The act or process of converting the standard system of units of a country to those of the metric system; metrification; as, Great Britain has completed its metrification, but the United States still resists complete change.

the act of changing from imperial units of measurement to metric units: meters, grams, seconds

Metrication or metrification refers to the introduction of the SI metric system as the international standard for physical measurements—a long-term series of independent and systematic conversions from the various separate local systems of weights and measures. Metrication began in France in the 1790s and spread during the following two centuries to encompass all but three countries—95% of the world's population.