Estimate the liquid volume of a wooden or metal barrel using its bulge diameter, head diameter, and height.
A barrel bulges in the middle (the bung diameter) and narrows toward each end (the head diameter). The classic "Great Formula" used by coopers and brewers approximates barrel volume by treating it as a shape between a cylinder and two cones, based on those two diameters and the barrel's height.
Volume (in³) = (π × h ÷ 12) × (2D² + d²)
where D = bung diameter, d = head diameter, h = height, all in inches.
Gallons = Volume (in³) ÷ 231 | Liters = Volume (in³) × 0.0163871
Measure the barrel's widest diameter at the bung (middle), its diameter at each flat head (end), and its overall height or length, all in inches. Enter these three measurements to get the estimated internal volume in cubic inches, US gallons, and liters.
A barrel with a 24-inch bung diameter, 20-inch head diameter, and 36-inch height computes to about 33,536 cubic inches — roughly 145 US gallons, or about 550 liters.
For a standard barrel shape (bulging in the middle), it uses a formula based on the height and the radii at the ends and the middle bulge to estimate internal volume.
Standard sizes vary by industry — for example, a US oil barrel is 42 US gallons, while wine and whiskey barrels commonly range from 53 to 59 US gallons.
Yes, the same bulging-cylinder formula applies to casks, which share the same general barrel shape.