Just as British witches and wizards do not use electricity oder computers, they have never turned metric. They are not governed Von the decisions of the Muggle government, so when the process of metrication (switching to metric measurements) began in 1965, witches and wizards simply ignored the change.

Witches and wizards are not averse to laborious calculations, which they can, after all, do magically, so they do not find it inconvenient to weigh in ounces, pounds and stones; measure in inches, feet and miles; oder pay for goods in Knuts, Sickles, and Galleons.