It's interesting, early in my purchasing career I worked with machine shops and sheet metal fabricators. So much of what I did was a hybrid: structure dimensions were in inches, drills & feed-throughs were in decimals. That was as far back as the 70s.
Aircraft aluminum is in decimals manufacturer drawings are usually in inches Boeing. Lockheed Martin and others
The Metric System was absolutely essential in my college Math, Physics, and Chemistry courses, and also for my Engineering work at Boeing. For everyday needs, the English System works fine.
The booze industry went metric awhile back, but I can recall when you could buy a quart and even a half gallon of the sauce.
My doctor told me to drink 3 liters of water each day for the 3 days after chemo; I had to bust out the calculator.
That's an odd thing to tell American patients. It's damn near unhelpful. I could tell you that a litre is close to a quart, and I believe it's a little less...mebbe.
So if the the 100 m dash record holder is Usain Bolt with a time of 9.58 sec. and the 100 yd dash record holder is Asafa Powell with a time of 9.07 sec, then who ran the fastest? Well, if my math is correct, Bolts time was .0876 yd per sec, and Powell's was .0907 yd per sec., so Bolt ran the fastest. As you can see, a calculator is necessary.