How this instrument works
Most cylinder-volume questions want an answer in cubic inches or cubic metres, but a drum, a fuel tank, or a rain barrel is bought, filled, and sold in gallons — a unit of liquid capacity, not a cube of length. This sheet runs the identical πr²h formula every cylinder obeys, then divides the result by 231, the exact count of cubic inches packed into one US liquid gallon, so Volume reports the figure a supplier's invoice actually uses instead of a number you would have to convert by hand.
That constant, 231, is not arbitrary. One often-repeated account traces it to the old English wine gallon, long described as the capacity of a cylindrical vessel roughly 7 inches across and 6 inches deep — a shape whose own volume works out to about 230.9 cubic inches. Law eventually rounded that figure to an exact 231, so the unit sitting on the far side of this conversion began life as a cylinder's volume too, a small loop worth noticing.
Set either length to zero and the drum holds nothing: a zero radius leaves no circular face to extrude, a zero height leaves a disk with no depth, and both collapse the formula to zero gallons cleanly. Scale a container up instead — doubling both radius and height at once — and capacity does not double. It multiplies by eight, because radius is squared and height is not, a fact that surprises anyone who assumes a drum twice the size holds twice as much.
- Enter the container's radius into the Radius field — inches is the default, and the unit menu beside it switches to feet or centimetres if that matches your tape measure.
- Enter its height into the Height field, using whichever unit you actually measured, matched to the same container.
- Read the result in the Volume field — it reports directly in US gallons, with litres and cubic feet one click away in the same unit menu.
- Adjust either length and Volume updates immediately, so a few candidate drum or tank sizes can be checked before ordering one.
Worked example — a 6-inch radius, 12-inch drum
Take a steel drum with a 6-inch radius and a 12-inch height — set Radius to 6 and Height to 12, both in inches. Squaring the radius gives r² = 36, and extruding that circular base straight up through 12 inches of height gives a cubic-inch volume of V = πr²h = π × 36 × 12 = 432π, about 1357.168 cubic inches before any unit conversion happens.
Divide by 231, the exact cubic inches in one US gallon, and 432π ⁄ 231 ≈ 5.875186 gallons — call it 5.875 gallons on a purchase order, which is exactly what the Volume field shows for these two inputs. Double every dimension instead, to a 12-inch radius and 24-inch height, and the answer does not simply double: it scales by 2³ = 8, so that larger drum holds about 47.0 gallons, eight times as much from a container that only looks twice the size.
Questions
How do I convert a cylinder's volume to US gallons?
Compute the cubic-inch volume with V = πr²h using the radius and height in inches, then divide by 231, the exact cubic inches in one US liquid gallon. For a 6-inch radius and 12-inch height that is 432π ⁄ 231 ≈ 5.875 gallons, the figure this sheet returns directly without a separate conversion step of your own.
Why is a US gallon defined as 231 cubic inches?
The number traces to the old English wine gallon, long described as the capacity of a cylindrical vessel about 7 inches across and 6 inches deep — a shape whose own volume works out to roughly 230.9 cubic inches. Law later rounded that figure to an exact 231, so the unit on the far side of this conversion began as a cylinder's volume itself.
Is a US gallon the same size as an imperial gallon?
No, and confusing the two is the most common error when reading a drum's capacity off a spec sheet. A US liquid gallon is 231 cubic inches, about 3.785 litres; an imperial gallon used in the UK and Canada is larger, about 277.4 cubic inches, or 4.546 litres. This sheet always returns US gallons — roughly 17 percent fewer than the imperial count for an identical container.
How does this differ from a plain cylinder volume calculator?
A plain cylinder volume figure comes out in cubic units — cubic inches, cubic feet, cubic metres — which is exact but not how anyone orders diesel, treatment chemicals, or paint. This sheet applies the identical πr²h formula and then divides by 231 automatically, so Volume reads straight in US gallons, the unit a drum, tank, or barrel's capacity is actually quoted in.
Does doubling a drum's radius and height double its gallon capacity?
No, it multiplies capacity by eight. Radius is squared while height enters the formula only once, so scaling both dimensions by the same factor scales volume by that factor cubed. A 12-inch radius, 24-inch height drum — double a 6-by-12 drum in every direction — holds about 47.0 gallons, eight times the smaller drum's 5.875 gallons, not twice.
What does the calculator return for a radius or height of zero?
Exactly zero gallons either way. A zero radius leaves no circular cross-section to extrude, and a zero height leaves a flat disk with no depth to fill, so both collapse the formula to nothing — the correct answer at that edge, not an error condition.