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.
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.
When the manuscript of Harry Potter and the Philosopher’s Stone was first accepted for publication in Britain, the copy editor advised me that all weights and measures would be changed to metric, which was the publisher’s standard practise. I refused to allow the change because, for the reasons stated above, there was no logic to the thing. However, this ought not to be taken as any kind of political statement on the part of the author. I am not anti-European; on the contrary, I am all for Britain being part of Europe, and I am part French myself. Nor do I have anything against the metric system, which is much more logical than the imperial, and which certainly makes baking much easier. However, I do find the old system much more picturesque, much quirkier, and therefore more appropriate to the kind of society I was describing.
The decision to keep the imperial system in the book had an unexpected sequel, which was an invitation to join the British Weights and Measures Association. As I do not agree that Britain ought to refuse to use the metric system (as many of this society’s members do), I was about to throw this invitation in the bin when I was struck by a sudden thought, and changed my mind. I know that what I am about to say does not reveal very good things about my character, but I had realised in a flash how much it would enrage my sister, Di, if I signed up. Di is never funnier than when infuriated, and among her many pet hates is the old-bufferish adherence to the old ways just for the sake of them, or because-by-God-it’s-British-and-no-Johnny-Foreigner-is-Going-To-Tell-Me-How-To-Measure-Suet-ness that such an organisation represents.
When my membership came out in the press, she exploded in a really satisfying outpouring of rage. I could hardly stop laughing long enough to tell her that I’d only joined to annoy her. This rendered her almost incoherent with indignation, which was possibly even funnier. Frankly, I doubt whether anyone has ever had as much fun for the price of a postage stamp.