Cylinder Volume and Tank Capacity: From Dimensions to Gallons
October 21, 2026
The volume of a cylinder is π × radius² × height. A tank 4 feet in diameter (radius 2 ft) and 4 feet tall has a volume of π × 2² × 4 = 50.27 cubic feet. Since one cubic foot holds 7.48052 US gallons, that equals about 376 gallons, or about 1,423 liters.
Skip the math and get your answer instantly:
Open the Volume Calculator →Step by Step
First, find the radius, which is half the diameter. Square it, multiply by π (3.14159), then by the height. Keep every length in the same unit. If the dimensions are in inches, divide cubic inches by 231 to get US gallons, since one US gallon is exactly 231 cubic inches.
Example in inches: a barrel 22 inches across and 34 inches tall has a volume of π × 11² × 34 = 12,924 cubic inches, which is 12,924 ÷ 231 = 55.9 gallons, close to a standard 55-gallon drum.
| Unit conversion | Factor |
|---|---|
| 1 cubic foot | 7.48052 US gallons |
| 1 US gallon | 231 cubic inches or 3.78541 liters |
| 1 cubic foot | 28.3168 liters |
| 1 cubic meter | 1,000 liters or 264.17 US gallons |
Partially Filled Tanks
A vertical cylinder filled to a depth d holds π × r² × d. If the 4-foot tank is filled to 3 feet, it holds π × 2² × 3 = 37.7 cubic feet, or about 282 gallons, three quarters of capacity.
Horizontal tanks are trickier because the volume does not rise linearly with depth. A horizontal tank filled to half its diameter is half full by symmetry, but at a quarter of the diameter it holds only about 20% of its volume. Use a tank chart or formula for precise readings.
Practical Uses
The same method sizes rain barrels, water heaters, silos, culverts, and concrete columns. For a round concrete pier 12 inches across and 4 feet deep, volume is π × 0.5² × 4 = 3.14 cubic feet, about 0.116 cubic yards.
For a hollow pipe, subtract the inner cylinder from the outer one, or calculate the inner volume to find how much liquid it carries.
Frequently Asked Questions
What is the formula for the volume of a cylinder?
V = π × r² × h.
How many gallons are in a cubic foot?
7.48 US gallons.
How do I find the volume if I only know the diameter?
Halve the diameter to get the radius, then use the standard formula.
How many cubic inches are in a gallon?
231 cubic inches in a US gallon.