Measurements (pottermore.com)

Just as British witches and wizards do not use electricity or computers, they have never turned metric. They are not governed by 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. Some of them (e.g., Arthur Weasley) would have been interested and intrigued; most would have been indifferent, and some (e.g., Lucius Malfoy) would have spat in the face of anyone who asked him for the length of his wand in centimetres, because they would have been revealing themselves as a blood traitor.

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; or pay for goods in Knuts, Sickles, and Galleons.