The volume of a cylinder in US gallons equals πr²h (in cubic inches) divided by 231, because one US liquid gallon is defined as exactly 231 cubic inches. To get this number from real-world measurements, you multiply π by the radius squared by the height, then divide by 231 when the inputs are in inches. If you measured in feet instead, multiply cubic feet by 7.4805 to land on gallons. If you measured in centimeters, divide cubic centimeters by 3785.41 to get US gallons. The free Cylinder Volume Calculator does the πr²h step for you the moment you stop typing, and you take the cubic output and apply the right conversion factor for your unit. That's the whole job in two short steps: find the cubic volume (in inches, feet, or centimeters), then convert to gallons using a fixed, defined constant. The rest of this guide walks through the formula, the conversions, a worked example, and the traps people fall into when sizing water tanks, fuel cylinders, drums, pipes, and pools.
Why Gallons Are the Common Unit for Cylinder Capacity
Almost every cylindrical container sold in the United States is labeled in gallons. A standard steel drum is a 55-gallon drum. A propane tank for a residential grill is rated around 20 pounds, which corresponds to about 4.7 gallons of liquid propane. A backyard fire pit propane tank runs closer to a gallon, and a paint can, an aquarium, and a pool filter all live in the same gallon universe. The reason is simple: gallons are the unit customers actually need to know, because the contents (water, fuel, paint, chlorine) are bought and dosed by the gallon. The underlying geometry does not change when the units do, but the conversion step is what most people forget. Sizing a container in gallons is really two problems stacked: a geometric problem (find the cubic volume) and a unit problem (translate cubic inches, feet, or meters into gallons). Doing them in the right order keeps mistakes out of the result.
The Formula: From πr²h to Gallons
The geometric core is one short formula. The volume of a cylinder is V = πr²h, where r is the radius of the circular base and h is the height measured along the axis. The intuition is direct: a cylinder is a circle swept straight up, so its volume is the area of that base circle (πr²) multiplied by how far the circle travels (h). Doubling the height doubles the volume, but doubling the radius quadruples it, because r is squared while h is not. That single asymmetry is the source of most estimation errors.
Once you have V in some cubic unit, the second step is unit conversion. There are three conversions worth memorizing because they cover nearly every practical case in the United States:
- Inches to gallons: divide cubic inches by 231, since 1 US liquid gallon = 231 in³ (defined exactly).
- Feet to gallons: multiply cubic feet by 7.48052, since 1 ft³ ≈ 7.48052 US gal.
- Centimeters to gallons: divide cubic centimeters by 3785.41, since 1 US gallon = 3785.411784 cm³.
Mix the units (radius in inches, height in feet, for example) and the cubic result is meaningless, because the radius and height must travel through the formula in the same unit. This is the single most common mistake, and it is why a tool that locks the two inputs to a single unit, or that prints the unit it used, makes the rest of the work easier.
How to Get the Answer in Gallons Using the Calculator
- Measure the radius (half the diameter) of the cylinder's base. If you only know the diameter, halve it first. Pick the unit you will use for both inputs and stick with it: inches, feet, centimeters, or meters are all fine.
- Open the Cylinder Volume Calculator and type the radius into the first field. Use that same unit for the next step.
- Measure the height of the cylinder along its central axis, then type that height into the second field using the same unit you used for the radius.
- Read the volume that appears below the fields the moment you stop typing. The result is in your unit cubed (in³, ft³, cm³, or m³), and the calculator also shows the base area, lateral (side) area, and total surface area alongside it.
- Convert the cubic result to US gallons using the matching factor from the table below: divide by 231 for cubic inches, multiply by 7.4805 for cubic feet, or divide by 3785.41 for cubic centimeters. For cubic meters, multiply by 264.17.
There is no submit button to press and no data is sent to a server, so the result is purely a function of the two numbers you typed. If the tool flags an entry, it is catching a bad value (such as a typo or a non-numeric input) so a misleading answer never appears; correct the input and the gallons follow.
Conversion Reference: Cubic Units to US Gallons
| Cubic unit you have | How to get US liquid gallons |
|---|---|
| 1 cubic inch (in³) | Divide by 231 (≈ 0.004329 gal) |
| 1 cubic foot (ft³) | Multiply by 7.48052 |
| 1 cubic meter (m³) | Multiply by 264.1721 |
| 1 cubic centimeter (cm³ / mL) | Divide by 3785.411784 |
| 1 liter (L) | Multiply by 0.2641721 |
These conversion factors are not approximations that drift with measurement precision; the 231-cubic-inch figure, in particular, is the legal definition of the US liquid gallon. The other figures are derived from it, so the chain is consistent across all the rows. If you ever need an imperial (UK) gallon instead, swap 231 for 277.419 and the rest of the math works the same way.
A Worked Example: A 30-Inch-Tall Tank
Suppose a cylindrical water tank has a base diameter of 24 inches and stands 30 inches tall. The radius is half the diameter, so r = 12 in. Plugging into the formula:
V = π × r² × h = π × 12² × 30 = π × 144 × 30 = π × 4320 ≈ 13,571.68 in³
Convert to gallons by dividing by 231:
13,571.68 ÷ 231 ≈ 58.75 US gallons
That single reading is enough to confirm the tank's marketed capacity. As a sanity check, note that a matching cone with the same 12-inch radius and 30-inch height would hold one third of that volume: about 19.58 gallons. If the actual tank reads anywhere near 19 gallons, you have a cone, not a cylinder.
Related Calculations That Come Up Often
Volume in gallons answers "how much fits inside," but the same cylinder is often sized for what covers the outside. The Cylinder Volume Calculator returns the base area (πr²), the lateral or side area (2πrh), and the total surface area (2πr² + 2πrh) alongside the volume, which covers paint coverage, sheet-metal layout, and insulation estimates in the same trip. For a quick visual of what that curved wall actually is, picture unrolling the side into a flat rectangle whose width is the base circumference 2πr and whose height is h; the area of that rectangle is exactly the lateral area.
The gallon relationship also pairs neatly with related shapes. A cone with the same base and height holds exactly one third of a cylinder's volume, which is why ice cream cones, traffic cones, and funnel-style hoppers all feel "small" for their footprint. If the question is the volume of a flat-ended tube (a hollow cylinder), subtract the inner πr²h from the outer πr²h to get the wall volume, and convert that to gallons only after the subtraction. For shipping a cube instead, the formula simplifies to V = s³, but the gallons conversion is identical once you have the cubic unit. These cross-checks are how experienced estimators catch a unit slip in seconds rather than after a tank overflow.
Practical Tips for Gallon Accuracy
Three habits keep the answer trustworthy. First, write the unit next to the number, every time; "12" sitting alone is the root cause of most mixed-unit errors. Second, round at the end, not in the middle; carry full precision through the πr²h step, then round the final gallon figure to the precision that actually matters for the job (a fuel-tank estimate can take ±2% rounding; a dosing calculation for a chemical cannot). Third, sanity-check with the cone rule: a cylinder is exactly three times the volume of a cone with the same r and h. If an estimate comes back wildly different from three times the matching cone's volume, a unit has been switched somewhere and it's worth re-measuring before pouring anything into the tank.