The grain bin capacity calculator above converts the dimensions of a round bin or a rectangular flat store, plus a grain depth, into the units grain is actually traded and settled in: bushels, short tons and metric tonnes. It adds the cone of grain that sits above the eave after filling, which is routinely several percent of the total and is routinely left out, and it uses your commodity's test weight to convert bushels into weight.
Arb Digital publishes this as a free tool for growers, elevator staff and anyone reconciling an inventory. It is worth being straightforward about what makes it different from our other volume tools, because the underlying geometry is not new. What this page adds is the grain layer on top of that geometry — the bushel conversion, the peaked cone, the test-weight-to-tons step and the honest treatment of packing — and that layer is the whole reason a separate page exists.
What This Grain Bin Capacity Calculator Does
The calculation runs in three stages. First it finds the volume of the level grain: a cylinder for a round bin, a rectangular prism for a flat store. Second it adds the cone above the eave, if there is one. Third it converts cubic feet into bushels and then into weight using the test weight you enter.
The boundary against our existing tools is worth stating plainly. The cylinder volume calculator gives the geometric volume and surface area of a cylinder in whatever units you like; it knows nothing about grain. The tank volume calculator gives liquid capacity in gallons and litres for cylindrical, capsule and cone-bottom tanks, and its cone points downward as a drain, not upward as a fill peak. Neither converts to bushels, neither models a peaked fill, and neither applies a test weight. If all you want is a cylinder's volume in cubic feet, those pages are the better route.
How to Use It
- Choose the shape — a round bin, or a rectangular flat store such as a shed or a bunker with vertical walls.
- Measure the grain, not the structure. The depth that matters is from the grain surface at the wall down to the floor the grain actually rests on, which sits above the concrete if there is a perforated aeration floor.
- Estimate the peak height from the level surface to the top of the cone. Enter zero if the bin was levelled after filling.
- Enter the test weight for your commodity. Shelled corn is 56 pounds per bushel; soybeans and wheat are 60.
- Read the total as an estimate. Grain in a bin is not a laboratory measurement, and the section on packing below explains why.
The Formulas and How It's Calculated
Level volume for a round bin is π/4 × D² × depth. For a rectangular store it is length × width × depth. The peaked cone adds π/4 × D² × peak ÷ 3 on a round bin, because a cone is a third of the cylinder that contains it.
The bushel conversion is the grain-specific step. A US bushel is defined as 2,150.42 cubic inches, which is 1.2445 cubic feet, so one cubic foot holds 0.8035 bushels. University of Nebraska–Lincoln's CropWatch guide on how to estimate bushels in a round grain bin folds this into a single shortcut, Bu = 0.628 × D² × H, which rounds the conversion to 0.8 bushels per cubic foot.
Work the defaults through both routes. A 36-foot bin with 18 feet of level grain has a level volume of 0.7854 × 1,296 × 18 = 18,322 cubic feet. At 0.8035 bushels per cubic foot that is 14,721 bushels; the CropWatch shortcut gives 0.628 × 1,296 × 18 = 14,650. The half-percent gap is the rounding in the conversion factor, and it is a fair illustration of the precision this whole exercise carries.
Add a 6-foot peak and the cone contributes 0.7854 × 1,296 × 6 ÷ 3 = 2,036 cubic feet, or 1,636 bushels — a full ten percent on top of the level fill. At 56 pounds per bushel the total of 16,357 bushels weighs 916,000 pounds, which is 458 short tons or 415 metric tonnes.
Packing: Why Your Bin Holds More Than the Geometry Says
Grain is not a fluid. Kernels resting under the weight of the grain above them settle, fines migrate into the voids between larger kernels, and the bulk density at the bottom of a deep bin is measurably higher than at the top. The result is that a tall bin holds more bushels than a straight geometric calculation predicts, and the effect grows with depth.
Published bin capacity charts sometimes build a packing factor into their figures for that reason, which is why a chart can sit a few percent above a calculated number for the same bin. This page applies no packing factor. It gives the geometric estimate, which is the conservative figure and the one that is unambiguous about what it contains. If you are reconciling against a chart or an elevator's number and find yourself a few percent short, packing is the first thing to check — and the size of the correction depends on the commodity, the depth, the moisture and the fill method, which is exactly why hard-coding one would be misleading. Iowa State University Extension's Ag Decision Maker file on storage capacity for grains, forages and liquids collects the standard conversions alongside the caveats.
Where the Peak Estimate Goes Wrong
The cone above the eave is the least certain part of this calculation and often the largest single source of error. Its height is set by the angle of repose — the steepest slope a heap of that material will hold — and that angle is not a constant. Dry, clean, round grain flows to a shallower angle. Damp grain, grain carrying fines and chaff, and grain with damaged kernels all hold steeper slopes. Corn and soybeans differ from each other, and the same corn differs between a wet harvest and a dry one.
Fill method matters just as much. Grain dropped through a single centre spout builds a symmetrical cone. Grain distributed by a spreader builds a much flatter surface, and grain filled off-centre builds a lopsided heap that is not a cone at all. The honest approach is to measure the peak height when you can — the difference between the grain at the wall and the grain at the centre — and to treat a calculated cone from an assumed angle as the roughest figure on the page. Our cone calculator handles the geometry directly if you want to work from an angle instead.
Test Weight, Moisture and What a Bushel Actually Means
A bushel is a volume, but grain is bought and sold by weight against a standard bushel weight for the commodity. Shelled corn's standard is 56 pounds; soybeans and wheat are 60. That is why the tons figure on this page needs a test weight input, and why entering the wrong commodity's figure produces a weight error of seven percent without changing the bushel count at all.
Moisture complicates it further, in two directions at once. Wetter grain is heavier per unit volume, so a bin of wet corn weighs more than a bin of dry corn. But grain is settled to a market moisture, and drying removes water and therefore weight, so the bushels that leave a bin after drying are fewer than the bushels that went in. Anyone reconciling a bin inventory against scale tickets has to account for that shrink explicitly; a capacity calculation alone will never balance the two. Yield estimates made before harvest with our crop yield calculator carry the same caveat.
Arb Digital publishes hundreds of free, no-signup tools covering agriculture, biology, chemistry, finance and everyday maths — no accounts, no stored data.
Browse All Free Tools Contact Arb DigitalCommon Mistakes to Avoid
- Measuring the bin instead of the grain. Capacity depends on grain depth, and a perforated aeration floor raises the true floor level.
- Ignoring the peak. On the default bin the cone is a tenth of the total, and leaving it out understates the inventory badly.
- Using the wrong test weight. Corn at 56 and soybeans at 60 pounds per bushel differ by seven percent in the tons figure.
- Expecting a chart figure to match exactly. Some charts include a packing factor and this page deliberately does not.
- Treating bushels in as bushels out. Drying removes water and therefore weight, so an inventory has to account for shrink.
Related Free Tools From Arb Digital
For pure geometry, the cylinder volume calculator and the cone calculator give volume and surface area without any grain assumptions, and the tank volume calculator covers liquid capacity including partial depths. For harvest planning see the crop yield calculator and the field work rate calculator. Unit work is handled by the volume converter and the weight converter. Browse the full free online tools hub for more.
Frequently Asked Questions
Find the cylinder volume as pi over four times the diameter squared times the grain depth, then multiply by 0.8035 bushels per cubic foot. University of Nebraska–Lincoln publishes a shortcut version, bushels equals 0.628 times diameter squared times depth, which rounds the conversion to 0.8.
A US bushel is defined as 2,150.42 cubic inches, which works out to 1.2445 cubic feet. Inverting that gives 0.8035 bushels per cubic foot, the figure used throughout this calculator.
Yes, unless the bin was levelled. A cone above the eave is a third of the cylinder that would contain it, and on a typical bin it can add ten percent or more to the total. Leaving it out understates the inventory substantially.
Because grain packs. Kernels settle under load and fines fill the voids between them, so bulk density rises with depth. This page gives the geometric estimate with no packing factor applied, which is the conservative figure.
Multiply bushels by the standard test weight for the commodity in pounds per bushel, then divide by 2,000 for short tons or by 2,204.62 for metric tonnes. Shelled corn is 56 pounds per bushel; soybeans and wheat are 60.
The geometry is the same, but this page adds the grain layer: the bushel conversion, the peaked cone above the eave, the test weight conversion to tons, and an explicit treatment of packing. A cylinder tool returns cubic feet and a tank tool returns gallons.
It changes the weight rather than the volume. Wetter grain is heavier per bushel, and drying removes water and therefore weight, so bushels going into a bin and bushels coming out after drying will not match without accounting for shrink.
This calculator gives a geometric capacity estimate using the published bushel conversion and applies no packing factor. Figures are for planning and inventory estimation only. Grain settlement is governed by the terms of your contract and by the weights and measures recorded at a licensed scale.
