source : http://www.us-metric.org/
The US Metric Association (USMA), Inc., is a national non-profit organization that was founded in 1916. USMA advocates completing the US conversion to the International System of Units, known by the abbreviation SI (“ess-eye”) and also called the metric system. The process of changing measurement units to the metric system is called metric transition or metrication.
In the 1970s, most British Commonwealth nations changed from the Imperial system of units to the metric system. Several of these nations rapidly and successfully made the metric transition, thanks to strong government encouragement and support. In addition, other countries have also made the change.
The chart (below) shows the chronology of the advance of metric usage around the world. A second chart (below) from Hector Vera shows the number of countries adopted the metric system.
Advance of metric usage in the world
The chart (below) shows when various countries adopted the metric system and is based on a survey conducted by USMA many years ago. The chart shows the countries’ names at the time; some have since changed names. And, because metrication is an evolutionary process that takes place over time, any attempt to assign a single year to a country’s conversion is only an approximation. Outside of what is shown on the chart (below) most other details of that survey have been lost. However, since the original development of the chart, the information on Jamaica has been added.
Number of countries that have adopted the metric system
A second chart and a table from Hector Vera (below) show the number of countries that have voluntarily adopted the metric system, both by year and cumulative.