Geometry & Shapes
Gallons in a Cylinder: US and Imperial Capacity
Calculate cylinder capacity from its internal dimensions, convert cubic inches or litres into gallons, and state which gallon definition is used.
Calculate a volume before converting units
Gallons measure volume, so first find the cylinder’s capacity from V = πr²h. For an upright cylindrical interior 20 inches in diameter and 30 inches high, radius is 10 inches. Volume is π × 10² × 30 = 3,000π in³, approximately 9,424.778 in³. The unit is cubic inches because all three length factors are in inches. This ideal capacity excludes wall thickness because the starting dimensions were internal.
Convert to US liquid gallons
One US liquid gallon equals exactly 231 cubic inches. The example therefore contains 3,000π/231, approximately 40.7999 US gallons. Divide the cubic-inch volume by 231; multiplying would move in the wrong direction. The result should be much smaller numerically because a gallon contains many cubic inches. Do not use a fluid-ounce factor or a length conversion in this step, since those would refer to a different unit relationship.
State an Imperial gallon result separately
One Imperial gallon equals exactly 4.54609 litres, whereas one US liquid gallon equals 3.785411784 litres. The same physical volume therefore has different numerical gallon values. For a cylinder holding exactly 100 litres, the results are about 26.4172 US gallons or 21.9969 Imperial gallons. A label saying only “gallons” leaves this ambiguity unresolved. The site’s general gallon converter uses the US liquid definition, so an Imperial result requires the separately stated factor rather than assuming a hidden regional setting.
Match the fill geometry
For the upright example, a 15-inch liquid depth is half the full height and therefore half the full volume, approximately 20.4000 US gallons. A partly filled horizontal cylinder does not have volume proportional to liquid depth except at special symmetric points. It needs a circular-segment method, and a domed end changes the relationship again. Record the cylinder’s orientation, internal dimensions, selected gallon definition and fill level so that another calculation can use the same physical model. If dimensions were measured in centimetres instead, convert cubic centimetres to litres before applying the appropriate litres-per-gallon factor; do not divide by 231.
Choose the solid and the measured dimensions
A three-dimensional measurement needs a geometric model before a formula can be selected. A circular cylinder needs radius and perpendicular height; a rectangular box needs length, width and height; a right circular cone needs radius and vertical height. The same word “height” can refer to different segments in a sketch, so label it explicitly. A cone’s slant height runs along its surface and cannot replace its perpendicular height in the volume formula. Similarly, a supplied diameter must be halved before entering a radius field.
Keep square and cubic units distinct
Surface area uses square units because it measures a boundary surface, while volume uses cubic units because it measures three-dimensional space. Convert all dimensions to the same length unit before multiplying. One cubic metre equals 1,000 litres, and one cubic centimetre equals one millilitre. A scale factor k applied to every length multiplies area by k² and volume by k³. This scaling check is particularly useful for catching a diameter entered as radius or a centimetre value entered alongside dimensions measured in metres.
State the limits of an ideal-shape calculation
A calculator evaluates the dimensions supplied; it does not inspect a container, survey a pile or determine the thickness of a wall. Use internal dimensions for capacity and the relevant external dimensions for an outside envelope. Keep any cut-outs, fittings, headspace or additional material allowances separate from the ideal geometric result. Retain π and unrounded intermediate values until the final step. When dimensions are approximate, report the result with suitable precision and describe the chosen shape model instead of implying that a long decimal proves an exact physical measurement.
Calculator for the supported step
Calculate cylindrical volume first using radius and height in one unit. Convert the resulting cubic-unit volume to the chosen gallon definition separately; the cylinder fields are not gallon inputs.
Sources and method
Related guides
Compare cube and cylinder volume and surface formulas, match equal-volume shapes correctly, and avoid confusing radius with a cube edge.
Calculate cylinder volume with the correct radius, invert for height, and explain why a partly filled horizontal cylinder needs another method.
Estimate rectangular or cylindrical container capacity, convert cubic units to litres, and account for internal dimensions, headspace and partial filling.