You may be accustomed to thinking that each circle has a single diameter. However, many realworld circular objects, such as rings, washers, and pipes, have two diameters: an inner diameter and an outer diameter. You can calculate the outer diameter of one of these objects in several ways; the way you choose will depend on the information you have.
The Diameter of a Circle

The diameter of a circle is a line drawn through the circle that passes through its midpoint. A diameter divides a circle into two equal haves. There are two primary formulas for the diameter of the circle. The first formula shows that a circle's diameter is equal to two times the circle's radius. The second shows that a circle's diameter is equal to the circle's circumference divided by the constant pi, which is always equal to approximately 3.14.
Inner and Outer Diameter

Picture a realworld circular object, such as a common washer that is used with nuts and bolts. A washer looks like a disc with a circular hole in the middle. The diameter of the central hole is the inner diameter. The diameter of the entire disc is the outer diameter. The shortest distance between a point on the inner circumference and a point on the outer circumference is the wall width.
Calculating Outer Diameter from Inner Diameter and Wall Width

If you know the inner diameter, I, and wall width, W, of an object like a washer or a pipe, you can calculate the outer diameter, O, using the following formula: O = I + 2W. For example, suppose that a washer has an inner diameter of half a centimeter and a wall width of threefourths centimeter. The calculation for outer diameter would look like this: O = 0.5 cm + (2 × 0.75 cm) = 2 cm
Calculating Outer Diameter from Circumference

You can use a piece of thread or tape to measure the outer circumference of an object like a washer. Simply wrap the piece of thread around the outside of the object, then straighten the thread and use a ruler to measure the length that was required to cover the whole circumference. The formula for the diameter of a circle with a known circumference is the following: O = C ÷ pi. Suppose that the measured outer circumference of a washer is two and a half centimeters. The calculation for the outer diameter would look like this: O = 2.5 cm ÷ 3.14 = 0.8 cm
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