What grain bin capacity really measures
A grain bin's capacity is a volume dressed up as a count. The bushel you buy and sell by is a unit of weight at a reference test weight, but the bushel a bin holds is a unit of volume: the US dry bushel is defined as 2,150.42 cubic inches, which is 1.24446 cubic feet. Divide the interior volume of the structure by that number and you have its capacity in bushels, whatever crop you put in it.
That single conversion explains why manufacturers quote a bin as, say, 27,000 bushels regardless of commodity, and why the weight it holds changes completely with the crop. The same 33,648 cubic feet holds 27,038 bushels of anything, but at 56 pounds per bushel that is 757 short tons of corn, while at 60 pounds per bushel it is 811 short tons of wheat or soybeans. Volume is fixed by the steel; weight is fixed by what you put in it.
Three numbers drive every answer on this page. The floor area, which for a round bin is π/4 × diameter squared. The height of the vertical grain column, which for a full bin is the sidewall or eave height. And the coned peak that forms above the eave when a bin is spout-filled to the roof, which is a genuine part of working capacity and is routinely left out of quick estimates.
Once you have the total, a measured grain depth turns the calculator into an inventory tool. Drop a tape from the roof hatch to the grain surface, subtract from the eave height, and you know the depth. The calculator converts that depth to bushels held and to a percent-full figure, which is what a lender, a crop insurance adjuster or your own marketing plan actually needs.
The formula, one term at a time
Start with the cylinder. A round bin is a vertical cylinder of grain, so its volume is cross-sectional area times height. The area of a circle is πD²/4, and π/4 is 0.785398 — the number that shows up in every bin capacity rule of thumb ever printed. Multiply by the sidewall height and you have the struck-level cubic feet.
Now the peak. Grain poured from a centre spout will not spread flat; it heaps at its angle of repose, the steepest slope a loose pile of that material sustains. Dry clean shelled corn sits at roughly 27 degrees, wheat close to the same, soybeans nearer 29 degrees because the round seeds roll but pack differently, and damp or trashy grain stands steeper because the particles interlock. The heap is a right circular cone whose base is the bin circle and whose height is the radius times the tangent of the repose angle: at 27 degrees, tan θ = 0.5095, so a 36-foot bin peaks 9.17 feet above the eave. A cone's volume is one third of base area times height, so the peak adds πD²/12 × h.
That cone is not a rounding error. On a 36-foot bin the peak is 3,112 cubic feet, or 2,501 bushels — about 9 percent of total capacity, and roughly the same as adding a whole extra 3-foot ring of sidewall. Manufacturers usually publish both a level capacity and a peaked capacity for exactly this reason.
Flat storage drops the cone and replaces the circle with a rectangle: length times width times depth. That is the honest figure for grain levelled with a bucket, and it is deliberately conservative for a building filled through a roof conveyor, where a ridge of grain stands above the levelled depth in a shape that depends entirely on how the conveyor was run. If you want that ridge counted, measure its own height and treat it as a separate prism.
Converting bushels to weight uses test weight, the pounds a struck bushel of that grain weighs. US No. 2 yellow corn carries a 54 lb/bu minimum with 56 lb/bu as the standard bushel weight; wheat and soybeans use 60, barley 48, oats 32. If your load's real test weight is well below the standard, the bin still holds the same volume — it just holds fewer pounds. The test weight calculator covers that conversion in detail.
Worked example: a 36 ft × 30 ft bin of corn, peaked
Take a common on-farm bin: 36 feet in diameter, six 5-foot rings for a 30-foot eave height, spout-filled to the roof with dry shelled corn at 56 lb/bu and an angle of repose of 27 degrees. Work it through on paper.
- Cross-sectional area. A = 0.785398 × 36² = 0.785398 × 1,296 = 1,017.88 ft².
- Volume to the eave. 1,017.88 × 30 = 30,536.3 ft³.
- Peak height. h = (36 ÷ 2) × tan 27° = 18 × 0.509525 = 9.171 ft.
- Peak volume. 1,017.88 × 9.171 ÷ 3 = 3,111.8 ft³.
- Total volume. 30,536.3 + 3,111.8 = 33,648.1 ft³.
- Bushels. 33,648.1 ÷ 1.24446 = 27,038 bu. Of that, 24,538 bu is below the eave and 2,501 bu is in the peak.
- Short tons. 27,038 × 56 ÷ 2,000 = 757.1 tons. In metric that is 686.8 tonnes in 952.8 m³.
Now suppose you drop a tape and find the grain surface 6 feet below the eave, so the depth is 24 feet. The grain is a plain cylinder at that point, well below the peak: 1,017.88 × 24 = 24,429.0 ft³, which is 19,630 bushels. Against a peaked capacity of 27,038 bushels the bin is 72.6 percent full. Against the level capacity of 24,538 bushels it is 80.0 percent full — which is why you must always say which capacity a percentage refers to.
At $4.50 corn that 19,630 bushels is $88,335 of inventory sitting on one floor. Feeding it into a break-even price calculator tells you whether holding it is earning anything.
How to read the number you get
Treat the calculated capacity as the geometric ceiling, then adjust for four physical realities before you trust it as an inventory figure.
Packing adds bushels. Grain compresses under the weight of the column above it, so the bushels per cubic foot in the bottom of a 40-foot column exceed those at the top. The effect is small in a farm bin and material in a tall concrete silo; commercial elevators apply published pack factors by commodity and depth, and USDA's Rural Development storage standards recognise the same effect. Geometry alone is therefore a slightly conservative estimate for deep storage, which is the direction you want an error to run.
Floor and equipment subtract bushels. A full perforated drying floor sits 12 to 18 inches above the concrete, and that whole slab of space is air, not grain — on a 36-foot bin, a 14-inch plenum is about 1,188 ft³ or 954 bushels gone. Sloped floors, sumps, stirring machines and the unload tube all take their cut. If your bin has a drying floor, measure the eave height from the top of the floor, not from the concrete.
The peak only exists if you filled it that way. A bin filled with a portable auger swung around the roof hatch, or one that has been levelled by a spreader, does not carry a full cone. If you cannot see the heap, use the level capacity and treat the peak as headroom.
Depth measured at the wall is not depth measured at the centre. On a partly unloaded bin the surface is cratered above the unload sump; on a filling bin it is peaked. Average several tape drops when the surface is visibly uneven — a one-foot error on a 36-foot bin is 818 bushels. Then sanity-check against your own harvest records: if the tickets say 23,000 bushels went into a bin the calculator says holds 27,038 and the bin looked full, one of the two numbers is wrong, most often the yield from the corn yield estimate calculator or the moisture, since wet bushels shrink as they dry.
Bushels per foot of depth and peak volume by bin diameter
| Bin diameter (ft) | Cubic feet per foot of depth | Bushels per foot of depth | Peak height at 27° (ft) | Bushels in the peak |
|---|---|---|---|---|
| 15 | 176.7 | 142.0 | 3.82 | 181 |
| 18 | 254.5 | 204.5 | 4.59 | 313 |
| 21 | 346.4 | 278.3 | 5.35 | 496 |
| 24 | 452.4 | 363.5 | 6.11 | 741 |
| 27 | 572.6 | 460.1 | 6.88 | 1,055 |
| 30 | 706.9 | 568.0 | 7.64 | 1,447 |
| 36 | 1,017.9 | 817.9 | 9.17 | 2,501 |
| 42 | 1,385.4 | 1,113.3 | 10.70 | 3,971 |
| 48 | 1,809.6 | 1,454.1 | 12.23 | 5,927 |
| 60 | 2,827.4 | 2,271.9 | 15.29 | 11,576 |
Derived from the same expressions the calculator runs: ft³/ft = 0.785398·D², bushels/ft = 0.631118·D², peak bushels = 0.053595·D³ at 27°. Round bins only.
Where the bushel comes from
The US dry bushel is the Winchester bushel, fixed at 2,150.42 cubic inches, and it is carried in the general tables of units in NIST Handbook 44. It is not the same as the imperial bushel used historically in the UK and Canada, which is 2,219.36 cubic inches — about 3.2 percent larger. Nor is it the same as the pound-based bushel used in trade, where a "bushel" of corn means 56 pounds regardless of the volume it occupies. This calculator uses the volumetric bushel for capacity and the trade test weight only to convert that capacity into tons.
Mistakes that put a bin measurement wrong
- Measuring the diameter outside the corrugations. Bin sheets are quoted by nominal diameter, and taping the outside of the ribs adds a few inches. Because capacity goes as diameter squared, a 4-inch overstatement on a 36-foot bin with a 30-foot sidewall adds about 455 bushels that are not there: 0.6311 × 36.333² × 30 = 24,995 against 24,538.
- Counting the eave height from the concrete when there is a drying floor. The plenum below a perforated floor holds air. Measure from the top of the floor sheets.
- Adding the peak to a levelled bin. A spreader or a levelling auger removes most of the cone. Use the level figure unless you have seen the heap.
- Applying one test weight to a mixed bin. If you filled with grain from several fields at different test weights, the volume is still right but the tonnage is a blend. Use a weighted average or weigh a load.
- Confusing wet bushels with dry bushels. Grain binned at 20 percent moisture and dried to 15 percent loses close to 6 percent of its bushels to water. Convert first with the grain moisture shrink calculator, then compare against bin capacity.
- Assuming the roof eave is level with the top ring. On bins with a stiffened or raised eave, the last few inches of sheet are above the point where the cone starts. Measure to the point where the roof line meets the wall.
Key terms
- Angle of repose
- The steepest slope a free-flowing pile of a material will hold without sliding. It sets the height of the cone above a bin's eave and the shape of any outdoor pile. It rises with moisture, fines and foreign material.
- Test weight
- The weight in pounds of one struck volumetric bushel of grain, measured with a standard kettle. It is a grading factor as well as a conversion factor: US No. 2 yellow corn requires a minimum of 54 lb/bu.
- Level (struck) capacity
- The capacity of a bin filled exactly to the eave with a flat surface. It is the conservative figure and the one to use for insurance and inventory unless the peak is confirmed.
- Pack factor
- A multiplier applied by commercial storage operators to account for grain compacting under the weight above it. It grows with depth and with finer, denser commodities.
When to reach for a different tool
This calculator handles the two shapes that hold most of the world's grain, but several storage problems need a different geometry or a different unit.
Outdoor ground piles and bunkers. A free-standing conical pile is the cone term alone: πD²/12 × h. An elongated pile with a ridge is a triangular prism with a half-cone at each end, so its volume is (base area × height ÷ 2 for the prism section) plus one full cone. Bunker walls change the shape again. Measure the actual heap rather than assuming a repose angle when the pile has been pushed with a loader, because a pushed face is steeper than a poured one.
Hopper-bottom bins. The cone below the sidewall adds volume rather than subtracting it. A hopper of the same diameter with a cone angle of 45 degrees adds πD²/12 × (D/2) — for a 15-foot hopper bin, another 442 cubic feet or 355 bushels. Add that to the cylinder above it.
Storage economics. Once you know the bushels, the question becomes whether they are worth holding: carry charges, interest, shrink, quality risk and the basis you expect to capture. Start from the cost of production in the crop break-even price calculator and net the drying and shrink costs against the market you are storing for.
For any figure that will be used in a crop insurance claim, a loan collateral inspection or a bin measurement for settlement, follow the measuring procedure the party requesting it specifies. Adjusters use standardised techniques for depth measurement and pack, and a geometric estimate is not a substitute for a measurement taken their way.