Every bushel decision on a farm — how much can be stored at harvest, when the bin is truly full, whether this year’s yield fits — starts with the capacity of the bin. This grain bin calculator turns three measurements into a bushel figure: the bin’s diameter, the height of the straight wall (eave height), and the roof peak, where the sloped cap holds a surprising bonus pile of grain. Enter the numbers and get the cylinder bushels, the peak bushels, and the total.
How Grain Bin Capacity Is Calculated
A bin filled level to the eave is a cylinder: volume = π × radius² × wall height. Grain heaped into the roof forms a cone: volume = (π × radius² × peak height) ÷ 3, where the peak height is set by the roof angle — peak height = radius × tan(roof angle). Add the two volumes and convert cubic feet to bushels with the standard factor: one bushel = 1.2445 cubic feet (equivalently, multiply cubic feet by 0.8036). This calculator runs exactly that math and reports each part separately, because the peak is the portion most often forgotten or overestimated.
Worked Example: An 18-Foot Bin
Take a bin 18 feet across with a 12-foot wall and a standard 30-degree roof. The cylinder holds π × 9² × 12 = 3,054 cubic feet, which is 2,454 bushels level full. The cone on top rises 9 × tan(30°) = 5.2 feet and adds (π × 81 × 5.2) ÷ 3 = 441 cubic feet — another 354 bushels. Total working capacity: about 2,808 bushels. Roughly one bushel in eight lives in the peak, which is why “level full” and “full” are different conversations at the elevator.
Bushels, Cubic Feet, and Test Weight
A bushel is a unit of volume — 2,150.42 cubic inches, or 1.2445 cubic feet — before it is a unit of weight. The weight of a bushel depends on the crop and its test weight: standard figures are 60 lb for wheat and soybeans, 56 lb for shelled corn and rye, 48 lb for barley, and 32 lb for oats. So the 2,808-bushel bin above holds about 157,000 lb of corn at standard test weight. When planning floor loads, transport, or drying capacity, convert with the test weight of the actual grain, not the nominal crop figure — a light crop packs fewer pounds into the same bushels.
Real-World Adjustments
Settling and packing: grain packs under its own weight; a bin that calculates to the brim may accept a few percent more after the first load settles, and fine material concentrates in the core. Cored bins: pulling the center cone out for aeration removes peak bushels — recompute with the level option after coring. Partial fills: for a partly filled bin, measure the grain depth at the wall and run the calculator with that as the wall height, ignoring the roof. Angle of repose: dry grain heaps at roughly 21 to 23 degrees, shallower than a 30-degree roof, so a bin filled through the center hatch tends to peak to the roof only if it is filled completely — under-filled peaks hold less than the full cone figure.
Common Mistakes
Using the diameter where the radius belongs quadruples the answer. Quoting the cylinder figure as the total misses the roof cone entirely — or, the opposite error, adding a full cone to a bin that will be cored. Mixing nominal bin sizes with measured ones matters too: an “18-foot” bin may measure 17.7 feet inside, and bushels scale with the square of the radius, so small measuring errors grow. Measure inside, to the wall, in two directions, and average.
Planning Storage Before Harvest
Work the calculation backwards to plan the season: expected yield in bushels, divided by a bin’s total capacity, tells you how many bins a crop needs before the combine rolls. Remember that the last bin is rarely filled to the peak, and that wet grain bound for the dryer should not be planned into long-term capacity at all. A realistic plan leaves the final bin’s cone as slack for the yield estimate being wrong in the pleasant direction — running out of space mid-harvest is far more expensive than a half-empty peak.
Related Tools
The Biology Calculators hub holds the rest of the farm and field set, including the Basal Area Calculator for timber and the Soil Calculator for volumes, cubic yards, and bags.
Frequently Asked Questions
How many bushels fit in a grain bin?
Capacity = (π × radius² × wall height + cone volume) ÷ 1.2445. An 18-foot diameter bin with 12-foot walls and a 30° roof holds about 2,808 bushels — 2,454 in the cylinder plus 354 in the peak.
How many cubic feet are in a bushel?
One bushel is 1.2445 cubic feet (2,150.42 cubic inches). To convert a volume in cubic feet to bushels, divide by 1.2445 or multiply by 0.8036.
How much grain is in the roof peak?
The cone above the eave typically adds 10 to 15 percent on a 30-degree roof. For an 18-foot bin it is about 354 bushels; the exact figure depends on the roof angle and how fully the peak is packed.
Does the crop change the bushel capacity?
The volume in bushels does not change, but the weight does: multiply bushels by test weight (corn 56 lb, wheat 60 lb, soybeans 60 lb, oats 32 lb) to get pounds.
How do I calculate a partially filled bin?
Measure the grain depth at the wall, use that as the height with the level-fill option, and ignore the roof. If the surface is peaked or dished, average the center and wall depths.
What is the angle of repose for grain?
Most dry grains heap at about 21 to 23 degrees. Since roofs are usually steeper (around 30 degrees), grain only fills the roof cone completely when the bin is filled right up through the peak.
Can I use this for a hopper-bottom bin?
Partly — compute the cylinder and roof as shown, then add the hopper cone separately with the same cone formula (π × radius² × hopper depth ÷ 3 ÷ 1.2445). The hopper on an 18-foot bin typically adds a few hundred bushels.