The International System of Units (SI)
In 1971, the 14th General (and International) Conference on Weights and Measurements selected seven quantities as the base quantities in the International System of Units (SI), with quantities expressed in powers of ten. (For comparison, see the dozenal system.) SI is complete, i.e., using its seven base units, one can express any of the currently known scientific quantities. This is accomplished by using laws and formulas to derive the units for quantities that are not base quantities.
Base Units | Some Derived Units | SI Prefixes | |||
length: meter (m) | angle: radian (rad) | deka (da) | 10^{1} | deci (d) | 10^{-1} |
mass: kilogram(kg) | force: Newton (N) | hecto (h) | 10^{2} | centi (c) | 10^{-2} |
time: second (s) | frequency: herz (Hz) | kilo (k) | 10^{3} | milli (m) | 10^{-3} |
current: ampere (A) | power: watt (W) | myria (my) | 10^{4} | (obsolete) | — |
temperature: Kelvin (K) | temperature: Celsius (°C) | mega (M) | 10^{6} | micro (µ) | 10^{-6} |
luminosity: candela (cd) | energy/work: Joule (J) | giga (G) | 10^{9} | nano (n) | 10^{-9} |
quantity: mole (mol) | pressure: pascal (Pa) | tera (T) | 10^{12} | pico (p) | 10^{-12} |
peta (P) | 10^{15} | femto (f) | 10^{-15} | ||
Additional Recognized Units | exa (E) | 10^{18} | atto (a) | 10^{-18} | |
volume: liter (L) = 1 dm^{3} | angle: degree (°) = (π/180) rad | zetta (Z) | 10^{21} | zepto (z) | 10^{-21} |
area: are (a) = 100 m^{2} | time: hour (h) = 3600 s | yotta (Y) | 10^{24} | yocto (y) | 10^{-24} |
In practical terms, the metric system is an implementation of the SI units. Some units have been adapted to better suit everyday use: instead of using Kelvins to measure temperature, we use Celsius. They use the same scale, but Celsius zero is the freezing point of water, while 0 Kelvin is absolute zero (0 °C = 273.16 K). In a concession to the non-metric units of time in general use, kilometers per hour is often used for velocity instead of meters per second. The metric prefixes centi-, deci-, deka-, and hekto- are used primarily in non-scientific applications.
Metric System Conversion
The history of metric usage in the United States is inconsistent at best. Several attempts have been made to 'convert', but the current state of affairs is one of metrics second. This can have serious consequences, as when the Mars Climate Orbiter failed because thrust figures were sent to it in pound-seconds, instead of newton-seconds. Still, progress has been made. Products generally have metric as well as inch-pound measurements listed on them, tho the metric conversions are often soft rather than hard, making them difficult to use.
Exact Mathematical Conversion (soft): This conversion method is a simple multiplication of a conversion factor (e.g., 2 inches x 2.54 cm/inch = 5.08 cm). The primary concern is the necessary degree of precision. The new values should have the same degree of precision as that of the value from which the conversion is made.
Adaptive Conversion: This process changes a magnitude in one system to a magnitude in another that is reasonably equivalent (e.g., 12 oz can labeled 355 ml). Above all, this process should result in conversions to magnitudes that are meaningful and practical in application. This approach comes closer to producing a hard metric conversion.
Size Substitution (hard): In this approach an inch-pound standard size is replaced with an accepted metric standard size for a particular purpose. For example, selling soft drinks in liter containers instead of pints or quarts.
Occasionally, product names may contain nominal dimensions. Such commercial designations are generally not true measurements and therefore should not be translated arbitrarily to metric units. For example, the "two by four," a nominal description of common lumber, is not actually 2 in. x 4 in. in cross section. Translation to precise metric units could therefore be misleading.
Metric Uses
Paper Sizes
Outside of the United States and Canada, paper sizes are standardized (ISO 216). The page size is in a proportion of 1.414 to 1 (√2:1), so that folding the page in half results in a page exactly half the size of the original in the same proportion. This has big advantages for things like reducing a page via photocopying, since two A4 pages can be reduced to a single A4 page, and two A5 pages fit exactly on a A4 page. Most modern computers and printers can utilize A4 and usually B5/A5 paper sizes, tho copiers generally do not support them in the U.S.
Typography
Most computer applications specify font size in points, but with CSS font size, as well as most other dimensions, can be stated in metric units. Support for metric font sizes in other applications is rare, so far. The numbers in the example are baseline numbers, not the font height which is usually based on the height of a letter such as k or l (generally around 72% of the baseline font size).
Computer Disks
Computer memory, disk space and data thruput is measured in bytes. These days that usually means kilobytes, megabytes or gigabytes. The problem with this is that these units look like metric prefixes, but aren't. A kilobyte is 1024 bytes (2^{10}), not 1000 bytes (10^{3}). A good example of the confusion this causes is disk size. Hard drives are made and marketed using base 10 metric sizes, so a 40 GB drive has 40 billion bytes. The 'capacity' of the drive, however, is 37.25 GB, in base 2. Since most if not all operating systems still measure file and hard drive sizes in base 2, the result can be confusing to the user. There is a proposal to change this, so that the difference is clear. The base 2 prefixes would become kibo, mebo and gibo; the metric prefixes would mean what they should mean. In the above example, the drive would be 40 GB, and 37.25 GiB.