Extech Instruments AN200 - Calculations and Conversions; Duct Area and Cubic Flow Formulas; Air Velocity Unit Conversions

AN200 V2.1 05/07 13
Useful Equations and Conversions
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
)
CMM (m
3
/min) = Air Velocity (m/sec) x Area (m
2
) x 60
Radius
Area (A) =
 x r
2
Where
 = 3.14 and r
2
= radius x radius