• You want the total capacity of a round vertical tank.
• You need to know how much water is in a partially filled round tank (using the water depth as h).
• You are sizing a rainwater barrel, fish tank, or chemical storage tank.

For a horizontal tank with a partial fill, you need a different formula that accounts for the circular segment.

How to Measure Correctly

Accuracy depends on measuring the inside dimensions, not the outside. For a tank with thick walls, outside measurements overestimate volume. Use these tips:

• Measure the inside diameter across the widest point of the opening or the bottom.
• Measure the water depth from the bottom to the water surface – not the tank height if it isn’t full.
• Use the same unit for both dimensions. If you measure in inches, convert to feet (divide by 12) before entering.
• For a tank with a domed or dished bottom, the cylinder formula only covers the straight-walled section; add the dome volume separately if needed.

Common Mistakes and Misconceptions

One frequent error is using the radius in the formula but entering the diameter in the calculator – or vice versa. The calculator asks for diameter, so do not halve it. Another mistake is mixing units: entering diameter in feet and depth in inches gives a wildly wrong answer. Always convert to the same unit first.

A common misconception is that a 4-ft diameter, 2-ft deep tank holds 100 gallons – it actually holds about 188 gallons. People often underestimate because they think of a 55-gallon drum (which is about 23 inches in diameter and 35 inches tall).

Also, remember that the volume of water in a partially filled tank is linear with depth only if the tank has straight sides. For a horizontal cylinder, the relationship is not linear – at half depth you do not have half the volume.

Limits of the Formula

The cylinder formula assumes perfectly straight sides and a flat bottom. Real tanks often have rounded or cone-shaped bottoms, which add or subtract volume. For a standard vertical tank with a flat bottom, the formula is exact. For tanks with dished heads (common in pressure vessels), you need additional geometry. Also, the calculator gives the geometric volume; if you are storing a liquid, the actual usable volume is slightly less because you cannot fill to the absolute brim without spilling. For safety, leave a few inches of freeboard.

Units and Conversions

This calculator works in feet and gives cubic feet, US gallons, and liters. Here are common conversions for reference:

• 1 cubic foot = 7.48052 US gallons
• 1 cubic foot = 28.3168 liters
• 1 US gallon = 3.78541 liters
• 1 imperial gallon = 4.54609 liters (different from US gallon)

If you need imperial gallons, divide the liter value by 4.54609. The calculator uses US gallons by default, which is the standard for most water tanks in North America.

Frequently Asked Questions

How many gallons is a 4 ft diameter by 2 ft deep round tank?
Using the formula, the volume is about 188 US gallons. That comes from the area of the circle (12.566 ft²) times the depth (2 ft) times 7.48052 gallons per cubic foot.

Does this work for a horizontal tank?
Only if the tank is completely full. For a horizontal cylinder partially filled, the volume is a circular segment times the length, which is a different calculation. This calculator is for vertical tanks where the water depth is measured from the bottom.

What if I measure in inches?
Convert your inches to feet by dividing by 12 before entering. For example, a 48-inch diameter is 4 feet. If you enter inches directly, the result will be in cubic inches, not gallons, and the conversion factors would be wrong.

Why does the calculator ask for water depth, not tank height?
Because you often want the volume of water in a partially filled tank. If you want the total capacity, enter the full height of the tank. The formula is the same – the height variable becomes the water depth.

Is the volume in US or imperial gallons?
The calculator gives US gallons. Imperial gallons are about 20% larger, so if you need imperial, multiply the US gallons by 0.83267 or divide the liters by 4.546.

Can I use this for a fish tank?
Yes, as long as the tank is cylindrical. For a rectangular tank, use length × width × height. For a bow-front or irregular tank, you need a different approach. This calculator is specifically for round tanks.

Related

• Aquarium Volume Calculator: How Many Gallons
• Aquarium Volume Calculator for Cylinder Tanks
• Hexagon Aquarium Volume Calculator

in d | Tank diameter | ft | 4 | 0
in h | Water depth | ft | 2 | 0
out vol_ft3 | Volume (cubic ft) | ft³ | (pi/4)*d^2*h | 2
out vol_us | Volume (US gallons) | gal | vol_ft3 * 7.48052 | 1
out vol_l | Volume (liters) | L | vol_ft3 * 28.3168 | 1