How Many Gallons Is My Tank? A Practical Volume Calculator Guide
If you've measured a water tank, propane cylinder, planter, or shipping box and need to know how many gallons or liters it actually holds, this guide walks through the exact formulas — for cubes, cylinders, spheres, cones, pipes, and even a partially filled tank — plus the unit-conversion step people usually get wrong.
Quick answerThe volume of a cylinder is V = π × r² × h, where r is the radius of the circular base and h is the height (or length, for a tank lying on its side). A tank with a 9-inch radius and a 24-inch height has a volume of π × 9² × 24 ≈ 6,107.3 cubic inches.
Most water tanks, propane cylinders, drums, and pipe sections are cylinders, which makes this the single most useful volume formula for everyday measuring jobs. The trick is getting the radius right, not the diameter — measure across the tank and divide by two, or measure around the circumference and divide by 2π if a direct diameter measurement is awkward.
Example cylinder volumes (illustrative measurements, inches)
Radius
Height
Volume (in³)
US gallons
6 in
18 in
~2,036.0
~8.8
9 in
24 in
~6,107.3
~26.4
12 in
36 in
~16,286.0
~70.5
These are example figures for common tank dimensions, not a spec for any particular product — always confirm with the manufacturer's rated capacity for anything safety-related, like propane or chemical storage.
Converting cubic inches to gallons and liters
Quick answer1 US gallon = 231 cubic inches (about 3.785 liters). To convert, divide your cubic-inch result by 231. For cubic feet, multiply by about 7.48 to get US gallons; on the metric side, 1,000 cubic centimeters = 1 liter and 1 cubic meter = 1,000 liters.
This is the step most people either skip or get backwards — dividing when they should multiply, or mixing up US gallons with UK (imperial) gallons, which are about 20% larger. A 6,107 in³ result divided by 231 gives 26.4 US gallons; the same volume in UK gallons would be roughly 22 (since 1 UK gallon ≈ 277.4 in³). This matters most for imported tanks or recipes that specify "gallons" without saying which kind.
Why it's easy to get wrongCubic inches, cubic feet, cubic centimeters, and cubic meters are all "cubic" volume units, but they don't convert by simply moving a decimal point the way linear units do — a cubic unit conversion factor is the linear factor cubed. Going from inches to feet, for example, is not ÷12 but ÷1,728 (12³).
Why a partially filled tank isn't a simple percentage
Quick answerFor a rectangular tank, partial volume is simply length × width × fill depth — straightforward and linear. For a horizontal cylindrical tank, the wetted cross-section is a circular segment, calculated with Area = r² × acos((r−d)/r) − (r−d) × √(2rd−d²), then multiplied by the tank's length — and this relationship is NOT linear with depth.
In plain terms: a horizontal cylindrical propane tank filled to half its height (depth) is close to 50% by volume, but a tank filled to a quarter of its height holds noticeably less than a quarter of its volume, because the bottom of a circle is "narrower" in cross-section than the middle. This is a common source of confusion when reading a dipstick or sight gauge on a horizontal tank and trying to estimate gallons remaining by eye.
Example scenario: imagine a horizontal cylindrical tank with a 12-inch radius and a 40-inch length, filled to a 6-inch depth (exactly half the 24-inch diameter). Because the circular-segment formula isn't linear, that 6-inch fill depth works out to almost exactly 50% of volume in this particular case — but at a 3-inch depth (a quarter of the diameter), the filled volume is closer to 20% of total capacity, not 25%. A tool that runs the exact segment formula avoids this kind of estimation error.
Sphere, cone, pipe, and other tank shapes
Quick answerA sphere's volume is (4/3) × π × r³; a cone is (1/3) × π × r² × h (one-third of a cylinder with the same base and height); and a pipe (hollow cylinder) is π × h × (R² − r²), the outer cylinder's volume minus the inner hollow's volume.
Sphere — useful for round storage tanks and spherical propane/gas containers.
Cone / frustum — a frustum (truncated cone, with two different-sized bases) fits buckets, planters, and funnels better than a plain cone.
Pipe (hollow cylinder) — subtracts the inner radius from the outer radius before applying the cylinder formula, giving the actual material or liquid-carrying volume of tubing, hose, and pipe.
Capsule — a cylindrical body capped with two hemispheres (which together make one full sphere) — the shape of many horizontal propane tanks and pill-shaped storage silos.
231 in³ = 1 US gallon — worth memorizing if you regularly estimate tank capacities from a tape-measure reading, since it turns any cubic-inch volume into gallons with one division.
Calculate any shape's exact volume and gallonsPick from 13 shapes — cube, box, cylinder, sphere, cone, pipe, capsule, frustum, and more — enter your measurements in inches, feet, centimeters, or meters, and get volume, surface area, a step-by-step solution, and instant gallon/liter conversion.Try the free Volume Calculator →
Frequently asked questions
How do I convert cubic inches to gallons?
Divide the cubic inches by 231 — that's how many cubic inches make up 1 US gallon (about 3.785 liters). A cylindrical tank with a volume of 4,620 cubic inches, for example, holds 4,620 / 231 = 20 US gallons. For cubic feet, multiply by about 7.48 to get US gallons instead.
What is the formula for the volume of a cylindrical tank?
Volume = π × r² × h, where r is the radius of the circular base and h is the height (or length, for a horizontal tank). For example, a tank with a 9-inch radius and a 24-inch height has a volume of π × 9² × 24 ≈ 6,107.3 cubic inches, which is about 26.4 US gallons.
How do you calculate the volume of a partially filled tank?
For a rectangular tank, partial volume is simply length × width × fill depth. For a horizontal cylindrical tank filled to depth d, the wetted cross-section is a circular segment: Area = r² × acos((r−d)/r) − (r−d) × √(2rd−d²), then multiply by the tank's length. This is why a horizontal cylindrical tank filled halfway by depth is NOT filled to 50% of its volume by simple proportion — the circular cross-section makes the math nonlinear.
What is the difference between volume and surface area?
Volume is the amount of three-dimensional space a solid occupies, measured in cubic units (in³, ft³, cm³, m³, or liters/gallons). Surface area is the total area of all of the object's outer faces, measured in square units (in², ft², cm², m²). A tank's volume tells you how much it holds; its surface area tells you, for example, how much material or paint is needed to cover it.
How do you calculate the volume of an irregular or hollow shape like a pipe?
For a pipe or any hollow cylinder, subtract the volume of the inner hollow from the volume of the outer cylinder: V = π × h × (R² − r²), where R is the outer radius, r is the inner radius, and h is the pipe's length. This gives the actual material volume, which is useful for weight estimates, or the actual liquid capacity for tubing and hose.
Methodology note: The formulas in this guide are standard geometric volume and surface-area equations (cube, cylinder, sphere, cone, frustum, pipe, capsule, and the circular-segment formula for partial cylindrical fill). Example numbers are illustrative only, for common measurements, not specifications for any real product. For safety-critical capacities — propane, fuel, chemical storage — always confirm with the manufacturer's rated capacity rather than a self-measured estimate.