**How do you measure something really, really small?**

If we are talking about a cell, then not with a ruler. Units must exist that can describe sizes many times smaller than the smallest marking on a ruler.

### Units of Measurement

The measurements that scientists use are based on the **International System of Units (SI),** which is a form of the metric system. The term *SI* is shortened from the French term *Le Système international d'unités*. It is the world's most widely used system of units, both in science and business. It is useful to scientists because it is based on multiples of 10. The SI was developed in 1960 from an older metric system and is used in almost every country.

The SI is not static, as the technology of measurement progresses, units are created and definitions are changed through international agreement among many nations. The international system of units is made up of a seven base units, shown in the **SI Base Units Table** below. From these seven base units several other units are derived.

Name |
Symbol |
Quantity |
---|---|---|

meter | m | length |

kilogram | kg | mass |

second | s | time |

ampere | A | electric current |

kelvin | K | thermal energy (temperature) |

mole | mol | amount of substance |

candela | cd | uminous intensity |

A prefix may be added to SI units to make a multiple of the original unit. An **SI prefix** is a name or symbol that is put before a unit of measure (or its symbol) to form a decimal or a multiple of the unit. For example, *kilo*- is a multiple of a thousand and *milli*- is a multiple of a thousandth, so there are one thousand *millimeters* in a meter, and one thousand meters in a *kilometer*. All prefixes are multiples of 10, as you can see from the **SI Prefixes Table** below. The prefixes are never combined; a millionth of a kilogram is a *milligram* not a *microkilogram*.

Name | Symbol | Factor of 10 | |
---|---|---|---|

tera- |
T | 1,000,000,000,000 (10^{12}) |
trillion (thousand billion) |

giga- |
G | 1,000,000,000 (10^{9}) |
billion (thousand million) |

mega- |
M | 1,000,000 (10^{6}) |
million |

kilo- |
k | 1000 (10^{3}) |
thousand |

hecto- |
h | 100 (10^{2}) |
hundred |

deca- |
da | 10 (10^{1}) |
ten |

deci- |
d | 1 (10^{-1}) |
tenth |

centi- |
c | 0.1 (10^{-2}) |
hundredth |

milli- |
m | 0.01 (10^{-3}) |
thousandth |

micro- |
µ | 0.00001 (10^{-6}) |
millionth |

nano- |
n | 0.00000001 (10^{-9}) |
billionth |

pico- |
p | 0.00000000001 (10^{-12}) |
trillionth |

### Summary

- The measurements that scientists use are based on the International System of Units (SI), which is form of the metric system. Based on multiples of ten, it is the world's most widely used system of units, both in science and business.

### Review

- What is SI?
- Why is it important that scientists use common units of measurement?
- Which one of the following units of measurement would be the most appropriate in determining the mass of a banana? Kilograms, micrograms, or grams.
- What is the standard SI unit for measuring volume, weight, time, and length?
- What is the shorthand unit for .000056 grams, 5,600,000,000 seconds, .56 liter, and 560 meter?