Grain Bin Calculator

Bin capacity is geometry plus a legal conversion. The cylindrical body holds pi times radius squared times eave height in cubic feet, and grain peaked into the roof adds a cone of one third pi times radius squared times cone height, where the cone height is the radius multiplied by the tangent of the roof pitch angle. Cubic feet become bushels by dividing by 1.2445, because the United States bushel is defined as 2,150.42 cubic inches. Weight then follows from the standard test weight for the crop: 56 pounds per bushel for shelled corn and sorghum, 60 for wheat and soybeans, 48 for barley and 32 for oats. Two practical checks belong with any result. First, the weight is enormous. A 24 foot diameter bin with an 18 foot eave holds close to 200 tons of corn, so foundation design and wall loading must come from an engineered specification rather than an estimate. Second, aeration is specified per bushel, commonly 0.1 to 0.25 CFM per bushel for holding and cooling and 1 CFM per bushel or more for natural-air drying, which means fan selection scales with capacity and with grain depth. The calculation also ignores compaction: grain packs under its own weight, and official capacity tables apply packing factors that add several percent to what a tall bin actually holds compared with the raw geometric volume.

The bushel behind this arithmetic is a British dry measure. The Winchester bushel, standardized in England and defined as 2,150.42 cubic inches, was carried into United States practice and kept after Britain adopted the larger imperial bushel in 1824, which is why US and imperial bushels differ to this day. Because grain was traded by volume but handled by weight, states and then the federal government fixed pounds-per-bushel equivalents for each crop, and the United States Grain Standards Act of 1916 created the national system of official grades, inspection and test weight determination that still supplies the 56 and 60 pound conversions. On-farm round steel bins are far more recent. Corrugated galvanized bins with perforated aeration floors spread across the Midwest after the Second World War, as combines replaced ear corn harvest, shelled grain required drying and controlled storage, and federal loan and reserve programs gave farmers a financial reason to hold crop on the farm instead of delivering everything at harvest. Agricultural engineers at land grant universities published the capacity tables, packing factors and airflow recommendations that turn bin dimensions into marketable bushels, and those extension tables remain the working reference for insurance valuation, grain inventory reporting and drying system design.