Formula Used
Bin floor area = π × (diameter ÷ 2)2.
Grain volume = floor area × grain depth × fill level.
Bushels = grain volume ÷ 1.244456.
Total airflow = bushels × target CFM per bushel × safety margin factor.
Static pressure estimate = 0.45 × depth factor × airflow factor × crop factor × compaction factor × floor factor × elevation factor + duct loss.
Brake horsepower = CFM × static pressure ÷ (6356 × fan efficiency).
The static pressure equation is a planning estimate. Always compare the final result with a real fan curve before purchase.
How to Use This Calculator
Enter the grain type, bin diameter, grain depth, fill level, and target airflow. Add the number of fans and expected fan efficiency. Include duct loss, floor open area, compaction, and elevation for a better estimate. Use manual pressure only when you already know the grain bed pressure. Press the calculate button. The result appears above the form and below the header section.
Example Data Table
| Use case |
Grain |
Bin diameter |
Depth |
Target airflow |
Typical note |
| Storage aeration | Corn | 36 ft | 18 ft | 0.10 CFM/bu | Temperature control |
| Fast cooling | Wheat | 42 ft | 22 ft | 0.25 CFM/bu | Post-harvest cooling |
| Natural air drying | Corn | 36 ft | 20 ft | 1.00 CFM/bu | Moderate drying |
| Low temperature drying | Soybeans | 30 ft | 16 ft | 1.50 CFM/bu | Stronger airflow |
Why Fan Size Matters
A grain bin fan controls more than comfort. It drives air through kernels, removes heat, and supports safe storage. A weak fan may leave wet pockets near the center. An oversized fan can waste energy and create uneven drying. Correct sizing starts with bushels, crop depth, airflow target, and static pressure.
Airflow and Grain Depth
Fan demand rises as grain depth rises. Deep grain beds resist air movement. Fine material, broken kernels, and compacted layers add more resistance. The calculator estimates bushels from bin diameter, fill depth, and fill level. It then multiplies bushels by the selected airflow rate. Low airflow suits maintenance aeration. Higher airflow supports faster cooling or drying.
Static Pressure Planning
Static pressure is the force the fan must overcome. It is commonly expressed in inches of water column. The estimate used here combines air speed through the floor, grain depth, crop resistance, floor restriction, and duct losses. This is a planning value. Final fan selection should be checked against the manufacturer fan curve at the required pressure.
Horsepower and Efficiency
Horsepower depends on airflow, pressure, and fan efficiency. Better efficiency reduces power demand. Safety margin helps cover dirty grain, weather changes, screens, transitions, and small measurement errors. The result shows total airflow, airflow per fan, estimated pressure, total brake horsepower, and suggested motor size. A higher standard motor size is often selected.
Using Results Wisely
Use the output as a design guide. Compare several airflow targets before buying equipment. For long term storage, gentle aeration may be enough. For harvest moisture removal, stronger airflow may be needed. Check electrical supply, fan type, noise, vent area, and roof exhaust capacity. Grain condition also matters. Level the grain surface. Clean fines when possible. Inspect floors and ducts. Monitor temperature and moisture during operation. Good fan sizing works best with regular checking and safe bin practices.
Practical Safety Notes
Never enter a bin while grain is moving. Lock out power before service work. Confirm guard placement, weather covers, and wiring ratings. Provide enough roof vents for the planned airflow. If grain smells musty or heats quickly, stop and investigate before forcing more air. Record readings daily so pattern changes become visible before losses grow quickly.
FAQs
What is CFM per bushel?
It is the airflow delivered for each bushel of grain. Low values help maintain storage. Higher values support cooling or drying. The correct target depends on crop, moisture, weather, and management goals.
Why does grain depth affect fan size?
Deep grain creates more resistance. The fan must push air through a longer packed bed. More depth usually means higher static pressure and more horsepower for the same airflow target.
Can I use one large fan instead of several smaller fans?
Yes, but compare fan curves and bin layout. Multiple fans may spread air better. One large fan can simplify controls. Electrical service, noise, transitions, and airflow balance should be checked.
What is static pressure?
Static pressure is resistance to airflow. Grain depth, crop type, fines, floor openings, ducts, and transitions add resistance. Fans must be selected at the required pressure, not free air output.
Why is fan efficiency needed?
Efficiency converts airflow and pressure into motor power. A less efficient fan needs more horsepower. The calculator uses efficiency to estimate brake horsepower and suggest a practical motor size.
Should I include a safety margin?
Yes. Grain is rarely perfect. Fines, uneven fill, weather changes, and duct losses can reduce actual performance. A modest margin helps prevent undersizing during real field conditions.
Does this replace a manufacturer fan curve?
No. Use this calculator for planning and comparison. Final fan selection should be matched to the manufacturer curve at the calculated airflow and static pressure point.
How much roof vent area do I need?
The calculator gives a simple free-area guide. Many designers start near one square foot per 1,000 CFM. Actual vent needs depend on vent type, screen restriction, weather, and roof design.