Grain Bin Capacity and Moisture: Volume, Weight and Shrink

A grain-bin measurement answers a space question: how much grain geometry is inside the bin. A moisture calculation answers a mass question: how much of a weighed lot remains after water is removed. The two checks belong in one harvest workflow, but they are not interchangeable. A measured bulk density can connect bin volume to an estimated mass; a standard commodity bushel weight cannot turn an unmeasured bin volume into a scale ticket.

Use the Grain Bin Capacity calculator to document geometry first. Use the Grain Moisture Shrink calculator only after you have an incoming mass and wet-basis moisture. Keep field grain that never entered the bin separate with the Harvest Loss Calculator.

For a round, flat-floor bin, measure the inside diameter and the grain depth at the sidewall. Describe the surface as level, a centred peak or a centred inverted cone. For a peak or depression, measure the vertical rise or drop from the sidewall grain level. A depression cannot extend below the floor at the centre, and hopper bottoms, off-centre piles, sloping floors and obstructions need their actual geometry rather than this round flat-floor approximation.

Diameter, depth, rise and drop must use one length unit. Diameter is squared, so a small relative diameter error has roughly twice the relative effect on calculated volume. Measure the inside, not the outside shell; keep peak height separate from sidewall depth; and take several readings when a surface is irregular. Obtain measurements without entering stored grain or climbing unsafely.

Drying follows a dry-matter balance: dry matter = incoming mass × (1 − incoming moisture / 100), then finished mass = dry matter ÷ (1 − target moisture / 100). Removed water is incoming mass minus finished mass. The formula assumes wet-basis moisture and water-only drying; it does not predict a safe storage target, grain quality, dryer energy or a buyer's policy.

A US dry bushel in the geometry formula is a volume of 1.2445 ft³ (0.03523907016688 m³). It does not specify moisture, crop, test weight or saleable mass. Market-equivalent bushels are a different calculation: measured or estimated mass divided by that crop's standard market bushel weight. Label the two outputs separately so an inventory estimate does not become a settlement claim.

If a representative sample gives a bulk density, estimated mass equals volume × density. Record the density's moisture and packing condition with the sample. Do not insert a standard bushel weight in its place. After drying, weigh the lot or measure density again before estimating its new bin volume; dried weight alone does not determine storage space.

Handling loss is a separate deduction from dried grain, so it includes dry matter and remaining water. Buyer pencil shrink, dockage, drying fees and grade discounts are contractual adjustments that may not equal physical water loss. Use the same lot, moisture basis and scale basis throughout, and do not add this tool's physical losses to a weight the buyer has already adjusted without checking for double counting.

• The round-bin cylinder and centred-cone method is a geometry approximation, not a structural rating or density recommendation.
• The 15% target is arithmetic for the worked example, not a universal storage or contract moisture recommendation.
• Check local grain-storage, sampling, safety and buyer guidance for the crop and conditions.
• A measured density or scale ticket improves an estimate but does not replace a contract's official quantity, moisture, dockage or discount rules.

• Using outside diameter: shell thickness makes outside dimensions too large for the grain cylinder. Measure the inside width.
• Putting centre depth into sidewall depth: enter the wall depth, then add the separate peak height or subtract the separate central drop.
• Calling volume bushels weighed bushels: volume measures space. Use representative measured bulk density before estimating mass, then a crop standard only for a separately labelled market-equivalent conversion.
• Using incoming density after drying: density depends on moisture and packing. Measure the dried condition before turning dried mass back into an estimated bin volume.
• Calling a moisture-point reduction a mass-loss percentage: use the dry-matter balance. The 25% to 15% example removes 11.7647% of incoming mass as water.
• Combining field, water and handling losses: field loss occurs before the incoming lot, water loss is physical drying, and handling or buyer deductions have their own basis.

The round-bin cylinder and centred-cone method follows Iowa State University Extension's storage-capacity guidance. The dry-matter equations and 10,000 kg moisture example follow Purdue Extension's wet-basis conversion method.

• Iowa State University Extension, Storage Capacity for Grains, Forages and Liquids (Ag Decision Maker C6-82) — cylinder/cone capacity method and dry-volume conversion context.
• Purdue University Extension, Converting Wet Corn Weight to Dry Corn Weight — wet-basis dry-matter balance and water-shrink arithmetic.