You may be accustomed to thinking that each circle has a single diameter. However, many real-world 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 real-world 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 three-fourths 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**