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Comment by db48x | Hacker News Reader

> 1in = ???

What do you need to convert those inches into? Why would you ever need to convert those inches into miles? Nothing you will ever do in life requires you to convert between inches (or feet and inches) and miles.

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A reservoir is 3 sq miles, the level rises by 7 inches. How many acre-feet of water have been added :)

Bonus point: how many cubic yards of water added?

The owner of the reservoir doesn’t bother to measure it in square miles, he measures the acreage. Then the acre–feet becomes a trivial computation. Yes, if pressed he might dredge up the obscure fact that a square mile is 640 acres, but then he would have to do long division to convert it, and the whole point is to avoid that kind of complexity.

Do you buy gravel by the cubic mile then?

Weird real-life unit conversions come up all the time, often when you need to calculate a net weight or price, and you have some big surface area (eg. a road, with width in yards + feet, thickness in inches, and length in miles), and your gravel supplier prices by ton, and you look up the density in a conversion table and see something like lbs/ft^3, and now you have a whole bunch of opportunities to make a mistake.

Gravel is purchased by the yard, not by the cubic inch or mile.

I'm certain it's by weight, but either way, I don't see that helping much.

Sellers price it by the weight, purchasers buy it by volume.

You have to lay 3miles of pavement, 5 feet wide, bricks are 5x10 inches. How many bricks to buy?

You have 4 square mile of land with dense forest. Trees grow 5 feet apart and are 10 inches thick on average. How many trees are there? How much wood can you expect?

There's 5 inches of snow on your 5000 square feet roof. How much does it weight? Can it break the roof? Should you clean it up or can it stay?

These things are mental-math in metric and notebook-math in imperial, and that sucks.

Of course you can work around that. People always avoid the bad parts of the tools they use and find workarounds. Like not using exceptions in C++ or comparisons in javascript. If it's a widely used tool you know it has to work somehow.

Thus Americans never convert between units unless absolutely necessary and write press articles like "Asteroid the size of 3 elephants missed Earth by 1000 football stadiums" :)

But let's not pretend it's not a problem.

You kind of mess it up from the beginning by assuming everything is based of 5 or 10 unit fractions for the math. Do it with 2,3,4,6,8 inch standards and metric becomes notebook and imperial becomes mental.

Do you mean because people just use metric in those contexts, or that everyone avoids math?

I assure you people regularly figure out how many small things fit in a large thing. I places where those things are measured in feet and miles, that's what they'd use.

I mean that in every context where a person would ever have to convert units or do long division, we invent a new set of units instead.

Others have given other examples, such as printers who divide the inch into points and picas. They do this because it makes figuring out how many characters fit in a line of type easier than if they measured the type in fractions of an inch. It lets them work with integers instead of fractions all day. Of course these days it’s all computer–driven and it hardly matters what units printers use; the computer does the counting and could just as easily do it in fractions of an inch.

Printing measures go smaller than an inch. The width of printed columns is measured in “picas.” 12 “points” equal one “pica”; 6 picas equal 1 inch. We still use those in image resolution measurements.

Yes, this is a great example of what I mean by everything having their own set of units!

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