How to Use the Grain Bin Calculator
1. Choose a task
Select Maximum Capacity, Current Bushels, Remaining Capacity or Bushels to Fill Height.
2. Choose storage shape
Select a round bin or rectangular or flat storage and enter inside dimensions.
3. Enter maximum and current depths
Enter eave height, roof clearance and the current grain depth at the wall.
4. Describe the grain surface and bottom
Choose level, peaked or inverted grain, then add full-cone or truncated hopper geometry when applicable.
5. Choose calculation assumptions
Select the U.S. dry-bushel basis, fill allowance, documented packing adjustment and pounds per bushel.
6. Review and share
Review stored, free and percent-full results, then copy, print, download or share the local URL scenario.
Formulas and Assumptions
For a round bin measured in feet, these Iowa State Extension field shortcuts give the level cylinder and centered peak directly:
Level bushels ≈ 0.628 × diameter² × level depth
Peak bushels ≈ 0.209 × diameter² × peak height
Round cylinder = π × radius² × depth
Rectangular level volume = inside length × inside width × depth
Cone or right-pyramid profile = base area × vertical height ÷ 3
Frustum = π × depth × (top radius² + top radius × outlet radius + outlet radius²) ÷ 3
Angle peak height = radius (or half the shorter rectangular span) × tan(angle)
Exact U.S. dry bushels = cubic feet × 1,728 ÷ 2,150.42
Current adjusted inventory equals geometric dry-volume bushels × the documented packing adjustment. Maximum usable capacity also applies the fill/headroom allowance. Test weight changes the mass result, not physical bin volume. Moisture is recorded only; use a separate documented shrink calculation when needed.
Common Round-Bin Capacity Reference
Farm basis: 0.8 U.S. bu/ft³; level grain only. “Per foot” is π × (diameter ÷ 2)² × 0.8. Select a row to load an 18-foot level example.
| Inside diameter | Bushels per foot | 18-foot level capacity | Use |
|---|---|---|---|
| 18 ft | 203.6 bu | 3,664 bu | |
| 21 ft | 277.1 bu | 4,988 bu | |
| 24 ft | 361.9 bu | 6,514 bu | |
| 30 ft | 565.5 bu | 10,179 bu | |
| 36 ft | 814.3 bu | 14,657 bu | |
| 42 ft | 1,108.4 bu | 19,950 bu | |
| 48 ft | 1,447.6 bu | 26,058 bu | |
| 60 ft | 2,261.9 bu | 40,715 bu |
Worked Grain-Bin Examples
30-foot bin with an 18-foot wall depth and 4-foot peak
Inputs: 30 ft inside diameter, 18 ft at the wall, 4 ft centered peak, flat floor, 100% packing. Radius = 15 ft. Cylinder = π × 15² × 18 = 12,723.5 ft³. Peak = π × 15² × 4 ÷ 3 = 942.5 ft³. Total = 13,665.9 ft³. At 0.8 bu/ft³ the result is 10,932.7 bu; at 0.803564 bu/ft³ it is 10,981.4 bu.
Partially filled 30-foot bin and remaining capacity
Inputs: 30 ft diameter, 18 ft eave, 12 ft current level depth, flat floor, no roof allowance. Current cylinder = π × 15² × 12 = 8,482.3 ft³, or 6,785.8 bu at 0.8 and 6,816.1 bu at 0.803564. The 18 ft level maximum is 10,178.8 or 10,224.1 bu, leaving 3,392.9 or 3,408.0 bu respectively.
30-foot bin with a 5-foot inverted withdrawal cone
Inputs: 30 ft diameter, 18 ft wall depth, 5 ft inverted depth. Cylinder = 12,723.5 ft³. Withdrawal cone = π × 15² × 5 ÷ 3 = 1,178.1 ft³. Current volume = 11,545.4 ft³, or 9,236.3 bu at 0.8 and 9,277.4 bu at 0.803564.
30-foot bin with a 6-foot truncated hopper
Inputs: 30 ft bin and hopper-top diameter, 3 ft outlet diameter, 6 ft hopper depth, 18 ft level grain above the rim. Hopper radii are 15 and 1.5 ft. Frustum = π × 6 × (15² + 15 × 1.5 + 1.5²) ÷ 3 = 1,569.2 ft³. Add the 12,723.5 ft³ cylinder for 14,292.7 ft³ total: 11,434.1 bu at 0.8 or 11,485.1 bu at 0.803564.
Common measurement mistakes
- Using outside instead of inside diameter.
- Using roof height instead of grain depth at the wall.
- Entering diameter where a source formula expects radius.
- Ignoring a raised floor, hopper rim or outlet reference.
- Treating an uneven surface as a perfect cone or pyramid.
- Rounding intermediate volume before the final answer.
Calculation Basis and References
The review checked geometry formulas, U.S. dry-bushel conversion, unit conversion, hopper partial-fill integration, reverse height solving, input limits and representative edge cases. No independent agricultural-engineering certification or review is claimed.
- Iowa State University Extension: Storage Capacity for Grains, Forages, and Liquids — round-bin geometry, 0.8 bu/ft³, the 0.628 diameter shortcut and one-third peak method.
- Iowa State University Extension: Grain Handling Equipment Safety — approximate 21° corn and 23° soybean angles of repose and grain-entry safety context.
- University of Minnesota Extension: Selecting Fans and Determining Airflow for Grain Bins — a worked round-bin capacity example using 0.8 bu/ft³.
- USDA Agricultural Marketing Service: Practical Procedures for Grain Handlers — 2,150.42 in³ U.S. bushel volume and test-weight definition.
- USDA Risk Management Agency: Corn Loss Adjustment Standards Handbook — example of a formal inventory method using crop-specific combined test-weight and pack factors; it is not a universal manual percentage.
Change history: v2.0.0 (31 August 2026) added task modes, rectangular storage, angle peaks, measurement diagram, frustum and partial hopper geometry, separated adjustments, target-height solving, reference table, share/print actions, revised examples and accessibility refinements. These references support the stated methods; they do not certify a particular structure or grain condition.
Grain Bin Capacity Calculator FAQs
How many bushels are in a 30-foot grain bin?
A 30-foot-diameter bin adds about 565.5 bushels per foot of level grain using 0.8 bu/ft³. At 18 feet level it holds about 10,179 bushels; a 4-foot centered peak adds about 754 bushels, for about 10,933 bushels.
How many bushels does one foot of grain add?
For a round bin, bushels per foot of level depth are approximately 0.628 × diameter² when diameter is in feet and the 0.8 bu/ft³ farm basis is used. A 30-foot bin therefore adds about 565.5 bushels per foot.
How do I calculate remaining bin capacity?
Estimate maximum usable bushels from eave height, roof allowance and hopper volume, then subtract adjusted current bushels. If a manufacturer-rated capacity is entered, this calculator uses that rating as the maximum for remaining capacity and percent full.
How do I estimate peak height from angle of repose?
For a centered round-bin peak, peak height equals bin radius × tan(angle). This tool starts at 21° for corn or 23° for soybeans, keeps the angle editable, and caps the peak at the available roof clearance.
How accurate is a grain-bin measurement?
Geometry can be precise, but field inventory is still an estimate. Inside-diameter error is squared, and uneven grain, corrugations, ducts, packing, floor elevation and uncertain hopper dimensions can move the result by several percent.
Does moisture or test weight change the estimate?
Neither changes physical bin volume. Pounds per bushel changes only reported mass here. Moisture is recorded in the summary but is not used as a shrink factor. Apply only a documented crop-specific inventory or packing method when one is required.
What is the difference between U.S. and Imperial bushels?
A U.S. dry bushel is 2,150.42 cubic inches, about 35.239 litres. An Imperial bushel is about 36.369 litres, roughly 3.2% larger. This calculator reports U.S. dry bushels.