Metric vs. Imperial: Should you use inches or millimeters for woodworking?

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Metric vs. Imperial: Does it really matter?

I want to make it clear that this is not about which system of measurement is “better”. Invariably, discussions about metric vs. imperial devolve into flame wars with zealots from each camp claiming superiority of measurement.

What I want to talk about are the real world advantages and disadvantages of working with each system in a woodworking shop.

I’ve lived in the U.S. my whole life and am accustomed to inches and feet, but converting to metric isn’t difficult and if everything suddenly switched to metric, we would all adapt and it honestly would make no difference to me and would hardly affect my daily life.

Americans like to embrace quirkiness

Let’s start with the obvious: The metric system is the standard worldwide, except for the United States and a couple other places. It’s important to note that just because something is widespread, it is not automatically better. Any Star Wars movie is widespread, easy to consume, and massively popular, but that doesn’t make it better than a small film like, say, last year’s Lady Bird. They both perform the same job of entertaining, just in different ways.

Conversely, just because the U.S. uses a quirky system of measurement shouldn’t make us the hipsters of the world clinging to our vinyl records and trying to convince ourselves about our superior sound quality. (Or our non-mainstream measurements in this case!)

When I was a kid in the 70s, there was a big effort for the U.S. to “go metric”. I think it was the same time this was happening in Canada and elsewhere, but I’m not completely sure. But for whatever reason, probably a lot of reasons, (probably mostly economics) the metric system never really caught on, but more significantly, it never caught on culturally here. Of course it is used exclusively in scientific applications and other important purposes in the U.S. but for the most part, we are perfectly content to go about our days using miles, gallons, tablespoons, inches, feet and even an occasional dash, pinch or a smidge. I’ll be honest though…I really have no idea how big an acre is.

Metric makes more sense when used in science.

And I think this is really important for non-Americans to understand: Imperial measurements in the U.S. are part of our culture. Most of us fully understand the metric system and agree how simple and logical a base ten system is, but we just love inches anyway! Ridiculing Americans as inferior or “out of touch with the times” for not using the metric system in our day to day lives is culturally disrespectful. It’s like disparaging someone who uses chopsticks because you grew up eating with a fork.

By the same token, most Americans would benefit from being more flexible with measurements and be willing to open up and adapt to metric when needed. Metric isn’t some Illuminati or communist conspiracy designed to tap our precious bodily fluids: it’s just a simplified method for assigning numerical values to things. We are all much better at counting by tens because most of us have ten fingers.

metric clock
Don’t you think metric time measurement would be a lot simpler?

My advice is to learn how to think in the system you are unfamiliar with rather than always try to convert. In other words, begin to develop a mental picture of how long a meter is. For instance, I’ve been using kilometers so long for running that I can easily visualize one kilometer as the distance to the supermarket or 5K as the distance to the community college. I have a mental picture of running 21 kilometers and can assure you that my legs will ache just the same as 13.1 miles! You see, I have no need to convert these to miles to know how far I run. They say you become fluent in a second language when you stop translating everything and just start thinking in that language.

Since I’ve been designing my woodworking plans in both metric and imperial, I’ve become very accustomed to metric measurements and have no trouble thinking in metric, but I’ve also noticed the things that seem to work smoother in each system.

Pros and cons of metric and imperial measurements

When designing projects in Sketchup, metric measurements are a breeze and a vast improvement over inches and feet. They are faster and I make fewer errors.

For instance, what’s half of 438mm? Well, half of 400 is 200 and half of 38 is 19. So 219 millimeters. Pretty basic stuff.

On the other hand, , say I need to cut a 17 ¼” board in half. Quick, what’s half of 17 ¼? Did you get it? Well, it’s 8 and ⅝”. I’m willing to bet most of us would have to think for more than a few seconds.

Using decimeters would be handy

Now in actual usage in the shop, here’s an advantage I see with the imperial system. It has a medium unit called a foot. This is so useful for breaking down measurements into something more tangible. It’s easy for most Americans to wrap our brains around my 6 foot 1 inch height. Or that my shop is 17’ 5” wide. But if I told you that my shop is 209” wide, most Americans would give me a blank look and not be able to really picture how small my space is. The number is too large and needs to be reduced. By having feet, we express a smallish number, then zero it in to just 1 to 12 smaller units, inches.

We even have a bigger unit, the yard, but most of us only use that when it relates to football fields, which themselves have become units of measurements.

In metric, we start with a tiny unit, the millimeter, which is close to a 32nd of an inch. For woodworking purposes, I usually only work to 1/16” tolerances, so the millimeter offers an even greater “standard” of precision.

Then we put ten of those together and move up to the centimeter, the next size chunk. Let’s consider this the mental subdivision similar to the inch unit (even though an inch is about 2.5 cm.)

Then what happens? Metric jumps all the way to the meter or 100 cm. We’ve completely skipped a middle measurement like the foot to help mentally break things up.

But the weird thing is that there IS such a measurement, the decimeter! I have no idea why the decimeter isn’t used, at least for casual spacial awareness.

I’ve learned that metric woodworking plans should be in millimeters only, to remove the decimal point and any potential confusion. My european friends tell me that centimeters are mostly used for clothing measurements and a few other applications. This makes a lot of sense and moving forward I am going to make sure all my metric plans are in millimeters only.

Dealing with big numbers

This leaves us with some big numbers that (at least my brain) has difficulty processing. Somehow a 5 and a half foot long desk is an elegant way to relate a measurement. A 1676 mm desk is accurate and useful, but somehow less graceful and one I would have trouble committing to memory without writing it down.

Which of these measurements is easier to remember without jotting down:

Wouldn’t it make more sense, spatially, to have a 16.7 decimeter desk in your office? Or better yet, a 16 and three quarter decimeter desktop? Fractions don’t have to be exclusive to imperial measurements. A half a decimeter sounds very practical. At the same time, there is no reason you can’t break down inches into tenths.

Decimal Inch Ruler
My proposed decimal inch scale

In fact, this could be very helpful, especially in woodworking where extremely tiny tolerances aren’t usually necessary. I propose a hybrid tape measure that uses inches and feet, but breaks the inches into tenths. It would make basic shop arithmetic and division much easier. I could quickly cut a 17.2” board into two 8.6” boards.

This is a metric/imperial tape measure:

Metric Imperial Tape Measure

Like almost all American tapes, It has inch designations and foot designations every twelve inches. So when I pull it out, no matter how long it is, I can quickly take a measurement and spout out, 2’ 9” or whatever for basic communication, but the inches continue the entire length of the tape, so for technical purposes, if my building plans call for 33”, I don’t have to use feet scale at all.

One other thing that is handy, especially for those of us who need glasses to see up close, the half, quarter, eighth, and sixteenth inch divisions use different lengths of lines. This makes them a little easier to read rather than if there were 16 lines all the same length.

In a shop situation, I find myself spending a bit more thinking when using the metric side. Every ten centimeters is marked in red. Remove the zero and these are decimeters, but since nobody is supposed to use decimeters, they indicate centimeters, which also aren’t used in woodworking, but strangely, we need them when using a tape measure otherwise there wouldn’t be enough room for all the millimeter numbers.

So if I need to cut a board 838mm, I need to pull the tape to the red 80 cm mark, then go to the 3 cm mark then count out 8mm. I think it would be handy of there were a 2mm measurement, so I would propose a quintimeter or pentameter, dividing each centimeter into fifths. This would be accurate enough for most woodworking projects and easier to dial in while actually building a project.

quintimeter ruler
My proposed quintimeter ruler.

Metric isn’t simpler for every situation

Again, the metric system is universal and much more logical and intuitive, easier to design with, and vastly superior when it comes to adding, subtracting, multiplying and dividing and should be the only system used for tech and scientific applications. But I feel that in the shop and in day to day conversational measurements, imperial has the edge. But that might just be for those of us who are more visually oriented.

Having two systems can require a little more work…I know that making two sets of plans for my projects is always a minor hassle. And sometimes we have to get redundant with our tools, but that’s mostly just with wrenches. Buying lumber can be a pain, especially when plywood is sold in metric thicknesses and a ¾” board is not actually ¾”. Having two systems of measurement can cause communication errors that might crop up between the two. But this is mostly hyperbolic.

These problems aren’t really that big a deal in the scope of things. For the most part the two systems seem to coexist in harmony and maybe having both adds a little spice to the world. If you need to convert a measurement, it literally takes a second. Making a small effort to understand and learn both systems can go a long way to keep everybody friendly with each other and make us all smarter in the process.

 

13 COMMENTS

  1. I’ve done a few projects in metric, just to see how I like it. I determined it’s like learning fluency in a language, you don’t translate every word to English, you understand the meaning in the other language. So, you don’t say, “Let’s see, I’d like this piece 20″ long, how many cm is that?” But you think in metric. Or as I heard once, “Imperial is OK, metric is OK, both concurrently is not” I have also had some hardware that is 6mm x 3″ with a 7/16″ hex head. Or RTA furniture that had a 10mm head in one part of the piece and a 7/16″ head in another part. Aaarg. And we all remember the Mars explorer that crashed because one group was using imperial and the other metric.

    One problem I had was that I think I’ve got 4 tape measures that are both imperial and metric. And one that’s just metric. They all read a bit differently. For example, at the 1 m + 1cm mark, one reads 110 (cm), the next 10 (mm, just having to rely you’ve passed the 1m mark), the other one 1 (cm, ditto), and another 1100 (mm).

    I spent a couple college summers doing civil engineering work (interstate highway construction). Those guys use feet and tenths and hundredths of a foot. So the measures have ten and 100 increments per foot. They used to tell the newbies to measure something 11″ and watch the bewilderment. And there were “stations” every 100 feet. So a given point would be noted “520 (station) + 50 (feet)”

  2. Some of the examples used to show the difficulty in remembering metric values are perhaps inadvertently biased. No one who works with metric as their “first language” would ever design a table that is 1676 mm wide. They are much more likely to round to 1700 mm, just the same as why the table in question is 5 1/2 feet wide. I grew up in Australia a decade after the metric conversion and imperial still pervades many aspects of our lives. The informal standard for people’s heights is feet and inches for instance, very people say they are 190.5 cm tall but instead say “6 foot 3”

    For measuring and cutting though I work in mm exclusively.

  3. Hi steve. Hope this txt finds you and your family in good health. Inches or meters, well some of us oldies in the UK still work in the old inches but the youngsters use the new, mm. The UK has been using the both systems since the seventies, I always covert the the new to the old, this maybe because the mm system was coming in as I left shcool. I manage to work with both, just. Best regards Wayne.

  4. In Australia we changed to the metric system in the 1970s.

    The construction industry here copied the UK in its metric conversion and uses millimetres for everything. From small things like a door frame, to a whole bulding, its size will be in millimetres. No metres with decimal points, or centimetres. I’ve seen floor plans for a supermarket with the length of the building as a 5-digit number, something like “47250”. No units cluttering up the drawing. In the title box in the bottom right-hand corner it says “All units in millimetres”

    (If you are wondering, that supermarket is 155 feet long.)

  5. I’ve seen metric rulers where the 2, 3, 7, and 8 mm marks are longer, making it pretty easy to visually find the right mark:

    | i | | i | i | | i |
    | | |
    0 1

    That’s kind of crude, but the best I can do with characters.

    • … and it lost all the spaces that were supposed to line up the 0, 5mm, and 1cm marks on the second line. I hope you still get the idea that there are always ways to make things better.

  6. Hi, I live in Costa Rica, which is a “metric system country” for most applications. As an engineer I use millimeters every day, but there are things that we simply do not use in metric, for instance the width and length of lumber is always in inches, as well as the diameter of conduit and all sorts of pipes, just to give you a couple of examples that imperial system is also used worldwide for certain applications.

  7. This is a courageous exercise. I was « born imperial » and now the country is metric. Therefore I have been using both systems but must admit the fractions measurement system (imperial) generates more mistakes in my projects. The introduction of 6 inches/mm digital calipers bought from hardware stores made me switched to metric (mostly): this because those calipers divide an inch into multiple of 10 anyway; I have not found any measuring tape in 1/10 of inches to transfer measurements from the calipers to a measuring tape . Since those calipers can either switch inch/metric, I went metric and can then transpose any measurement from a calipers in metric to a metric measuring tape in metric. It works for me but I am not a skilful Carpenter…

  8. In my 70s now, I grew up with imperial but now use metric as well. Main rule is to use either but not both. My Portuguese wife grew up with metric system and the funny thing is, they tend to use centimeters in conversation i.e. 80 centimeters but, on drawings would use 800mm. Here in the UK, we seem to be in an in between world with 4″ x 2″ studs but 150mm x 25mm boards. Plasterboard (dry wall), last time I bought small sheets were 6ft long x 90cm wide. Always seems strange to me when some Americans use terms like 4 quarter, instead of 1″.

  9. In Hellas (Greece) we use imperial (inches only) for certain objects like TV diagonal or wheel diameter. In most cases I say “One-sixty” meaning 1,6m or “fourty-five” meaning 45cm. Millimeters come in picture where more accuracy is needed and in manuals and alike “booklets” where both accuracy is needed along with less “confusion” of the final recipient (that’s the client).
    Either measuring system is fine if you “grew up” with it. (You can’t an old dog, new tricks…). Measuring systems are like languages, one can learn-use them fluently but you can NEVER TRANSLATE word by word, errr I mean exact measurements. I ‘ll use the office picture as an example, you ‘ll not find in Hellas a desk that’s 1676mm (5 1/2) but you will find 1,60m. Also by looking metric BMW plans, there are standard dimensions for some surfaces like standard desks (1,2 x 0,6 meters) and sheets of wood (2,5 x 1,2 meters). I mention those examples as a problem of translation 😉
    Finally “Imperials” have yards and some other sub-units they dont use, like “Metrics” have the decimeters that are not used. As said before you learn to measure as you grow up like you learn you native language by listening to your environment (parents usually).

  10. Australian here. You make a good point about the foot. Conversationally, we’ll use the foot when talking about some things like height or when approximations are good enough. However, whenever we actually start measuring we use metric.
    Also, I’ve found converting plans isn’t practical. Because the units don’t quite align, the 0.1mm differences start adding up and nothing fits properly at the end.

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