It is easy to calculate the diameter of a circle, if you know the other dimensions: the radius, the circumference or the area. You can calculate it even when you only have one circle design. If you want to know how, read on.
Steps
Method 1 of 2: Calculate the Diameter of a Circle from Radius, Circumference or Area
Step 1. If you know the radius, simply double it to get the diameter
The radius is the distance from the center to the edge.
Example
If the radius is 4cm, the diameter is 4cm x 2 = 8cm.
Step 2. If you know the circumference, divide it by π
The value of π is approximately 3.14, and you should use the calculator to get an accurate result.
Example
If the circumference of your circle is 10cm, the diameter is 10cm / π = 3.18cm.
Step 3. If you know the area, first divide it by π and then take the square root and multiply by two
This is the inverse formula of the one used for the area calculation: A = πr2.
Example
The area of the circle is 25 cm2, divide this value by 3.14 and you get 7.96 cm2; then calculate the square root: √7.96 = 2.82 cm. You have found the radius value which you now need to double to get the diameter, which in this case is 2.82cm x 2 = 5.64cm.
Method 2 of 2: Calculate the Diameter of a Circle from a Drawing
Step 1. Draw a horizontal line from point to point inside the circle
Use a ruler to do this. You can draw it up or down, or anywhere.
Step 2. Call the points where the line touches the circumference "A" and "B"
Step 3. Draw two overlapping circles, using point A and B as the center for each of them
They must overlap as in a Venn diagram.
Step 4. Draw a vertical line through the two intersection points of the circles
This line is the diameter of the circumference.
Step 5. Measure the diameter with a ruler or digital compass for greater accuracy
Done!
Advice
- Using geometric formulas and equations becomes easier with practice. Ask for help from someone who has experience with circles or other geometric figures. You will most likely find that geometry problems will seem less difficult with experience.
- Learn to use the compass. This is a very useful tool, for example, to draw the diameter of a circle as shown above. You can also use one with fixed tips, similar to the regular one.
