# Guide to SI Units

The International System of Units (SI) has been adopted in the publications of several scientific and technical societies in the United States and other countries. It is expected that in due course that these units will come into general use. The SI units and conversion factors applicable to this book are given below. For further information, see “The International System of Units (SI),” National Bureau of Standards Special Publication 330, U.S. Government Printing Office, Washington, D.C. 20402.

Quanity | SI Unit | Symbol |
---|---|---|

Length | meter | m |

Mass | kilogram | kg |

Time | second | s |

Electric current | ampere | A |

Temperature* | kelvin | K |

* (Temperatures may also be expressed in °C)

Quantity | Unit | Symbol | Formula |
---|---|---|---|

Force | newton | N | kg·m/s^{2} |

Pressure | pascal | Pa | N/m^{2} |

Energy, heat, etc. | joule | J | N·m |

Power | watt | W | J/s |

Frequency | hertz | Hz | I (cycle)/s |

Radioactivity | becquerel | Bq | I (decay)/s |

Absorbed dose | gray | Gy | J/kg |

To convert from: | to: | multiply by: |
---|---|---|

Length, Area, Volume | ||

inch | meter (m) | 2.540 × 10^{-2} |

foot | meter (m) | 0.3048 |

yard | meter (m) | 0.9144 |

mile | kilometer (km) | 1.609 |

centimeter | meter (m) | 10^{-2} |

angstrom | meter (m) | 10^{-10} |

square inch | meter^{2} (m^{2}) |
6.452 × 10^{-4} |

square foot | meter^{2} |
9.290 × 10^{-2} |

square mile | kilometer^{2} (km^{2}) |
2.590 |

cubic foot | meter^{3} (m^{3}) |
2.832 × 10^{-2} |

Mass | ||

pound | kilogram (kg) | 0.4536 |

ounce | kilogram (kg) | 2.835 × 10^{-2} |

Energy | ||

calorie | joule (J) | 4.187 |

erg | joule (J) | 1.00 × 10^{-7} |

MeV | joule (J) | 1.602 × 10^{-13} |

ton (TNT equivalent) | joule (J) | 4.2 × 10^{9} |

Miscellaneous | ||

density (lb/ft^{3}) |
kg/m^{3} |
1.602 × 10 |

pressure (psi) | pascal (Pa) | 6.895 × 10^{3} |

radiant exposure (cal/cm^{2}) |
J/m^{2} |
4.187 × 10^{4} |

speed (ft/sec) | m/s | 0.3048 |

speed (miles/hour) | m/s | 0.4470 |

dose (rads) | gray (Gy) | 1.00 × 10^{-2} |

dose rate (rads/hour) | Gy/s | 2.778 × 10^{-6} |

curie | becquerel (Bq) | 3.700 × 10^{10} |

The only multiples or submultiples of SI to which appropriate prefixes may be applied are those represented by factors of 10^{n} or 10^{-n} where n is divisible by 3. Thus, kilometer (10^{3}m or 1 km), millimeter (10^{-3}m or 1 mm), and micrometer (10^{-6}m or 1 μm). The centimeter and gram are not used in the SI system, but they are included in the metric system proposed for adoption in the United States