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Arc Length of a Segmental Arch

In my article on building a sunburst I described how to find the circumference of a circle given a specific radius. I then divided that circumference in half, because I was working with a half-round arch. This gave me the length measured along the arches curve, otherwise known as the arc length. Finding the arc length of a segmental arch is a little more difficult, but a construction calculator makes it incredibly easy.

Carpenters today frequently work with segmental arches, and they often need to determine the arc length of an arch. That’s the only way to order or cut trim material accurately. The steps on a construction calculator are easy—whether you use a CMPro or BuildCalc, the key strokes are the same, even though the keys are in slightly different positions. I’m using BuildCalc for these screenshots, because that’s what works best on my iPad!

Using this example:

1. Enter 48 in. and press Run.

2. Enter 6 in. and press Rise

3. Press the Arc key. The first result is the angle of the arc—which has rare use for carpenters (more on that later).

4. Pres the Arc key a second time. That’s the length of the arc.



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