Calculating the diameter of a circle is easy if you know one of its other dimensions: the radius, the circumference, or the area. Even if you don't know any of the dimensions, you can also do that if you have the drawing of a circle. If you want to know how to calculate the diameter of a circle, follow the steps below.
Method 1 of 2: Calculate the Diameter of a Circle Using Radius, Circumference, or Area
Step 1. If you know the radius of the circle, double it to get the diameter
The radius is the distance from the center of the circle to the edge. For example, if the radius is 3 cm, the diameter of the circle will be 3 cm x 2, or 6 cm.
Step 2. If you know the circumference of the circle, divide it by π (Pi) to find the diameter
π is roughly equivalent to 3.14, but you will have to use the calculator to get more accurate results. For example, if the circumference of the circle is 10 cm, the diameter will be 10 cm / π, or 3.18 cm.
Step 3. If you know the area of the circle, divide it by π and take its square root to get the radius; then multiply it by 2 to get the diameter
This is accomplished by manipulating the formula for the area of a circle (A = πr2) to get the diameter. You can change it to r = √ (A / π) cm. For example, if the area of the circle is 25 cm2, divide it by π and get the square root. This equates to 2.82 cm, so the diameter of the circle will be 2.82 x 2 = 5.64 cm.
Method 2 of 2: Calculate the Diameter of a Circle from Your Drawing
Step 1. Draw a horizontal line inside the circle, from one side to the other
Use a ruler or a straight edge to do this. It can be at the top, near the bottom, or anywhere in the middle.
Step 2. Name the points where the line crosses the circle “A” and “B”
Step 3. Draw two overlapping circles; one using point A as the center and the other using point 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 will mark the diameter of the circle.
Step 5. Measure the diameter with a ruler or, for greater accuracy, with a pair of digital calipers
It is done!
- Familiarize yourself with the use of a compass (the drawing tool, not the music tool). This is a very useful tool in many applications, including drawing the diameter of a circle, as noted above. Sometimes a divider (a compass-like instrument) can be used for these purposes.
- Using geometric formulas and equations will get easier with practice. Ask someone who has worked with circles or any other geometric shape for help. With a little experience, you will likely notice that the geometry questions seem less difficult.