Grain Bin Capacity Calculator - Bushels & Cubic Feet

What is a grain bin capacity calculator?

A grain bin capacity calculator finds how much grain a cylindrical storage bin can hold. It uses two measurements, diameter and height, to work out the bin's volume in cubic feet and its storage capacity in bushels. Farmers and grain elevator operators use it to plan harvest logistics, size equipment, and decide how much grain they can store on site.

How to calculate grain bin capacity

The volume formula

A grain bin is a cylinder, so its volume follows the standard cylinder formula:

$V = \pi \times r^2 \times h$

Here, $V$ is the volume in cubic feet, $r$ is the radius of the bin in feet, and $h$ is the height in feet. The radius is half the diameter: $r = d \div 2$.

The diameter used in the formula should be the inside diameter of the bin, not the outside diameter. Steel bin walls are usually 2 to 3 inches thick. On a 30-foot bin, ignoring wall thickness can overstate capacity by several hundred bushels.

Converting cubic feet to bushels

Once the volume is known, it converts to bushels with a fixed factor:

$\text{Bushels} = \text{Cubic feet} \times 0.8$

This factor comes from the legal definition of the bushel as a unit of volume. One bushel equals 2,150.42 cubic inches, a measure known as the Winchester bushel, adopted in the United States in the 18th century. One cubic foot holds 1,728 cubic inches, so one cubic foot equals about 0.8036 bushels. The grain trade rounds this to 0.8 for everyday use.

The 0.8 factor is an average that works well for corn. Other grains pack to different densities inside a bin, so their bushels-per-cubic-foot figures differ slightly. The table further down lists common values.

Example calculation

Consider a bin 30 feet in diameter and 24 feet tall.

1. Radius: $r = 30 \div 2 = 15$ feet.
2. Volume: $V = \pi \times 15^2 \times 24 \approx 16{,}965$ cubic feet.
3. Capacity: $16{,}965 \times 0.8 \approx 13{,}572$ bushels.

This is the bin's rated capacity: the theoretical maximum if it were filled to the eave with perfectly level grain.

Rated capacity versus usable capacity

The formula gives a theoretical maximum. Real bins usually hold 90 to 95 percent of that number, because of several factors.

• Moisture content. Wet grain swells and packs more loosely than dry grain. A common rule of thumb is that capacity drops by about 1.2 percent for each percentage point of moisture above the standard rate (15 percent for corn). Corn stored at 18 percent moisture, for example, would fill a bin to roughly 3.6 percent below its rated capacity.
• Grain type. Soybeans, oats, and other grains pack differently than corn, changing how many bushels fit in the same space.
• Equipment inside the bin. Aeration tubes, sumps, and sweep augers take up space that grain cannot occupy. A 12-inch-diameter perforated aeration tube running the full 25-foot height of a bin has a radius of 0.5 feet, giving a volume of about $\pi \times 0.5^2 \times 25 \approx 19.6$ cubic feet, or roughly 15.7 bushels of lost space. That is a small fraction of a large bin's total capacity, but larger equipment, such as sumps and transition cones, can add up to 2 to 3 percent.
• Grain piling. Grain naturally peaks in the center as a bin fills. Bins without a spreader rarely fill perfectly level.
• Safety headspace. Operators typically leave room near the top for airflow and to avoid overflow while loading.

A commonly used planning figure is 92 percent of rated capacity for a flat-bottom bin with standard equipment.

Cone tops and hopper bottoms

If grain is allowed to peak above the sidewall, the peak forms a cone that adds extra volume:

$V_{cone} = \frac{1}{3} \times \pi \times r^2 \times h_{cone}$

For a 30-foot diameter bin with a 5-foot peak, that adds about 1,178 cubic feet, or roughly 942 bushels, on top of the cylindrical volume. Hopper-bottom bins work the opposite way: the narrowing base typically reduces usable capacity by 8 to 12 percent compared with a flat-bottom bin of the same height.

Grain bin capacity by grain type

To estimate capacity for a specific grain, multiply the bin's volume in cubic feet by the value for that grain instead of the default 0.8.

Frequently asked questions

How accurate is the calculator compared with a real bin?

The formula itself is exact geometry, but a real bin's usable capacity is usually 90 to 95 percent of the calculated figure. The gap comes from equipment inside the bin, uneven grain piling, and safety headspace, not from an error in the math.

Can this be used for grains other than corn?

Yes. The volume in cubic feet stays the same no matter what is stored. Only the bushels-per-cubic-foot factor changes. Use the table above to substitute the correct factor for wheat, soybeans, oats, or other grains.

How should bin diameter be measured?

The inside diameter gives the most accurate result. It can be measured directly inside an empty bin, or estimated by measuring the outside diameter and subtracting twice the wall thickness, typically 4 to 6 inches for steel bins. Measuring across the corrugation ridges on the outside wall overstates the diameter and inflates the capacity estimate.

Does the calculator handle cone tops or hopper bottoms?

No, it covers the straight-sided cylindrical section only. A cone top adds volume, calculated separately with the cone formula above. A hopper bottom removes volume near the base. Manufacturer capacity charts already account for both and are the most reliable source for bins with complex shapes.

Why does moisture content matter so much at harvest?

Freshly harvested grain often carries more moisture than the standard rate used for grading. Wetter kernels are larger and pack less tightly, so a bin fills up faster than the rated capacity suggests. This is why bins can run out of room sooner than expected during a wet harvest.

References

1. National Institute of Standards and Technology, Handbook 44: Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices, for U.S. customary bushel and volume definitions.
2. Hellevang, K. J., Grain Drying, Handling and Storage Handbook, North Dakota State University Extension.
3. U.S. Department of Agriculture, Federal Grain Inspection Service, official grain standards and test weights.