how to find the diameter of a circle

Download Article

Calculating a circle's diameter is easy if you know any of the other dimensions of the circle: the radius, the circumference, or the area. It's also possible if you don't know any of the above dimensions but you do have a drawing of the circle. If you want to know how to calculate the diameter of a circle, just follow these steps.

Calculation Help

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 its edge.^[1]

Example
If the radius of the circle is 4 cm, then the diameter of the circle is 4 cm x 2, or 8 cm.
2

If you know the circumference of the circle, divide it by π to get the diameter.

π is equal to approximately 3.14

but you should use your calculator to get the most accurate results.^[2]

Example
If the circumference of the circle is 10 cm, then the diameter is 10 cm/π, or 3.18 cm.
3

If you know the area of the circle, divide the result by π and find its square root to get the radius; then multiply by 2 to get the diameter. This goes back to manipulating the formula for finding the area of a circle, A = πr², to get the diameter. You can transform this into r = √(A/π) cm.^[3]

Example
If the area of the circle is 25 cm², divide that by π and find the square root.
This equals cm 2.82cm, so the diameter of the circle is 2.82 x 2 = 5.64cm.

1

Draw a horizontal line inside the circle from one edge 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 between.
2

Label the points where the line crosses the circle points "A" and "B."
3

Draw two overlapping circles, one using A as the center and the other using B as the center. Be sure the two circles overlap like a Venn diagram.
4

Draw a vertical line through the two points at which the outer circles intersect. This line marks the diameter of the circle4.
5

Measure the diameter.

Measure it using a ruler,

or for greater accuracy, a pair of digital calipers. You're done!

Add New Question

Question

What is diference between radius and diameter?

The radius is half the diameter.
Question

If I have the circumference, how can I calculate diameter?

Divide the circumference by pi.
Question

The radius of a dinner plate is 6 cm; what is the diameter?

Chris (Byankno1)

Top Answerer

Since the radius is half the diameter, to get the diameter, you'd multiply the 6 cm by 2. The diameter of this circular dinner plate is 12 cm.
Question

What is the area of a circle?

The area of a circle is area=πr2, or pi * the radius squared, make sure to first square the radius then multiply that by pi.
Question

How do I trace a circle with a diameter of 4 cm?

You have a couple options. Find something that is 4 cm in diameter (or slightly smaller) and draw around it. Use a compass set to a radius of 2 cm. Use a drawing program and set the circle properties so that it has a diameter of 4 cm. Finally, use metric graph paper and plot the graph of x2 + y2 = 4. (1 unit = 1 cm).
Question

How do I find the area of a circle that has a perimeter of 36 cm?

Divide the circumference (perimeter) by pi. That gives you the diameter. Half of that is the radius. To get the area, square the radius, and then multiply by pi.
Question

How do I calculate the circumference of a circle to the diameter?

C = 3.14 x d
Question

What is the diameter of a ball with a volume of 288?

The diameter is equal to the cube root of the following: 6 times the volume divided by pi.
Question

How can I find the coordinates of a diameter?

The diameter is formed from the center of one side of a circle to the other, meaning that there are no coordinates since the diameter is always located in the center of the circle.
Question

If my radius is 20ft, how can I find the diameter?

Double the radius.

Show more answers

Ask a Question

200 characters left

Include your email address to get a message when this question is answered.

Submit

Video

Using geometrical formulas and equations gets easier with practice. Ask for help from someone who has worked with circles or any other geometrical figures. You're likely to find that questions of geometry will seem less challenging with a little experience.
Familiarize yourself with the use of a compass (the drawing tool, not the direction-finder). This is a very useful instrument in many applications, including drawing a circle's diameter, as outlined above. A divider (an instrument similar to a compass) can sometimes be used in such cases.^[4]

Thanks for submitting a tip for review!

Things You'll Need

Calculator
Pencil with an eraser
Compass
Ruler
Digital calipers(optional)

About This Article

Article SummaryX

To calculate the diameter of a circle, multiply the radius by 2. If you don't have the radius, divide the circumference of the circle by π to get the diameter. If you don't have the radius or the circumference, divide the area of the circle by π and then find that number's square root to get the radius. Then, you can just multiply the radius by 2 to find the circle's diameter. Read the article to learn how to calculate the diameter of a circle using a ruler!

Did this summary help you?

Thanks to all authors for creating a page that has been read 3,381,868 times.

Did this article help you?