SI Units

As you probably know, temperature in the United States is measured in degrees Fahrenheit, while most of the rest of the world uses the Celsius system. Similarly, units of mass, distance, volume, and many other measurements have two, three, or more different systems used in nations around the world. Imagine the confusion if scientists in the USA, England, Germany, Russia, Japan, and other countries were all working on a project, with each measurement being recorded in several systems!

To provide one universal set of units, the Système Internationale (French for International System) was established by the 11th General Conference on Weights and Measures in 1960. Most of the units in the SI come from the metric system used in Europe, Asia, and other nations outside the US. With the SI standard, even research done in America is recorded in these units, meaning no conversion is necessary to share data with scientists from other nations, saving time and data accuracy.

The table below provides SI units for many common measurement types. In future sections, we may occasionally depart from the SI; in these cases, we will provide conversion equations and explain why the alternate unit is preferable.

Measurement	SI Unit (abbreviation)
Distance	Meter (m)
Volume	Liter (l)
Mass	Kilogram (kg)
Amount of Substance	Mole (mol)
Time	Second (s or sec)
Force	Newton (N)
Pressure	Pascal (pas)
Temperature	Kelvin (K)
Electric Current	Ampere (A or amp)
Electric Charge	Coulomb (C)
Energy / Work	Joule (J)

Dozens of other SI units exist for more uncommon measurements, including light intensity and thermal conductivity. For a complete list and formulas for conversion between SI and other unit types, see our extensive "Reference" section.

Example Problem 1 Identify whether the following units are from the SI. If not, give the equivalent SI unit. A. Mile B. Gram C. Gallon D. Yard E. Joule F. Degree Celsius Answers: A. No--the mile is the US measurement of distance. The SI unit is the meter. B. Yes--the gram is the SI unit of weight. C. No--the gallon is the US measurement of liquid volume. The SI unit is the liter. D. No--the mile is another US unit of distance. The SI unit is also the meter. E. Yes--the Joule is the SI unit of energy and work (they are interchangeable). F. No--the Degree Celsius is the metric unit of temperature. The SI unit is the Kelvin.

The SI system provides a convenient mechanism for changing the size of the basic units. For example, expressing the distance between Paris and Beijing in meters would be awkward. To convert a basic unit into larger and smaller measurements, such as the distances between atoms or stars, you may attach a prefix to the unit. The prefixes for SI units are listed in the table below.

Prefix	Multiplication Factor
Yotta- (Y)	One Septillion (1 x 1024)
Zetta- (Z)	One Sextillion (1 x 1021)
Exa- (E)	One Quintillion (1 x 1018)
Peta- (P)	One Quadrillion (1 x 1015)
Tera- (T)	One Trillion (1 x 1012)
Giga- (G)	One Billion (1 x 109)
Mega- (M)	One Million (1 x 106)
Kilo- (k)	One Thousand (1 x 103)
Hecto- (h) *	One Hundred (1 x 102)
Deca- (da) *	Ten (1 x 101)
Deci- (d) *	One Tenth (1 x 10-1)
Centi- (c) *	One Hundredth (1 x 10-2)
Milli- (m)	One Thousandth (1 x 10-3)
Micro- (μ)	One Millionth (1 x 10-6)
Nano- (n)	One Billionth (1 x 10-9)
Pico- (p)	One Trillionth (1 x 10-12)
Femto- (f)	One Quadrillionth (1 x 10-15)
Atto- (a)	One Quintillionth (1 x 10-18)
Zepto- (z)	One Sextillionth (1 x 10-21)
Yocto- (y)	One Septillionth (1 x 10-24)

* Rarely used; most often, 2 decaliters is expressed as 20 liters, for example. The example problems will usually give answers as 20 liters.

Note that kelvins, moles, and seconds are not used in this system. Time measurements have a different system, which you should be familiar with! Kelvins and moles are simply written in scientific notation (3.7 x 10 K) or using the name of the number in longhand (two thousand kelvins). The table of units in the Reference section notes which units are not "expandable."

Example Problem 2 Convert the following measurements to or from the condensed SI notation. Example: 2 kilojoules = 2000 joules, .0045 liters = 45 milliliters or 4.5 centiliters. A. Seven kilometers B. Three trillion, seventy million newtons C. 4.00006 gigapascals D. 17 septillionths of a gram Answers: A. Seven thousand meters B. 1.07 teranewtons C. 4000060000 pascals D. 17 yoctograms (now that's a tiny particle!!)

Feel free to bookmark or print these tables (a more extensive copy of the units table is found in the "Reference" section) to use as handy guides. Next, we'll take a look at significant figures and accuracy in measurement; while you may be able to interpret an attogram, it's useless if your weighing apparatus is only good to the thousandths place.

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