If you do woodworking and DIYing in inches, a solid understanding of fractions is essential. Being able to calculate that half of 4 1/4" is 2 1/8", or that 1 1/2 + 1 3/16 = 2 11/16" is basic shop math that will keep your projects moving quickly.
But often, bringing fractions into the process is, well, completely unnecessary. Let's say you have a board that you'd like to divide into equal parts. You could measure it, then bust out a pencil, paper, and the calculator app, and eventually have to Google a decimal-to-fraction converter to figure out the size of each section. Then, you'd have to find that crazy number on your ruler, and carefully add the units together to mark out your parts. Or... you could just do this.
Often, when it comes to DIY projects, we create our own circles by starting from a center point. Whether a compass, string guide, or specialized cutting jig, a circle emerges from our pencils or saws precisely because we've created an established and consistent distance from a single origin.
But, what happens when the shape already exists, and you need to know how to find the center of a circle? You can do it in less than a minute without any specialized math, memorizing a formula, or even knowing what pi is.
Over the weekend, I was working in the garage when I found myself in a familiar position. I needed to transfer a pencil line from one face of a piece of stock to the one around its corner. Sounds simple enough to do with a square, but I've had this problem before. Sighting the line isn't accurate enough, and a traditional try or combination square isn't of much help here. Here's why: