Calculating the diameter of a circle is easy if you know any of its other dimensions: radius, circumference, or area. It is also possible if you only have the circle design. If you want to learn how to calculate the diameter of a circle, follow these Steps.
Method 1 of 2: Calculating the Diameter of a Circle Using Radius, Circumference, or Area
Step 1. If you know the radius of the circle, multiply by 2 to get the diameter
The radius is the distance from the center of the circle to the edge. For example, if the radius of a circle is 4 cm, then the diameter is 4 cm x 2 = 8 cm.
Step 2. If you know the circumference of the circle, divide by π to get the diameter
Pi (π) is equal to approximately 3, 14, but you should use your calculator to get a more accurate result. For example, if the circumference of the circle is 10 cm, then the diameter is 10 cm/π, or 3.18 cm.
Step 3. If you know the area of the circle, divide the value by π, then find the square root and multiply by 2 to get the diameter
This Method uses the following formula to find the area of the circle: A = π x r2. For example, if the circle area is 25 cm2, divide the value by π, 25 cm2/π = 7.96. Then find the square root √7.96 = 2.82 cm. To get the diameter, multiply the result by two. 2 x 2.82 = 5.64 cm. Therefore, the diameter of the circle is 5.64 cm.
Method 2 of 2: Calculating Circle Diameter from a Drawing
Step 1. Draw a horizontal line inside the circle from edge to edge
Use a ruler or other straight object to do this. It can be at the top, near the bottom or in the middle.
Step 2. Call the points where the line crosses the circle "A" and "B"
Step 3. Draw two overlapping circles, one using A as the center and the other using B as the center
The circles should overlap, as in the Venn diagram.
Step 4. Draw a vertical line through the two points where the circles overlap
This line marks the diameter of the circle.
Step 5. Measure the diameter
Measure using a ruler, or for greater accuracy, use a digital caliper. Ready!
- Familiarize yourself with using a compass. It is a very useful tool in many things, including drawing the diameter of a circle, as shown above.
- Using geometric formulas and equations becomes easier with practice. You will find that geometry questions and questions will seem less challenging with a little experience.