Measuring a grain bin comes down to three physical measurements: the bin’s diameter, the height of the grain inside, and the shape of the grain surface (peaked, level, or coned down). With those numbers, you can calculate volume in cubic feet and convert it to bushels using a simple multiplier. Here’s how to do it step by step.
The Core Formula for Round Bins
Most grain bins are cylindrical, so you’re calculating the volume of a cylinder. Multiply the diameter by itself, then multiply by 0.7854 (which accounts for the circular shape), then multiply by the grain height in feet. That gives you cubic feet. To convert cubic feet to bushels, multiply by 0.8.
For example, a bin that’s 36 feet in diameter with grain leveled at 25 feet deep: 36 × 36 × 0.7854 × 25 = 25,446 cubic feet. Multiply by 0.8 and you get about 20,357 bushels.
A useful shortcut is to first calculate your “bushels per foot,” which tells you how many bushels each foot of grain depth holds. The formula is: diameter × diameter × 0.7854 × 0.80385. For that 36-foot bin, one foot of depth holds about 818 bushels. Once you know that number, you just multiply it by the grain height any time you need to re-measure.
Taking Accurate Physical Measurements
Diameter is the easy part. Use your bin’s spec sheet, or measure across the inside floor from wall to wall. Grain height is where things get tricky, because the surface is rarely flat.
To measure height, lower a weighted tape measure from the top access hatch to the grain surface. Take readings at multiple points if possible: near the center, near the sidewall, and at least one point in between. The average of these readings gives you a more accurate depth than a single measurement. For the straight sidewall portion of grain (the part below the peak or above any cone), measure from the bin floor to where the grain meets the wall.
Accounting for Peaked Grain
When a bin is filled from the center, grain naturally forms a cone-shaped peak. That peak holds more grain than a flat surface would, so you need to account for it separately.
If you can measure the peak height (the distance from the base of the cone to the tip), divide that height by 3, then multiply by your bushels-per-foot number. You divide by 3 because a cone holds exactly one-third the volume of a cylinder with the same base and height.
If you can’t easily measure the peak height, there’s a rule of thumb from the Iowa Department of Agriculture: divide the bin diameter by 2, then multiply by 0.4 for corn or 0.5 for soybeans. That estimates the peak height based on each grain’s natural angle of repose. Soybeans form a steeper peak than corn, which is why the multiplier is higher. Once you have that estimated peak height, divide by 3 and multiply by bushels per foot, just like before.
Accounting for a Cone Down
After grain has been unloaded from the center, the surface dips into an inverted cone. This means less grain than a flat surface would suggest, so you need to subtract volume.
Measure the depth of the cone (the distance from the sidewall grain level down to the lowest point in the center). Multiply two-thirds of that depth by your bushels-per-foot figure, then subtract that from the total.
There’s also a shorthand approach: take the depth at the sidewall and subtract one-third of the cone depth. Use that adjusted number as your effective grain height and calculate as if the surface were level. Both methods give you the same result.
Why Your Calculation May Undercount
The formulas above assume grain sits loosely at its standard test weight. In reality, grain at the bottom of a deep bin gets compressed by the weight above it. This compression, called the “pack factor,” means a bin actually holds more grain than the basic volume formula predicts.
Pack factors vary by grain type and bin depth, but they typically range from about 3% to 6% for common grains in tall commercial bins. Doubling the grain height increases the pack factor by roughly 0.5% to 1.2%. Soybeans in elevators typically pack 3.2% to 4.7% beyond their loose volume, while sorghum packs 4.1% to 5.6%. For a standard on-farm bin under 40 feet tall, the pack factor is smaller but still worth noting if you want a precise count for insurance or sale purposes.
To apply a pack factor, simply multiply your calculated bushels by 1 plus the percentage. If you calculated 20,000 bushels and estimate a 3% pack factor, the adjusted figure is 20,600 bushels.
Sensor Options for Continuous Monitoring
If you’d rather not climb a bin every time you need a reading, electronic level sensors can take measurements automatically. The main technologies available today are radar, ultrasonic, weight-and-cable (yo-yo style), and 3D scanners.
Non-contact radar is widely considered the most reliable option for grain. It uses a focused 80 GHz signal that cuts through dust, humidity, temperature swings, and condensation without losing accuracy. You mount it at the top of the bin and it measures the distance down to the grain surface.
For bins where the grain surface is uneven, with peaks, sidewall buildup, or irregular draw-down patterns, 3D scanners offer the most complete picture. These systems send acoustic pulses that bounce off multiple points on the grain surface and build a topographic map. They report not just a single level reading but an actual volume estimate based on the surface shape, which makes them more accurate than single-point sensors in real-world conditions.
Ultrasonic sensors are a more affordable non-contact option, though they work best in controlled environments and for distances up to about 50 feet. Weight-and-cable systems physically lower a probe to the grain surface, which is straightforward but involves moving parts that need maintenance.
Safety When Entering a Bin
If you’re climbing into a bin to take manual measurements, grain engulfment is a serious and frequently fatal hazard. OSHA requires several precautions before anyone enters a grain bin. All powered equipment connected to the bin, including augers and conveyors, must be turned off and locked out so grain cannot move while someone is inside. Every person entering must wear a body harness with a lifeline secured before entry. An observer must be stationed outside the bin with no other task except tracking the person inside and being equipped to provide assistance.
Never walk on grain to make it flow, and never enter a bin where grain has bridged (formed a crust over an air pocket) or where buildup on the walls could collapse. The air inside must be tested for toxic gases and sufficient oxygen before entry. Each entry requires a written permit confirming all these precautions are in place. These aren’t suggestions; they’re federal workplace safety standards.