How to Calculate Grain Bin Capacity (Bushels and Tons)

Introduction

Knowing exactly how much grain your bins hold is essential for marketing, logistics, and storage planning. Yet many farmers guess at bin capacity, leading to overfilled bins, inaccurate inventory, and missed marketing opportunities. The good news is that calculating bin capacity requires only basic geometry and your grain's test weight.

This guide explains the cylinder volume formula, how to convert cubic feet to bushels, how peaked and level grain affect capacity, and provides test weight values for major crops. We include a complete worked example for a typical farm bin.

The Cylinder Volume Formula

Most grain bins are cylinders. The volume of a cylinder is: V = π × r² × h, where r is the radius (half the diameter) and h is the height of the grain. All measurements must be in the same units — typically feet, giving cubic feet.

For example, a bin 30 feet in diameter and 20 feet tall (to the eave) has a radius of 15 feet. Volume = 3.1416 × 15² × 20 = 3.1416 × 225 × 20 = 14,137 cubic feet. This is the volume up to the eave, not including any peaked grain above the eave.

One bushel (US) is legally defined as 1.2445 cubic feet. To convert cubic feet to bushels: Bushels = Cubic feet ÷ 1.2445. For our example: 14,137 ÷ 1.2445 = 11,360 bushels to the eave.

Accounting for Peaked Grain

When you fill a bin with a center unloading auger, the grain forms a cone above the eave. This cone adds significant capacity. The volume of a cone is: V = (1/3) × π × r² × h_cone, where h_cone is the height of the peak.

The angle of repose for most grains is 20–25 degrees. For a 30-foot diameter bin, a typical peak height is 4–6 feet. Using 5 feet: Cone volume = (1/3) × 3.1416 × 15² × 5 = 1,178 cubic feet = 947 bushels. Total bin capacity = 11,360 + 947 = 12,307 bushels.

If you level the grain (which you should for proper aeration), you lose the cone volume but gain more uniform airflow. Many farmers fill to the eave, level, then add a small peak for maximum capacity while maintaining some aeration.

Converting Bushels to Tons: Test Weight Matters

Bushels are a volume measure, not a weight measure. To convert bushels to tons, you need the test weight (pounds per bushel) of your grain. Standard test weights: corn 56 lb/bu, wheat 60 lb/bu, soybeans 60 lb/bu, grain sorghum 56 lb/bu, barley 48 lb/bu, oats 32 lb/bu.

Actual test weight varies with growing conditions. Drought-stressed corn may test 52–54 lb/bu; high-moisture corn can test 58–60 lb/bu. Always use your actual tested weight for accurate tonnage.

Formula: Tons = Bushels × Test weight (lb/bu) ÷ 2,000. For our 12,307-bushel bin of 56 lb/bu corn: 12,307 × 56 ÷ 2,000 = 344.6 tons. For 60 lb/bu wheat in the same bin: 12,307 × 60 ÷ 2,000 = 369.2 tons.

Worked Example: A 36-foot Diameter Bin of Corn

You have a grain bin 36 feet in diameter with 24-foot eave height. You fill it with corn testing 55 lb/bu at 15.5% moisture. The peak above the eave is 6 feet. You want to know bushels, tons, and value at $4.20/bu.

Step 1 — Cylinder volume: radius = 18 ft. V = 3.1416 × 18² × 24 = 3.1416 × 324 × 24 = 24,429 cubic feet.

Step 2 — Cone volume: V = (1/3) × 3.1416 × 18² × 6 = 2,036 cubic feet.

Step 3 — Total volume: 24,429 + 2,036 = 26,465 cubic feet.

Step 4 — Bushels: 26,465 ÷ 1.2445 = 21,266 bushels.

Step 5 — Tons: 21,266 × 55 ÷ 2,000 = 584.8 tons.

Step 6 — Value: 21,266 × $4.20 = $89,317.

Step 7 — Drying shrink note: at 15.5% moisture, drying to 15% for storage results in approximately 0.59% weight shrink (see moisture shrink article for details).

Common Mistakes and Pro Tips

Using diameter instead of radius. The formula uses radius squared. A common error is plugging the diameter directly into the formula, which gives 4× the correct volume. Always divide diameter by 2 first.

Forgetting the cone/peak. The peaked grain can add 5–10% to bin capacity. If you are calculating for aeration or inventory, decide whether you are calculating level or peaked capacity.

Using standard test weight instead of actual. Test weight can vary 5–10% from standard, especially in drought years. Get your grain tested at the elevator or with a portable moisture tester.

Ignoring core/center support. Some bins have a center support column that displaces grain. Subtract its volume: π × (column radius)² × grain height. For a 2-foot diameter column in our 36-foot bin: 3.1416 × 1² × 24 = 75 cu ft = 60 bushels — small but not zero.

Overfilling for aeration. Grain above the aeration floor's effective zone does not dry properly. Keep peaks modest (3–4 feet) if you plan to aerate, or level the bin completely.

Conclusion

Grain bin capacity is simple geometry: cylinder volume (πr²h) plus cone volume for the peak, divided by 1.2445 to get bushels, multiplied by test weight and divided by 2,000 to get tons. Measure your actual bin dimensions, use your actual test weight, and decide whether you are calculating level or peaked capacity. Accurate bin capacity means better marketing and storage decisions.

For quick bin capacity calculations, use our Grain Bin Capacity Calculator. The Grain Storage Calculator helps with storage cost planning, and the Moisture Shrink Calculator accounts for drying losses.

Frequently Asked Questions

How many bushels are in a grain bin?

Calculate cylinder volume (π × radius² × height) in cubic feet, then divide by 1.2445. A 30-foot diameter, 20-foot tall bin holds about 11,360 bushels to the eave, plus ~950 bushels for a peaked roof.

How do I convert bushels to tons?

Multiply bushels by the test weight in pounds per bushel, then divide by 2,000. Corn at 56 lb/bu: 1,000 bu × 56 ÷ 2,000 = 28 tons.

What is the standard test weight for corn?

The USDA standard is 56 pounds per bushel at 15.5% moisture. Actual test weight ranges from 52 to 60 lb/bu depending on growing conditions and hybrid.

Does grain moisture affect bin capacity?

Yes, indirectly. Higher moisture grain has higher test weight (more pounds per bushel) but the same volume. A bin holds the same number of bushels regardless of moisture, but wet grain weighs more per bushel.