Most farmers look at bin capacity the same way they look at truck beds: pick the size that holds what you need, add a margin, done. But grain bin sizing is more complicated than that. The number on the spec sheet is a rated capacity based on peaked fill at standard test weight. Your actual usable capacity will be lower, sometimes significantly lower.
The gap between rated and real capacity comes from three places: the geometry of peaked vs level fill, moisture shrink if you're drying in the bin, and practical clearance at the top. If you size based on rated capacity alone, you'll run out of room when it matters most. This guide walks through the math and the tradeoffs so you can size bins that work in real conditions, not just on paper.
Why Bin Sizing Is Not Just About Bushels
A 30-foot diameter bin holds about 23,000 bushels at peaked fill. A 36-foot bin holds about 33,000 bushels. That's a 44% increase in capacity from a 20% increase in diameter. This is the single most important concept in bin sizing: capacity scales with the square of the radius, not linearly with diameter.
The rated capacity you see on a spec sheet assumes peaked fill, which requires a spreader or leveler to build a cone of grain above the eave. Without a spreader, you get level fill, and you lose 15-25% of the rated capacity depending on the bin's height-to-diameter ratio. Most farmers without spreaders are storing closer to 80-85% of the rated number.
Rated capacity also assumes standard test weight: 56 lb/bu for corn, 60 lb/bu for soybeans, 60 lb/bu for wheat. If your crop comes in light (52 lb/bu corn after a drought year), you get more bushels in the same bin because you're measuring volume, not weight. But that extra volume doesn't help you at the elevator, where they pay by weight. The inverse is also true: heavy corn at 58 lb/bu means fewer bushels fit.
The practical result is that you need to think about usable capacity, not rated capacity. If you need to store 30,000 bushels and you're not running a spreader, a bin rated for 33,000 bushels is not enough. You need to size up to account for level fill, moisture shrink, and headspace.
The Geometry: Why Diameter Matters More Than Height
Bin volume comes from the cylinder formula: V = π × (D/2)² × H, where D is diameter and H is eave height. To convert cubic feet to bushels, divide by 1.2445 (the cubic feet per bushel). The squared radius term is why diameter dominates capacity.
A 30-foot bin with 22 feet of eave height holds about 15,500 cubic feet, or 12,500 bushels at level fill. A 36-foot bin with the same eave height holds about 22,300 cubic feet, or 17,900 bushels. That's a 43% increase in capacity from adding 6 feet to the diameter. If you added 6 feet to the height instead (keeping diameter at 30 feet), you only get to 17,000 cubic feet, or 13,700 bushels. A 9% increase.
This is why most bin sizing decisions come down to diameter first, then height. Diameter gives you capacity. Height gives you flexibility (easier to unload with an underfloor system, better aeration uniformity). But if you need more bushels, you need more diameter.
V = π × (D/2)² × H
Bushels = V ÷ 1.2445
Where D = diameter (ft), H = eave height (ft), V = volume (cubic feet)
Grain Bin Capacity Calculator
Calculate bushel capacity for flat-bottom and hopper-bottom grain bins. Enter diameter, eave height, and commodity to get level fill, peaked fill, and partial fill volumes with weight estimates.
Peaked Fill vs Level Fill: The 20% You Might Not Get
Peaked fill adds a cone of grain above the eave. The height of that cone depends on the angle of repose, which varies by commodity. Corn typically sits at about 25 degrees, soybeans at 29 degrees, wheat at 28 degrees. The volume of the cone is (1/3) × π × (D/2)² × h, where h is the cone height.
For a 36-foot bin, the cone height is roughly (D/2) × tan(angle). At 25 degrees, that's about 8.4 feet. The cone adds about 2,850 cubic feet, or 2,300 bushels. That's where the difference between 17,900 bushels (level) and 20,000+ bushels (peaked) comes from.
But you only get peaked fill if you have a spreader or if you're filling slowly from the center and letting the grain naturally pile. If you're using a spout that swings around the perimeter, you get something closer to level fill. The grain might peak slightly in the center, but not enough to hit the rated capacity.
The other issue with peaked fill is aeration. Air doesn't move well through a peaked pile because the path length varies too much. If you're planning to aerate or dry in the bin, level fill works better. Many operators use a spreader to fill, then level the pile before starting fans. That defeats the capacity advantage of peaked fill.
Corn: 25°
Soybeans: 29°
Wheat: 28°
These are approximations. Actual angle depends on moisture, foreign material, and kernel size.
Moisture Shrink Eats Your Capacity
If you're storing wet grain and drying it in the bin, you lose bushels to physical shrink. The formula is: dry_bu = wet_bu × (100 - wet%) / (100 - dry%). If you put 20,000 bushels of 20% moisture corn in the bin and dry it to 15%, you end up with about 18,800 bushels. You lost 1,200 bushels, or 6%.
This is not negotiable. The moisture you remove is weight you remove, and since bushels are measured by volume but sold by weight, the shrink shows up as fewer bushels. The dryer the target moisture, the worse the shrink. Going from 20% to 14% loses 7.1%. Going from 25% to 15% loses 11.8%.
The practical impact is that if you need to deliver 30,000 bushels of dry grain, you need to harvest more than 30,000 bushels of wet grain. If your bin holds 30,000 bushels at level fill and you're drying from 20% to 15%, you can only deliver about 28,200 bushels after shrink. You need to size the bin for the wet bushels, not the dry bushels.
dry_bu = wet_bu × (100 - wet%) / (100 - dry%)
Example: 20,000 bu at 20% moisture, dried to 15%
dry_bu = 20,000 × (100 - 20) / (100 - 15) = 18,824 bu
Shrink: 1,176 bu (5.9%)
Grain Moisture Shrink Calculator
Calculate physical grain shrink from drying vs. elevator-applied shrink. See the hidden margin in your elevator's shrink factor and the dollar impact at current commodity prices.
Matching the Auger to the Bin
A 50,000 bushel bin is useless if your auger takes two days to fill it. Auger capacity is rated in bushels per hour at a reference angle (usually 45 degrees for portable augers, vertical for permanent systems). Capacity derates as angle increases because the flights have to lift the grain higher per revolution.
A 10-inch auger might move 3,000 bu/hr at 45 degrees but only 2,400 bu/hr at 60 degrees. If you're filling a 30,000 bushel bin, that's the difference between 10 hours and 12.5 hours. For a single bin, that might not matter. For a multi-bin setup where you're moving grain all day, it adds up.
The other constraint is unloading. If you have an underfloor unload system with a 6-inch auger, it might only pull 1,500 bu/hr. If you need to turn the bin around in 8 hours to make room for the next load, you're capped at 12,000 bushels. A bigger bin doesn't help if you can't empty it fast enough.
A rough sizing rule: for harvest-time filling, you want auger capacity equal to at least 10% of bin capacity per hour. A 30,000 bushel bin should have a 3,000 bu/hr auger. For unloading, 5-7% per hour is usually enough unless you're running a high-volume commercial operation.
Auger & Conveyor Sizing Calculator
Find the right auger diameter for your target bushels per hour. See capacity derating by angle and commodity, HP requirements, and time to fill a bin.
Thinking About Expansion
The most expensive parts of a grain bin are the concrete pad and the aeration system. The steel rings are relatively cheap. This is why most farmers size bins for future needs, not current needs. If you think you'll need 40,000 bushels of storage in three years, build the 40,000 bushel bin now. The incremental cost is small compared to pouring a second pad.
The concrete pad has to carry the full weight of the bin and the grain, plus snow load and wind load. A 30-foot bin with 20,000 bushels of corn weighs about 600 tons. The pad is typically 6-8 inches thick with rebar and a compacted gravel base. If you pour a pad for a 30-foot bin and later decide you need a 36-foot bin, you have to pour a new pad. That's $8,000-12,000 plus site work.
The aeration system is similar. If you design for 1 CFM per bushel (a common target for humid climates), a 30,000 bushel bin needs a 30,000 CFM fan. A 40,000 bushel bin needs a 40,000 CFM fan. You can't just bolt a bigger fan onto an undersized plenum. The ductwork and perforated floor have to be sized for the airflow from the start.
The general advice is to size bins for your expected needs 3-5 years out, or for the capacity you'll need when you retire and a successor takes over. Underbuilding is expensive. Overbuilding by one size is usually cheap insurance.