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. Because the area grows with the square of the radius, even a few inches of extra diameter adds noticeable volume on a large bin.

Converting cubic feet to bushels

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

$\text{Bushels} = \text{Cubic feet} \times \frac{1{,}728}{2{,}150.42} \approx \text{Cubic feet} \times 0.8036$

This factor comes from the definition of the bushel as a unit of volume. The US bushel is the Winchester bushel, a measure of 2,150.42 cubic inches. One cubic foot holds 12 × 12 × 12 = 1,728 cubic inches, so one cubic foot equals about 0.8036 bushels. The calculator uses this exact ratio. Rounding it to 0.8 gives results about 0.4 percent lower.

Because the bushel measures volume, the same factor applies to every grain. What changes from grain to grain is the weight of a bushel, called the test weight. For example, USDA No. 1 hard red spring wheat must have a test weight of at least 58.0 pounds per bushel, according to the same NIST article.

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: $16{,}965 \times 0.8036 \approx 13{,}632$ bushels.

The calculator shows both results to two decimal places: 16,964.60 cubic feet and 13,632.14 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. A real bin usually holds somewhat less, for several reasons.

• Moisture content. Wet grain packs differently from dry grain, so the amount that fits changes with moisture.
• Equipment inside the bin. Aeration tubes, sumps, and sweep augers take up space that grain cannot occupy. A 12-inch-diameter aeration tube running the full 25-foot height of a bin has a radius of 0.5 feet. Its volume is about $\pi \times 0.5^2 \times 25 \approx 19.6$ cubic feet, or roughly 15.8 bushels of lost space. That is a small fraction of a large bin.
• 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.

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 947 bushels, on top of the cylindrical volume. Hopper-bottom bins work the opposite way: the narrowing base holds less grain than a flat floor would at the same height.

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 somewhat lower than 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 bushel is a unit of volume, so the result in bushels is the same for corn, wheat, soybeans, or any other grain. To estimate weight, multiply the bushels by the test weight of the grain in pounds per bushel.

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. 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, Office of Weights and Measures. Determining Reference Test Weight per Bushel Value of Grains. Test weight per bushel — the Winchester bushel measure of 2,150.42 cubic inches; USDA No. 1 hard red spring wheat minimum test weight of 58.0 pounds per bushel.