Calculating the diameter of a circle is very easy if you know the size of the other dimensions of the circle: radius, circumference, or area. You can still calculate the diameter if no other dimensions are known, but this circle must be drawn. To find out how to calculate the diameter of a circle, follow these steps.
Step
Method 1 of 2: Calculating the Diameter of a Circle Using the Radius, Perimeter, or Area
Step 1. If the radius is known, multiply it by two 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 its diameter is 4 cm x 2, or 8 cm.
Step 2. If you know the perimeter, divide by to get the diameter
is equal to about 3, 14, but use a calculator to get the most accurate result. For example, if the circumference of a 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, find the square root and then divide the result by to get the diameter
Back to the formula for finding the area of a circle, A = r2 to get the diameter. For example, if the area of the circle is 25 cm2, find the square root to get 25 cm2 = 5 cm. Then, divide the result by. 5cm/π = 1.59 cm, so the diameter of the circle is 1.59 cm.
Method 2 of 2: Calculating the Diameter of a Circle By Drawing a Circle
Step 1. Draw a horizontal line inside the circle from one end to the other
Use a ruler or straight edge to draw it, it can be the top, near the bottom, or anywhere in between.
Step 2. Name the point where the line crosses the circle point with "A" and "B"
Step 3. Draw two overlapping circles, one using A as the center and the other using B as the center
Make sure the two circles overlap like a Venn diagram.
Step 4. Draw a vertical line through the two points where the circles intersect
This line marks the diameter of the circle.
Step 5. Measure the Diameter
Measure with a ruler, or digital calipers for better accuracy. Finished!
Tips
- Get in the habit of using a compass (a tool for drawing, not a finder). The compass is a very useful tool in many ways including drawing the diameter of a circle as described above. A divider (a device similar to a compass) can sometimes be used as well.
- Using geometric formulas and equations will be easier than drawing. Ask someone who works with circles or other geometric shapes for help. Gradually, geometric questions will seem less challenging.
