How this instrument works
A round grain bin's storage volume breaks into simple geometric pieces: a cylinder for the straight-wall section, plus a cone for any peaked top or hopper-bottom fill above the eave line. This instrument computes volume in cubic feet from bin diameter and the two height measurements, then converts that volume to bushels — the unit grain is actually bought, sold and reported in — using the legally defined bushel-to-cubic-foot relationship.
A bushel is a fixed legal volume, defined as exactly 2,150.42 cubic inches. Dividing by 1,728 cubic inches per cubic foot gives 1.244 cubic feet per bushel, the conventional grain-storage conversion factor published across grain-handling references — not a rounded approximation but the direct result of the bushel's exact legal definition. Dividing a bin's cubic-foot volume by 1.244 converts it straight to bushels of storage capacity.
For a flat-bottomed, flat-topped bin, capacity is just the cylinder: pi x radius-squared x wall height, divided by 1.244. Many working bins also carry grain piled into a cone above the eave line once filling continues past the flat top, adding a cone volume — pi x radius-squared x cone height, divided by 3 — on top of the cylinder volume before the same bushel conversion is applied. Setting cone height to 0 reduces the calculation to the flat-top case automatically.
- Enter the bin's inside diameter into Bin diameter (ft).
- Enter the height of the straight cylindrical wall into Straight (cylinder) wall height (ft).
- If grain is piled above the eave into a cone, enter that additional peak height into Cone/peak height (ft) — leave it at 0 for a flat-bottom, flat-top bin.
- Read Volume (cubic feet) for the raw geometric volume, and Grain capacity (bushels) for the same volume converted to bushels.
- For a hopper-bottom bin, the same cone term can represent the hopper cone's height if you're estimating usable fill capacity above the hopper rather than the hopper itself.
Worked example — 20 ft flat-bottomed bin, 10 ft wall
Enter 20 into Bin diameter (ft), 10 into Straight (cylinder) wall height (ft), and leave Cone/peak height (ft) at 0 for a flat-bottomed, flat-topped bin. Volume (cubic feet) reads 3141.5927: pi x 10^2 x 10 = pi x 1,000 = 3,141.5927 cubic feet (radius is half the 20 ft diameter, so r = 10 ft).
Grain capacity (bushels) reads 2525.40: 3,141.5927 / 1.244 = 2,525.40 bushels of storage capacity — a realistic figure for a mid-size on-farm bin of this size, and a number a grower can check directly against the bin manufacturer's published capacity rating for the same dimensions.
Questions
Where does the 1.244 cubic-feet-per-bushel conversion come from?
It comes directly from the bushel's exact legal definition of 2,150.42 cubic inches: dividing by 1,728 cubic inches per cubic foot gives 1.244 cubic feet per bushel. It's not an agronomic estimate or a rounded field rule — it's the same conversion factor published across grain-handling and storage references, derived purely from the legal volume definition.
What if my bin has a flat bottom and flat top, no cone?
Leave Cone/peak height (ft) at 0 — the formula's cone term drops out entirely (pi x r^2 x 0 / 3 = 0), so the result is just the cylinder volume converted to bushels. This is the simplest and most common case for a basic flat-bottomed storage bin filled level with the eave.
How do I handle a bin with grain piled into a cone above the eave?
Enter the straight wall height (up to the eave) into the cylinder field and the additional peak height of the piled cone into the cone field — the instrument adds the cone's volume to the cylinder's volume before converting the total to bushels. This models the common on-farm practice of filling a flat-top bin past the eave into a cone-shaped peak for extra capacity.
Does this calculator work for hopper-bottom bins?
It computes the same cylinder-plus-cone geometry either way, so you can use the cone field to represent a hopper cone if you're estimating the usable volume above a hopper-bottom bin's cone section, though the hopper's own sloped volume below the straight wall would need to be accounted for separately if you need total bin capacity including the hopper.
How does this compare to a simplified field estimate like 0.8 bushels per cubic foot?
Some quick field guides use a rounded 0.8 bushels-per-cubic-foot shortcut (roughly the reciprocal of 1.244) for fast mental estimates, but that rounding can shift a large bin's estimated capacity by dozens of bushels. This instrument uses the more precise 1.244 cubic-feet-per-bushel factor derived from the bushel's exact legal definition, for a more accurate result than the field shortcut.