# 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.

QUANTITY |
UNIT |
SI SYMBOL |

Length |
Meter | m |

Mass |
Kilogram | kg |

Time |
Second | s |

Electric Current |
Ampere | A |

Temperature |
Kelvin | K |

Substance |
Mole | mol |

Luminous Intensity |
Candela | cd |

## 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.

UNIT |
SYMBOL |
QUANTITY |
SI BASE UNIT |

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)(m^{2})(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, m^{3}/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.

QUANTITY |
SI UNIT |
CUSTOMARY U.S. UNIT |
CONVERSION FACTOR |

Length |
millimeter (mm) | inch (in) | 1 in = 25.4 mm |

Pressure (above atmospheric) |
bar (bar g or bar a) | pounds per in^{2} (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 (dm^{3}/sec) |
cubic feet per minute (cfm) | 1 dm^{3}/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 |