^{The Metric System}

Important dates in the history of the modern metric system (S.I.):

1670  metric system originated on about this date. Gabriel Mouton, a French vicar

1790  Thomas Jefferson proposed a decimal-based measurement system for the United States.

1792  The U.S. Mint was formed to produce the world's first decimal currency (the U.S. dollar consisting of 100 cents).

1866  The use of the metric system made legal (but not mandatory) in the United States by the Metric Act of 1866 (Public Law 39-183).  This law also made it unlawful to refuse to trade or deal in metric quantities.

1975  The Metric Conversion Act of 1975 (Public Law 94-168) passed by Congress. The Act established the U.S. Metric Board to coordinate and plan the increasing use and voluntary conversion to the metric system. However, the Act was devoid of any target dates for metric conversion.

1979  BATF requires wine producers and importers to switch to metric bottles in seven standard [liter and milliliter] sizes.

1983  The meter is redefined in terms of the speed of light by the 17th CGPM, resulting in better precision but keeping its length the same.

1988  The Omnibus Trade and Competitiveness Act of 1988 amended and strengthened the Metric Conversion Act of 1975, designating the SI metric system as the preferred measurement system, and requiring each federal agency to be metric by the end of fiscal year 1992.

1991  President George Bush signed Executive Order 12770, Metric Usage in Federal Government Programs directing all executive departments and federal agencies implement the use of the metric system. The Executive Order is also available as an appendix to: Interpretation of the SI for the United States and Federal Government Metric Conversion Policy

1994  The Fair Packaging and Labeling Act (FPLA) was amended by the Food and Drug and Administration (FDA) to require the use of dual units (inch-pound AND metric) on all consumer products.

1996  As of July 1996 all surface temperature observations in National Weather Service METAR/TAF reports are now transmitted in degrees Celsius.

2001 April 09  U.S. Stock Exchanges changed to decimal trading. The Securities and Exchange Commission has ordered that all stocks must be quoted in dollars and cents rather than fractions by this date. The switch to decimal trading brought the U.S. in line with the rest of the world's major exchanges. This follows the change of the Canadian Stock Exchanges to decimal trading in 1996.

One of the principal advantages of the Metric System is its use of only one unit (standard of size) for each parameter (thing that is being measured). The metric units are interrelated in such a way that only seven fundamental standards are necessary. These fundamental standards are identified with an * in the tables below.

The 7 Standard Metric Units

Second - time
kilogram - mass
degrees kelvin - temperature
meter - distance

candle (candela) - light

ampere - current and resistance

mole - amount of substance

MECHANICS

HEAT

LIGHT

ELECTRICITY & MAGNETISM (There are no English electrical units)

Each physical quantity (length, mass, volume, etc.) is represented by a specific SI unit. That unit is made larger or smaller by addition of a prefix to the stem unit.

Commonly used metric system units and symbols:

*The gram (g) is the stem unit to which other prefixes are added.
**The kelvin (K) is the SI base unit of thermodynamic temperature.
***The capital el (L) is preferred as the symbol for liter in the USA; however the lower case el (l) also is correct and is used in many metric countries.

Derived Units

Commonly used metric prefixes

Some special relationships:

• 1 milliliter = 1 cubic centimeter
• 1 milliliter of water has a mass of approximately 1 gram
• 1 liter of water has a mass of approximately 1 kilogram
• 1 cubic meter of water has a mass of approximately 1 metric ton

Legal/official (exact) definitions of inch-pound units as set by U.S. law:

• 1 inch = 25.4 millimeters
• 1 pound = 453.592 37 grams (453.6 g)
• 1 gallon = 3.785 411 784 liters (3.79 L)

Note: In Canada the inch and the pound are defined identically, but 1 Canadian gallon = 4.546 09 liters.

Approximate conversion factors between inch-pound units and the International System of Units (SI):

• Multiply inches by 2.54 to get centimeters (this conversion factor is exact)
• Multiply feet by 0.305 to get meters
• Multiply miles by 1.6 to get kilometers
• Divide pounds by 2.2 to get kilograms
• Multiply ounces by 28 to get grams
• Multiply fluid ounces by 30 to get milliliters
• Multiply gallons by 3.8 to get liters
  

Some baseline temperatures in the three temperature scales

Temperature conversions between the three temperature scales:

kelvin / degree celsius conversions (exact):

• kelvins = degrees celsius + 273.15
• degrees celsius = kelvins - 273.15

degree Fahrenheit / degree celsius conversions (exact):

• degrees F = degrees C x 1.8 + 32.
• degrees C = (degrees F - 32.) / 1.8

A degree celsius memory device:

There are several memory aids that can be used to help the novice understand the degree Celsius temperature scale. One such nemoic is:

When it's zero it's freezing,
when it's 10 it's not,
when it's 20 it's warm,
when it's 30 it's hot!

Or, another one to remember:

30's hot
20's nice
10's cold
zero's ice

Examples of everyday item equivalences for metric units of length, mass, volume, and temperature

(Drawings excerpted from the book, Quick Guide to the Metric System, by Valerie Antoine, USMA Executive Director)

More detail is available in The SI Brochure at

or at