Whether you are reporting inventory, sizing a sale, or insuring stored crop, you need to turn bin and pile dimensions into bushels and tonnes. This calculator handles round bins, rectangular flat storage, and conical piles, converting volume to bushels with the standard factor and to tonnes using grain test weight. It is used by grain producers, co-op managers, crop insurance adjusters, and lenders who need a quick verification of reported stocks.
How it works
Each shape gives a volume in cubic feet, which converts to bushels and then to weight:
round bin = π × radius² × depth (+ optional cone: (1/3)π r² peak)
flat store = length × width × depth
cone pile = (1/3) × π × radius² × peak height
bushels = cubic feet / 1.2445
tonnes = bushels × test weight (lb/bu) / 2204.62The 1.2445 cubic-feet-per-bushel factor is the industry standard for free-flowing dry grain, and test weight (56 lb/bu corn, 60 lb/bu wheat and soybeans) sets the conversion from volume to mass.
Example and tips
A round bin 18 ft in diameter filled to 20 ft of corn holds π × 9² × 20 = 5,089 cubic feet, about 4,090 bushels, and at 56 lb/bu roughly 104 tonnes. For a peaked fill, add the roof cone. For free-standing piles, if you cannot measure the peak, estimate it from the angle of repose (about 23 degrees for corn) — the tool can do this from the base diameter. Always confirm tonnage with a scale where money changes hands.
Standard test weights by grain type
Test weight is the single biggest driver of the difference between volume capacity and mass:
| Grain | Test weight (lb/bu) |
|---|---|
| Corn (shelled) | 56 |
| Soybeans | 60 |
| Hard red winter wheat | 60 |
| Soft red winter wheat | 60 |
| Barley | 48 |
| Oats | 32 |
| Grain sorghum (milo) | 56 |
Oats are notably light — the same bin that holds 4,000 bushels of corn weighing about 102 tonnes holds 4,000 bushels of oats at only 58 tonnes. Enter your grain’s actual test weight at the current moisture level for the most accurate mass estimate, especially if grain is wet or has high foreign material content, which reduces packing density.
Measuring a flat storage
Rectangular flat storage is the easiest shape to measure but harder to fill uniformly. Grain piles into a peaked or sloped heap rather than filling to the wall uniformly. A practical approach is to measure the average fill depth at several points across the floor and use the average as your depth input. For planning purposes — sizing a new flat store before it is filled — enter the design depth that corresponds to the safe load limit for your floor structure.
Bin capacity on sloped or uneven ground
Round bins and flat stores on ground that is not perfectly level carry slightly less grain than a perfectly horizontal calculation suggests, because the grain surface is level but the bin is tilted. For bins on a concrete pad the error is usually negligible. For temporary structures on uneven terrain, measure the fill depth at the lowest eave point rather than an average to avoid overstating inventory.