### The International System of Units

The International System of Units (SI) is a modern version of the metric system used in many scientific disciplines. The following table shows some common SI units used in the agricultural industry along with their symbol and the quantities they measure.

SI Unit | Symbol | Quantity Measured |
---|---|---|

Gram |
| Mass |

Joule |
| Work, Energy, and Heat |

kelvin |
| Temperature |

Liter |
| Volume |

Meter |
| Length, Area, Volume |

Mole |
| Amount of Substance |

newton |
| Force |

Pascals |
| Pressure, stress, tensile strength |

Second |
| Time |

Watt |
| Power |

SI, like the metric system is built around a base ten system. The following table shows some SI prefixes as well as their symbol and the value they represent.

Prefix | Symbol | Value |
---|---|---|

kilo |
| 10 |

centi |
| 10 |

milli |
| 10 |

micro |
| 10 |

nano |
| 10 |

pico |
| 10 |

The following applet can help better understand how to convert between the various prefixes of SI

Starting Value: | |

Convert to: | |

Result: |

### United States Customary Units

U.S customary units are units of measurement derived from earlier English units that are commonly used in the United States today. Often referred to as standard units or common units, this is the primary system of measurement especially when it comes to length, area, and volume measurements in the United States. Most scientific applications use the metric system and many formulas are more convenient to use with SI units since it is conveniently based around factors of 10. As a result it is often necessary to convert between the two systems of measurement for making calculations or applying the results of calculations to a certain application. Since customary units are not based around factors of 10, it is not as convenient to convert between different units that measure the same quantity. The following table shows quantities to be measured some common customary units used.

Quantity Measured | Common Units |
---|---|

Mass/Weight | ounce (oz), pound (lb), ton |

Weight-time Rates | pound per unit time (lb/s, lb/min, lb/hr, lb/day, lb/year) |

Work, Energy, and Heat | British thermal unit (BTU), inch pounds (in. lb), foot pounds (ftlb), killowatt hour (kwhr), horsepower hour (hphr) |

Temperature | degrees Fahrenheit, degrees Celsius |

Volume | cubic inch (cu in.), gallon (gal), cubic foot (cu ft), cubic yard (cu yd), acre foot (acre ft) |

Volume-Flow rates | cubic feet per unit time (cu ft/s, cu ft/min,cu ft/s,cu ft/hr), gallon per unit time (gal/s, gal/min, gal/hr) |

Length | inch (in), foot (ft), yard (yd), mile |

Area | circular mil (cir mil), square inch (sq in.), square foot (sq ft), square yard (sq yd), acre |

Density (Mass/Volume) | pound per unit volume (lb/cu in, lb/ cu ft, lb/gal) ton per unit volume (ton/cu ft, ton/cu yd) |

Velocity | feet per unit time(fps, fpm, fph), mile per unit time (mpm, mph) |

Pressure | inch of water (in H2O), foot of water (ft H2O), inch of mercury (in. Hg), ounce per square inch (oz/sq in), ounce per square foot (oz/sq ft), pound per square inch (lb/sq in), pound per square foot (lb/sq ft) |

Time | second (s), minute (min), hour (hr), day, month, year |

Power | kilowatt (kw), horsepower (hp), foot pound per unit time (ftlb/s, ftlb/min, ftlb/hr), BTU per unit time (Btu/s, Btu/min, Btu/hr) |