Simple Method to Determine the Radius of an Arch

An arc is a section of the outline, or circumference, of a circle. This common shape appears in many forms, like the arch of a bridge or the top of certain windows.

If you want to create a copy of an arc, you can either trace it or build one like it from another circle the same size. Tracing is easy, but if you want to build your own circle, you need to find the radius of that circle. If it's complete or nearly complete, it's not hard to find the center, and that will let you measure its radius and diameter. But if the arch is only one small segment of the whole circle, finding those values might seem hopeless. It's easier than it looks, though: all you need is a tape measure and a pencil. Well, maybe a calculator, too...



What You'll Need
• Tape measure
• Pencil
• Calculator (optional)