Metrication, or metrification, is the process of converting from the various older systems of units used throughout the world (especially the "Imperial" or "American" systems that derived from the United Kingdom), to the metric or SI system.
Metrication involves not only changing the units with which quantities are measured, but also redesigning standards to use rational metric sizes. For example, a 1 inch is a rational size for a bolt if the bolt is to be measured in inches, but the metric equivalent of 25.40 mm is not a rational size, and thus the metric size of 25 mm is substituted. This requires the redesign not only of the bolt, but also of many other products and structures that would contain the bolt.
All countries in the world use the metric system to a greater or lesser extent, and most have abolished the use of non-metric units for almost all purposes. However in a few specific areas, non-metric measures retain worldwide dominance. These are notably air and sea transport, where the knot remains the prime unit of velocity for maritime navigation, and for safety and flexibility reasons aircraft flying heights are universally calculated in feet rather than metres.
The main exception to standardised metrication is the United States. Although metrication is the official policy of the United States, the progress of metrication has been much slower in the United States than in the rest of the world. The nonmetric units continue to be frequently used in everyday life, and in commerce and engineering. However, change has happened: most products within the U.S. are now labelled with both metric and nonmetric units; and an increasing number of companies and government agencies are switching to metric standards. The U.S. continues to use only miles for road distance signs. Originally U.S. legislation set October 2000 as a deadline by which states must undertake construction work and statistics in metric for states to be eligible for Federal funding, but that requirement has since been rescinded. Some states, such as California, have experimented with metric road signs, but there are as yet no plans for large-scale conversion. Metric units are generally used in scientific applications in the USA with some few exceptions like BTUs and calories.
Europe generally uses metric units for almost all purposes, and uses metric standards. Although some nonmetric units are still popularly used in some countries, they have no legal status. The United Kingdom and Ireland are the main exception, which are still in the process of phasing out the legal status of nonmetric units. Thus they use a mixture of metric and nonmetric units for different purposes. The United Kingdom is currently in the process of abolishing the use of nonmetric units; as of 2000, all loose goods sold by mass must be sold using metric units, a process that has been resisted by some leading to acts of vandalism such as the defacement of metric road signs. UK policy is to eliminate almost all nonmetric units by 2009, except for road signs. Ireland is generally more advanced than the United Kingdom, but still lags behind the rest of Europe; conversion of speed limit signs to kilometres per hour was not completed until 2001. Metrication for all EU member states is required under EU law, with a target date of 2009 by which all products in Europe must be sold only by metric units, with some limited exceptions.
Canada has converted to the metric system for most purposes, including temperature in weather reports, speed limits, road signs, and sizes of most products. However there is still significant use of nonmetric units and standards in some sectors of the Canadian economy, mainly due to the close proximity to the United States. Notable among these are stationery and construction lumber and gyproc. Retrofitting metric sized wallboard on old 16" spaced studs can be significantly difficult.
Australia and New Zealand have largely converted to the metric system, but nonmetric units are still sometimes used in popular conversation, especially to measure body height or mass. (Many Australians know their height in feet and inches but their mass in kilograms.)
With the ever increasing importance of global trade, increasing harmonization of units of measurement and standards, and thus further advances in metrication, is both necessary and inevitable. Metrication generally requires legislative action, i.e. legal requirements to use metric units in commerce, and the eventual prohibition of the use of nonmetric units. This is because businesses which convert to metric standards frequently cannot compete with those which do not, due to the preference of consumers for the familiar. Those countries which have attempted to engineer a voluntary conversion to the metric system, such as the United States, have been largely unsucessful compared to those which have made the metric system legally compulsory.
Use of multiple standards of units lead to loss of hundreds of millions of dollars when the Mars Climate Orbiter crashed into Mars. The manufacturer of the spacecraft had designed the navigation system to be programmed in English measurements. But the navigation team at NASA's Jet Propulsion Laboratory in Pasadena, California incorrectly assumed the spacecraft required metric units.