5
Useful Equations and Conversions
Volume Measurements
To determine CFM (cubic feet per minute) or CM M (cubic meters per minute) in a duct, the
area of the duct must first be measured (use the equations below). Then multiply an air
velocity measurement by the area measurement to obtain CFM or CMM.
Area equation for rectangular or square ducts
Area equation for circular ducts
Cubic equations
NOTE:
Measurements made in
inches
must be converted to
feet
or
meters
before using the above formulae.
Unit of Measure Conversion Table
m/s
ft/min
knots
km/h
MPH
1 m/s
1
196.87
1.944
3.6
2.24
1 ft/min
0.00508
1
0.00987
0.01829
0.01138
1 knot
0.5144
101.27
1
1.8519
1.1523
1 km/h
0.2778
54.69
0.54
1
0.6222
1 MPH
0.4464
87.89
0.8679
1.6071
1
Width (W)
Height (H)
Area (A) = Width (W) x Height (H)
CFM (ft
3
/min) = Air Velocity (ft/min) x Area (ft
2
)
Radius
Area (A) = x r
2
Where = 3.14 and r
2
= radius x radius
