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Grain Mass from Volume

Also known as bushels to tonnes · test weight conversion · grain bulk density · tonnes in a bin

m=ρ Vm = \rho \, V

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See also
Round Bin Capacity — Where the volume comes from.Grain Cone Volume — The cone on top, which needs the same density applied to it.Auger or Conveyor Transfer Time — The mass this produces is what the auger has to move.

Learning zone

Bulk density turns a measured volume into a saleable quantity, and grain's bulk density is the quantity the trade calls test weight. It is not the density of a kernel; it is the density of a heap of kernels, air gaps included, and it varies with the crop, the variety, the moisture, the dockage and how the sample was poured.

The legal test weights are where the familiar numbers come from. Wheat and soybeans are 60 lb per bushel, corn and rye 56, barley 48, and oats 32. Dividing each by the bushel's volume gives bulk densities of roughly 772 kg/m³ for wheat and soybeans, 721 for corn, 618 for barley and 412 for oats, with canola near 670. The spread is enormous — oats occupy nearly twice the bin space of wheat for the same mass — and it is the reason a single "bushels to tonnes" factor cannot exist. A bin that holds 4,000 bushels of wheat holds 4,000 bushels of oats too, and half the tonnage.

The classic mistake is a units confusion hiding inside a word. A bushel in a grain contract is a MASS: 56 pounds of corn is a bushel of corn whether it occupies 1.24 ft³ or, being light and badly filled, rather more. The bushel as a volume survives only in bin capacity work, which is exactly where this calculation sits. So a bin computed at 4,090 volumetric bushels of corn holds 4,090 contract bushels only if the grain makes the standard test weight, and light corn in a poor year fills the same bin with fewer saleable bushels than the geometry promises.

Two further effects are worth carrying. Moisture cuts both ways: wetter grain is heavier per kernel but packs less well, and over the normal range higher moisture usually LOWERS test weight, which is why drying often raises it. And test weight is itself a grading factor, not merely a conversion — it correlates with kernel plumpness and soundness, so a low figure both reduces the tonnage in the bin and attracts a discount on what comes out of it. Measuring it on the actual lot, rather than assuming the standard, is the difference between an inventory and a guess.

Grain Mass from Volume
m=ρ Vm = \rho \, V
Vρm
Where
  • mm= Mass of grain (t)
  • ρ\rho= Bulk density (kg/m³)
  • VV= Volume (m³)
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