Measure the length of a curve by treating the curve as part of a complete circle. Once the diameter of the circle is known, it is possible to calculate the length of the curve.

**Determine diameter**Use a straightedge tool to extend a line from either edge of the curve towards the center. Where these two lines meet is the radius of the circle. Multiply the radius by 2 to get the diameter.

**Find the angle of the arc**Center a protractor in the middle of the circle so that the zero edge overlays the diameter line. Align the rotating edge of the protractor with the top of the curve. This is the angle of the curve.

**Solve a formula**Multiply the diameter by pi, and then multiply the solution by the angle. The result is the length of the curve.