Units Used in Hydraulic and Pneumatic Calculations | Conversion Factors

Standard International (SI) Units

Standard International Units define the modern metric measurement system. All other metric units of physical properties are derived from these seven basic units.

Length Meter m
Mass Kilogram kg
Time Second s
Electric Current Ampere A
Temperature Kelvin K
Substance Mole mol
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Named SI Units Commonly Used in Hydraulic and Pneumatic Calculations

The International System of Units assigns special names to 22 units derived from the Standard Units. Below are the Named SI units most commonly used in hydraulic and pneumatic calculations, expressed in SI Units.

Hertz Hz Frequency s-1
Radian rad Angle m/m
Newton N Force and Weight (kg)(m)(s-1)
Pascal Pa Pressure and Stress (kg)(m-1)(s-2)
Watt W Power (kg)(m2)(s-3)
Degrees Celsius ˚C Temperature (relative to 273.15 K, freezing point for water) K

Additional units were derived from both SI Units and Named Units. Examples relevant to hydraulics and pneumatics include flow (volume per time, m3/s) and rotational speed (angle per second, rad/s). 

Conversions Between SI Units and US/English Units

Industries in the U.S. have not adapted SI units or the metric system. Below are definitions of metric and U.S. quantities commonly used in hydraulic and pneumatic calculations, as well as conversion factors.

Length millimeter (mm) inch (in) 1 in = 25.4 mm
Pressure (above atmospheric) bar (bar g or bar a) pounds per in2 (PSIG or PSIA) 1 bar = 14.5 PSI
Pressure (below atmospheric) mm of mercury (mm Hg) inches of mercury (in Hg) 1 in Hg = 25.4 mm Hg
Flow (liquid) liters per minute (l/min) U.S. gallons per minute (U.S. GPM) 1 U.S. GPM = 3.79 l/min
Flow (gas) cubic decimeters per second (dm3/sec) cubic feet per minute (cfm) 1 dm3/sec = 2.12 cfm
Force newton (N) pound (f) or lb (f) 1 lb (f) = 4.44 N
Mass kilogram (Kg) pound (m) or lb (m) 1 kg = 2.2 lb (m)
Time second (sec) second (sec)  
Volume (liquid) liter (l) gallon (U.S. gal) 1 U.s. gal = 3.79 l
Temperature degrees Celsius (˚C) degrees Fahrenheit (˚F) ˚C = 5/9(˚F) - 32; ˚F = 9/5(˚C) +32
Torque newton - meters (Nm) foot - pounds (ft-lbs)  
Power watt (W) horsepower (HP) 1 HP - 746 W
Rotational Speed revolutions per minute (RPM) revolutions per minute (RPM)  
Freequency hertz (Hz) cycles per second (cps) 1 Hz = 1 cps
Displacement milliliters per revolution (ml/rev) cubic inches per revolution (cir) 1 ml/rev = 0.061 cir
Kinematic Viscosity centistokes (cSt) Saybolt Universal Seconds (SUS or SSU) 1 cSt = SUS + 4.635
(approximate for SUS 225 & over)
Velocity meters per second (m/s) feet per second (fps) 1 m/s = 3.28 fps