Cylinder Volume Calculator (Gallons)
Enter the diameter (or radius) and height (or length) of your cylinder to get the full capacity in US gallons, Imperial gallons, and liters. Switch between vertical and horizontal tank orientation, add a fill depth for partial-fill calculations, and choose your preferred dimension units. Results update instantly as you type.
Formula
Worked example
A vertical cylinder with a 24-inch diameter and 36-inch height: radius = 12 in. Cross-section area = pi * 12^2 = 452.39 in^2. Volume = 452.39 * 36 = 16,286 in^3. Divide by 231 to get 70.5 US gallons.
How to calculate cylinder volume in gallons
The volume of a cylinder is found with the formula V = pi x r^2 x h, where r is the radius (half the diameter) and h is the height. This gives a result in cubic units. To convert cubic inches to US gallons, divide by 231 (there are exactly 231 cubic inches in one US liquid gallon). For cubic feet, multiply by 7.481. Imperial (UK) gallons are slightly larger: one UK gallon equals 277.42 cubic inches, so divide the cubic-inch volume by 277.42 instead. For liters, multiply cubic inches by 0.016387.
Vertical vs. horizontal cylinders
A vertical cylinder stands upright and fills from the bottom. Calculating partial fill is straightforward: the filled volume is simply pi x r^2 x fill-height, which is a direct proportion of the total capacity. A horizontal cylinder lies on its side, so the cross-section seen from the end is a circle and liquid fills a circular segment. At 50% fill depth the segment is a half-circle and exactly half the capacity is filled, but the relationship is not linear: the tank fills relatively slowly near the bottom and top, and fastest near the midpoint. This calculator uses the exact circular segment formula for horizontal partial fills.
US gallons vs. Imperial gallons
The US liquid gallon and the Imperial (UK) gallon are not the same. One US gallon equals 231 cubic inches or about 3.785 liters. One Imperial gallon equals approximately 277.42 cubic inches or about 4.546 liters. This means an Imperial gallon is about 20% larger than a US gallon. Fuel economy figures, tank labeling, and container sizes differ between countries, so always confirm which gallon definition applies to your context. This calculator shows both.
Practical uses: tank capacity and weight of water
Knowing a tank volume in gallons helps with plumbing sizing, irrigation planning, aquarium stocking, chemical mixing, fuel storage, and many other tasks. Once you have the gallon figure you can quickly estimate the weight of water: one US gallon of fresh water weighs approximately 8.34 lb (3.78 kg) at room temperature. For salt water, use about 8.56 lb/gal. For diesel fuel, use roughly 7.1 lb/gal. For propane (liquid), use about 4.2 lb/gal. Multiply the full-capacity gallons by the appropriate density to get the total weight when full.
Common cylinder size reference
| Description | Diameter | Height / Length | US Gallons |
|---|---|---|---|
| Pint mason jar | 3.5 in | 5.5 in | 0.3 |
| 1-gallon jug | 6 in | 11 in | 1.0 |
| 5-gallon bucket | 11 in | 15 in | 5.0 |
| Hot water heater (40 gal) | 18 in | 52 in | 40 |
| 55-gallon drum | 23 in | 35 in | 55 |
| 100-gallon stock tank | 36 in | 23 in | 100 |
| 500-gallon propane tank | 37 in | 120 in | 500 |
| 1,000-gallon septic tank | 60 in | 96 in | 1,177 |
Approximate capacities for standard cylindrical tanks and containers.
Frequently asked questions
How many gallons are in a cylinder?
Calculate the volume in cubic inches using V = pi x r^2 x h (r = radius, h = height), then divide by 231 to get US gallons. For example, a 24-inch-diameter, 36-inch-tall cylinder has a volume of pi x 12^2 x 36 = 16,286 cubic inches, which is 16,286 / 231 = about 70.5 US gallons.
What is the formula to convert cubic inches to gallons?
Divide the cubic-inch volume by 231 to get US liquid gallons. Divide by 277.42 for Imperial (UK) gallons. Multiply by 0.016387 for liters.
How do I calculate how many gallons are in a horizontal tank at a given fill depth?
For a horizontal cylinder you need the circular segment formula. The filled cross-section area is r^2 x arccos((r - d) / r) - (r - d) x sqrt(2rd - d^2), where r is the radius and d is the fill depth. Multiply that area by the tank length to get the filled volume in cubic inches, then divide by 231 for US gallons. This calculator handles that automatically when you enable partial fill for a horizontal tank.
Is a UK gallon the same as a US gallon?
No. A US liquid gallon is 231 cubic inches (about 3.785 liters). An Imperial (UK) gallon is 277.42 cubic inches (about 4.546 liters). The Imperial gallon is roughly 20% larger. Always check which gallon your tank label or specification uses.
How much does a full cylinder tank weigh?
Multiply the capacity in US gallons by the liquid density. Fresh water weighs about 8.34 lb/gal, salt water about 8.56 lb/gal, diesel about 7.1 lb/gal, and gasoline about 6.1 lb/gal. Do not forget to add the empty tank weight when calculating structural loads.
What is a circular segment and why does it matter for horizontal tanks?
A circular segment is the region between a chord and the arc it cuts from a circle. When a horizontal cylindrical tank is partially filled, the surface of the liquid forms a chord across the circular cross-section, and the filled portion is a segment. Because the relationship between fill depth and volume is nonlinear, a simple proportion of total capacity underestimates volume near the bottom and top of a horizontal tank. The exact formula is needed for accurate readings.