Why All the Fractions?

Why All the Fractions?

I am a novice woodworker. I have looked at a lot of different plans for simple projects, and I would like to know why measurements are very seldom in whole numbers? It seems like fractions are the rule: 12-1/2, 13-7/8, 5-1/8 and so on. Your help would be appreciated. – Earl Maddalena

Rob Johnstone: Sadly, we use an archaic system of measurement developed from some dead monarch’s appendage. That is where it all started to go wrong. There is no doubt that dealing with fractions is a troublesome and tedious fact that our system of measurement requires.

When I went to school to become a luthier, I was informed on the first day of school that we would be using the metric system henceforth. I panicked — as I had never really used it before. After literally about two minutes using it, I wondered why anyone would use any other system … it was so incredibly easy and user-friendly.

The shortest answer to your question is that the relative proportions of the components of woodworking projects simply vary to a nearly infinite degree — hence, they don’t fall on even measurements.

Tim Inman: If a 2 X 4 were really two inches by four inches, we would probably see more “whole” dimensions. But, due to the nature of wood, sawmills, accountants and marketing, a board is seldom what it is claimed to be, dimensionally. A one-inch board is usually more like 3/4 when it is dried, planed and ready to use. Architectural standards usually specify the overall dimensions for seating, cabinets and other interior appointments in whole numbers. We furniture makers work “backwards” from these standards – and end up with fractions.

Chris Marshall: And then, to top all of this reasoning off, there’s the matter of cumulative error we introduce into our project part dimensions. We cut a little bit short here, plane or rout away too much there, and before you know it, even those initial fractions actually become different fractions as a project transforms from numbers on paper to reality. Fractional dimensions are an inevitable part of American woodworking, it seems. I’ve just learned to just grin and bear them.

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