The process of metrication started in 1965, by decision of the Muggle British government, so that Great Britain would switch from the Imperial system of weights and measures to the Metric one. Unlike their fellow Muggles, the British wizarding community, at the time led by Minister Nobby Leach, simply ignored the change, as they were not averse to laborious calculations which they could, after all, do magically. That being said, wizards and witches weigh in ounces, pounds and stones; measure in inches, feet and miles; and pay for goods in Knuts, Sickles, and Galleons.