I despise the metric system. No, that's not quite accurate. I'm an engineer in the field of computational photonics, so I gladly use the metric system all the time for work. But in my personal life, I banish the metric system to a footlong ruler in the junk drawer.

On metric

Proponents of the metric system always cite two main points:

1. Ease of conversion and calculation, since everything is based on a decimal system.
2. The rest of the world uses it.

To me, these are completely irrelevant. In daily life, quantities are measured, replicated, and rarely involved in calculations. If there are calculations, they involve addition and subtraction, which are as easy in whatever unit system you choose. 93 inches minus 45 inches is no harder to compute than 236 cm minus 114 cm. If you were foolish enough to use a combination of feet and inches in the initial measurement when you knew you needed to perform arithmetic, that is on you. Or else they involve multiplication and division by small integers. On this point, the metric system is inferior since unless you want to divide by 2, 5, or 10, you're going to have the same mental gymnastics as in the imperial system. For everything else, you're going to pull out a calculator anyways.

I agree that the metric system is great for calculation, and avoids many common errors when performing conversions when units are not related by powers of 10. For these purposes, I wholeheartedly support and even use the metric system. I suppose a fundamental assumption in the discussion on systems of units is that there should only be one. And on that point, I emphatically disagree. Just as one should use the right tool for the job, so should one use the right units for the task.

On the argument that the rest of the world uses it: I don't care. I don't live in the rest of the world. America did fine using SAE hardware sizes for decades. We survived on wire gauge and standard pipe threads and two-by-fours. Our cakes and souffles turn out a-ok with teaspoons and tablespoons. Oh, but you say, how will we import nuts and bolts from China then? This, I will reply, presupposes that we should do such a thing. In an age when global supply chains are seen as a risk, and with a growing made-in-America sentiment, I simply don't see why we should cater to international market forces on something as inherently sovreign as a system of units. I would go so far as to mandate SAE sizing for auto components, to encourage American manufacturing. I, too, hate having to stock metric and SAE socket sets in all my toolboxes. Why don't we keep using the one that has served us just fine since forever ago?

On definitions

Oh, but you say, the definitions of imperial units are based on arbitrary things, like King Henry's armspan. Well, yes, as they should be, in a way. Units should be human centric, because, well, they are used by humans. A foot is great because it is easily relatable to the length of your foot. Sure, there's variation, but you'll have a pretty good sense for how big a foot is. If you need precision, that's why the tape measure was invented. The precise definitions don't really matter, so long as they are defined. I'm even fine with (nay, fully supportive of) the redefinition of imperial units in terms of metric units. What matters to me is the magnitude of the units. I like that an inch is as big as it is, so that things you can grasp in your hand measure a couple of inches. I like that a foot is as long as it is so that things that can fit in your house are a couple of feet. A yard is about as long as a pacing step, so you can estimate distances by pacing. That there are multiple units of length, with a small degree of usage domain overlap, is ideal to me. More on this later.

On numbers

I hate numbers; I'm terrible with them. I know, it's a funny thing for me to say, as a scientist or engineer, but I just can't stand numbers. Maybe that's why I made a career out of making computers deal with them for me. But the fewer and simpler the numbers I have to deal with, the better. I imagine I am not alone in this sentiment.

I cannot remember a number like 463.23. If you tell it to me, I'll forget in a matter of seconds. It's simply too many digits, and jeez, there's even a decimal point! 4, 6, did the decimal point go here? One digit occured twice I think... you get the point. To metric proponents, unit conversion is something to be avoided, because you may make a mistake in the conversion calculation. Well, what about the mental tax of dealing with longer digit strings and decimal points, and the attendant transcription errors? This merely shifts the errors to a different sort.

As an engineer, numbers are half, nay, less than half the battle. The devil is in the tolerances, or sigfigs. One rarely, in daily life, ever has to deal with anything tighter than 1% tolerance. If you do, you're doing it wrong. Of course, you can't measure a board to greater than 1% tolerance and expect to build a house. But, why are you measuring such a dimension to replicate it? For all things like this that matter, the goal is almost always replication, and for that, measuring is an unnecessary (and error-prone) detour. Cut to a template, use a jig, make marks, whatever you need to do. Just don't use a tape measure. In cooking, even that most precision-demanding discipline of baking does not require 1% accuracy. To specify 175 mL of an ingredient is a sigfig and a half too many. Why am I even looking at a 3 digit number?

I see that I have been focusing on units of length, so let's look at weight. The difference between a pound and a kilogram is about a factor of 2, so they are, for all intents and purposes, equivalent in their domains of application. I would happily use kilograms in lieu of pounds. The issue now, is what happens if you need a smaller unit. In imperial units, you go down to the ounce, a factor of 16 down. In the metric system, one does not use the hectogram, or the decagram. No, one must go down by a factor of 1000 to the gram. So if I want to express a quarter of a kilogram, now all of a sudden I have to deal with 250 of something. Fuck. that. shit. I don't want to deal with 250 of anything. That is simply too big a number to be juggling. How heavy is a gram anyways? Do you know? I guarantee unless you're a drug dealer, it is unlikely you will be able to estimate a gram to within a factor of 5. This, to me, is the true issue with metric units. It is a decimal system, sure. But it does not deal in factors of 10. The units, for all practical purposes, are set apart by factors of 1000. And 1000 is waaay too big.

I mentioned earlier that I like my units to have some small overlap in domains of application. That is, for small numbers of pounds, I could use ounces without ending up with huge numbers. Same goes for feet and inches, yards and feet, etc. The point is, I want there to be commonly used units that are set apart by about an order of magnitude. And I plan on using all of them. And there's nothing you can do about it.

On graduation

Let's talk temperature for a moment; this reliable starter of flame wars. People love to say that Celsius is so logical because zero is freezing and 100 is boiling. I'm sorry, boiling? You were dead half the scale ago. Also, I live at a half mile of elevation, so whose boiling are you talking about? I do not concern myself with boiling temperatures except when cooking, and at that point, I'm dealing with a menagerie of high temperatures, like having to remember that the chicken needs to reach 165 F. No. I can feel a 1 F temperature difference, and dammit, I want my scale to reflect that. A scale where 0 is intolerably cold and 100 is intolerably hot and a change by 1 can be felt, is perfect. The funny thing is, one end of the scale is not arbitrary. There is an absolute zero. But fuck if I want that to impact my daily measurements. Let's accept that the scale is inherently arbitrary and pick the one that is human-relevant.

On humanity

This gets at the central issue I have with the metric system: it is not human centric. It is computation centric. I am not a computer, nor do I strive to be. When you tell me someone is 6' 3", at no point am I trying to add the quantities together to form a total. I know my height, 5' 10", which is close to 6', so 6' 3" is like half a head taller. That's it. End of computation. Most measurements are used for comparison, and if that is not enough, an intuitive feel for how much it is is all that is required. The doctrinarian approach of accepting only the metric system is a denial of the world we live in, of the fact that we are humans. "Oh haha, those silly people used stones as a weight measure! How primitive!" Well, yes, when you eat a staple grain everyday, it's nice to have a unit that is divisible by seven so you can easily figure out how much to buy on market day. "But the pint isn't even standardized! It's different in different countries!" Look, if you're serious about drinking, it won't even matter. And if you're not, you're not ordering a pint.

Metric absolutists are basically a bunch of pedants that like the theoretical ideal of measurement without understanding what it means to have to do it for 8 hours a day. Why don't we all speak Esperanto, while we're at it. I for one am glad that imperial units are still in widespread use, and I will resist metrification at every opportunity I get. I hope I don't live to see the day that metric becomes the only unit system allowed. Until then, I'm going to keep buying 1/4-20 bolts, and all the M6 I find are going straight in the trash.