Imperial or Metric for Woodworking
Let me start off by saying the metric system is great. I live in the United States so I use imperial on a daily basis, but when I bake I measure everything in grams because it is so much easier. If I need to cut a recipe in half I would rather measure half of 20g instead of finding half of a 3/4 tablespoon.
The 6 inch rule, one of my favorite tools as described in my post about my top 3 must have tools. That being said, the other side is in 32nds and 64ths. I never use it, it’s scary to even look at.
The metric system is great for granularity and accuracy, no fractions necessary. It’s easy to convert units because everything is on the same base. Imperial is all over the place. 12 inches in a foot, 3 feet to a yard, 5280 feet per mile. Then if you want to go small there are inches and each inch is divided in half over and over so it’s 1/2, 1/4, 1/8, 1/16, 1/32, 1/64, and so on. Doesn’t really make sense from a math perspective, and it is annoying to add 37 7/16 to 28 5/8.
That being said, metric is not always superior, especially in woodworking. The imperial system was developed based on the human body. An inch is the width of your thumb, a foot is the size of your foot, and a yard is the distance covered in a stride. A mile as established by the Romans was one thousand strides which equated to 5,000 feet, but then was changed when the British standardized measurements to base the mile on a furlong. To dive even deeper, depending on the trade there are other specific units of measurement such as hands to measure horses or fingers to measure fabric. If you want to know more about obscure imperial measurements check out this website.
The point is, imperial units of length are based on the human body. Does that matter? Not really since it’s arbitrary anyway. I don’t know about you but my thumb is not an inch and my foot is less than 12 inches. Though these measurement systems did make sense in a time when we did not have measurement devices but still needed a way to understand dimensions. To be honest, I still use the old fashioned way from time to time like pacing out a rough distance for a new garden path when my wife has an idea for a new project.
So what does all that have to do with a system of measurement, well hold your horses and I’ll tell you. Some may argue against me on this, but woodworking is not exact. Accuracy to a measurement doesn’t really matter, what does matter are proportions and proportions are where the imperial system shines. The metric system uses decimals to divide up larger units of measurements making it easy to be accurate to a number. The imperial system uses proportions of sizes in relation to each other. It’s a subtle difference but an important one. Proportions within the piece and proportions to us humans. Here’s what I mean.
Say you build a chair. Chairs have human in their center, literally. We want our chairs comfortable and to do that we have to have the proportions of the chair accurate to our bodies. Have you ever sat in a chair where the seat was too short? It feels like you are going to fall out. Or a chair where the legs are too long without a spot to rest your feet? If you haven’t experienced this recently just go n the way back machine and remember what it was like as a kid, dangling your legs in the air. It isn’t comfortable. Chairs have to have the correct proportions based on our bodies.
How long is the distance from the bottom of your foot to the bend in the back of your knee? And how long is the bend in your knee to the bottom of your pelvis? Those are important measurements and anatomy tells us it’s simple proportions. The lower body is the same length as the upper. The knee divides the lower body in half, therefore from the feet to the knee is a quarter of your length and from your knee to your pelvis is another quarter leaving the last half being your upper body, neck, and head.
Now we know how big a chair needs to be. The space from floor to seat should be the same as the depth of the seat and the back should be double that measurement (well more like 1.5 times to take out room for your head).
Simple right? Using proportions like that are great for validating your design will work. If something is off in the proportions the chair won’t be comfortable, so fix it before you spend time making a chair that will end up in the firepit. Now these are rough guidelines and your chair doesn’t need to exactly follow the measurements, but they are a good rule of thumb to follow.
The same principle goes for other pieces of furniture. Things look nice in good proportions, but if something is off it’s immediately obvious. Don’t take my word for it, take a look at these two sofa tables and you judge which one looks better.
Notice how the table on the left is a bit awkward compared to the size of the sofa? It doesn’t fill the space, isn’t centered on anything, the sofa or the room, and is just a bit awkward for the space. The table may be fine, but in this instance it’s not right. This type of problem exists when furniture is bought or made to fit a size specification without thinking about the space it will occupy.
The picture on the right is much better. The table fills the space and looks like it belongs behind that sofa. I know, it’s nerdy but it’s things us woodworkers think about. This is just one example, but you’ll find the same type of imbalance elsewhere in unevenly or erratically spaced shelving. Big furniture manufacturers like Ikea know this which is why their furniture is so pleasing. It has the right proportions and clean lines to show them off.
When designing a piece, think through the proportions and how a human will interact with it. Ask yourself, will it look top heavy? Maybe make the legs more substantial. Will it be uncomfortable for the user, too high or too low? Will it fit the space it’s going in (sometimes clients think they know how big their table should be and sometimes clients are wrong.) Thinking through these proportional issues early will result in a much better end result.