Duct airflow calculator
Aeration Fan Calculator
OSU Biosystems Engineering

This calculator provides an estimate of static pressure as air flows through ducts in a grain aeration or closed-loop fumigation system.  Calculations are based on air at 60 F and standard atmospheric pressure through constant crosssection ducts.   If more accurate estimates are needed consult a professional engineer.  This calculator is provided as is and OSU assumes no responsibility for results obtained. The parameters needed to estimate stastic pressure are discussed below in the order that they appear on the calculator web page.

Enter the dimensions of the duct in inches. First select the cross-section shape of the duct, and then fill in the dimensions in the boxes below the selected duct shape in inches.


Enter the Length of Duct in feet. The total length of duct should be entered here.  If the complete duct system consists of different sizes or shapes of ducts connected together, the pressure drop through the complete duct system can be calculated by calculating the pressure loss for each section of duct and then adding them together.

Select duct material.  Pressure drop due to friction is partially dependant on the roughness of the inside of the duct.  As the surface roughness increases so does pressure drop.  Roughness values for each material are:

Duct material

roughness (ft)

PVC or Plastic




Concrete or wood


Corrugated plastic drain pipe


Enter the number of each type of fitting in the boxes below.  Bends and discontinuities in the ducts also increase the resistance to airflow, and thus the static pressure, in a duct system.  There is also a static pressure due to air entering or exiting a duct system.  The minor losses from all fittings are added and used in the final calculation of static pressure for the duct system.

Fitting type

Minor loss per fitting

90 degree elbow


30 or 45 degree elbow




Ball Valve (fully open)


Duct entrance


Duct exit



Enter the flow rate in cubic feet per minute through the duct.  As airflow rate increases so does the static pressure per unit length of duct. 

CALCULATE button uses the information entered above to calculate the following estimates using the indicated equations/methods:


Velocity in duct: CFM/Duct cross-sectional area plus appropriate unit conversions.


Reynolds number: This dimensionless number is used to calculate the friction factor (f) and also determine if the flow is laminar (Re<2100) or turbulent (Re>2100). 

Re=Density*diameter*velocity/viscosity;    Density=2.38X10-3Slugs/ft3, Viscosity=3.74x10-7lb-s/ft2.  For non-circular ducts, “diameter” is replaced by hydraulic diameter Dh= 4*area/perimeter.


Reynolds friction factor (f): If Re<2100 then f=Re/64, otherwise

f=0.25/[Log(roughness/3.7D + 5.74/Re0.9)]2


Static pressure in duct is calculated using the following equation:

Static pressure = density*V2/(2) [f*L/D* + sum of minor losses]