How the grain bin calculator works
A standard farm grain bin is a vertical cylinder, so its capacity comes straight from the volume of a cylinder. Measure the inside diameter of the bin and the depth of grain, both in feet, and the capacity is:
Volume (ft³) = π × (diameter / 2)² × grain depth
Capacity (bushels) = Volume (ft³) × 0.8036
The 0.8036 factor converts cubic feet to bushels. One U.S. dry bushel is defined as 2,150.42 cubic inches, which is 1.2445 cubic feet, so one cubic foot holds 1 / 1.2445 = 0.8036 bushels. This is the same factor USDA and university extension services use for round-bin capacity charts.
The calculator reports the level-full capacity: the amount of grain if the top surface is flat at the depth you entered. It also multiplies bushels by the crop's test weight to give an approximate stored weight, since 56 lb/bu corn and 60 lb/bu wheat weigh very differently for the same volume.
Why grain-bin capacity matters
Knowing a bin's true capacity drives harvest logistics, marketing, and insurance decisions. Producers size on-farm storage against expected yield so they can hold grain for a better basis instead of selling at harvest lows. Elevators and crop insurers need accurate stored quantities, and lenders financing grain inventory want the numbers to match. Overestimating capacity leads to trucks with nowhere to unload; underestimating it wastes storage you already paid to build.
Common bin sizes and reference capacities
These level-full capacities use the same formula the calculator applies (cubic feet × 0.8036):
- 18 ft diameter, 16 ft grain depth: π × 9² × 16 = 4,072 ft³ ≈ 3,272 bushels.
- 24 ft diameter, 20 ft grain depth: π × 12² × 20 = 9,048 ft³ ≈ 7,271 bushels.
- 30 ft diameter, 24 ft grain depth: π × 15² × 24 = 16,965 ft³ ≈ 13,633 bushels.
- 36 ft diameter, 30 ft grain depth: π × 18² × 30 = 30,536 ft³ ≈ 24,539 bushels.
A quick rule of thumb: every foot of grain depth in a 24 ft bin is about 363 bushels (π × 12² × 0.8036).
Real-world uses and how to measure correctly
This calculation is used to plan how much of a crop can be stored on-farm, to estimate the volume left in a partially filled bin before selling, to check that a bin site can hold a field's yield, and to convert a physically measured grain level into bushels for records. To use it correctly:
- Measure the inside diameter of the bin wall, not the outside — corrugated steel and stiffeners add a few inches that are not grain.
- Measure grain depth from the floor (or the top of the aeration flooring, since that space holds no grain) to the flat grain surface.
- Pick the crop so the weight estimate uses the right test weight; the bushel count itself does not change with crop.
- Read the bushels output for capacity, and use the weight output for truck and axle-load planning.
How to sanity-check your result
Do an order-of-magnitude check: a mid-size 24 ft bin should land in the several-thousand-bushel range, not hundreds or hundreds of thousands. If your bushel number looks ten times too big, you probably entered the diameter where the radius belongs, or used inches instead of feet. Remember volume scales with the square of diameter, so doubling the diameter roughly quadruples the capacity — a 36 ft bin holds far more than 1.5× a 24 ft bin at the same depth. You can also cross-check against a manufacturer's capacity chart, which lists bushels per ring for standard bin sizes.
Limitations and things this formula assumes
The formula assumes a perfect vertical cylinder with a flat grain surface. Real capacity differs when grain is peaked — a filled center cone adds roughly one-third of a full cylinder of that peak height, so a heaped bin holds more than the level-full number. Hopper-bottom bins lose a little usable volume in the cone floor, while flat-floor bins with aeration ducts lose the duct height. The calculator does not account for pack (grain settling and compressing under its own weight over time), which can nudge true bushels slightly higher. It also assumes the depth you enter is actual grain depth, not total bin height. For structural loading, foundation design, or grain that is out of condition, consult the bin manufacturer or an agricultural engineer rather than relying on a capacity estimate alone.
Bushels to weight: test weights and precision
Bushel capacity is a volume, so it is fixed by the bin geometry, but the weight of grain depends on the crop and its moisture. Standard market test weights are 56 lb/bu for shelled corn and grain sorghum, 60 lb/bu for wheat and soybeans, 48 lb/bu for barley, and 32 lb/bu for oats. Wetter grain weighs more per bushel and also shrinks when dried, so a bin filled with 20% corn will show fewer marketable (dry) bushels after drying to 15%. When planning truck loads, round weights up and leave margin for legal axle limits; when marketing, use dry-basis bushels for the numbers that actually sell.