How To Determine The Diameter Of A Circle
How to Calculate the Diameter of a Circle
Three Methods:
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.
Steps
Calculation Help
Calculating the Diameter of a Circle Using the Radius, Circumference, or Area

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.
Example
If the radius of the circle is 4 cm, then the diameter of the circle is 4 cm x 2, or 8 cm. 
If you know the circumference of the circle, divide it by π to get the diameter.π is equal to approximately 3.14but you should use your calculator to get the most accurate results.
Example
If the circumference of the circle is 10 cm, then the diameter is 10 cm/π, or 3.18 cm. 
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.
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.
Calculating the Diameter of a Circle from a Drawing of a Circle

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.

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

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.

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

Measure the diameter.Measure it using a ruler,or for greater accuracy, a pair of digital calipers. You're done!
Community Q&A

QuestionIf I have the circumference, how can I calculate diameter?Top AnswererDivide the circumference by pi.Thanks!

QuestionThe radius of a dinner plate is 6 cm; what is the diameter?Top AnswererSince 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.Thanks!

QuestionWhat is the area of a circle?wikiHow ContributorCommunity AnswerThe area of a circle is area=πr2, or pi * the radius squared, make sure to first square the radius then multiply that by pi.Thanks!

QuestionHow do I find the area of a circle that has a perimeter of 36 cm?Top AnswererDivide 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.Thanks!

QuestionWhat is the diameter of a ball with a volume of 288?Top AnswererThe diameter is equal to the cube root of the following: 6 times the volume divided by pi.Thanks!

QuestionHow do I calculate the circumference of a circle to the diameter?Top AnswererC = 3.14 x dThanks!

QuestionHow do I trace a circle with a diameter of 4 cm?wikiHow ContributorCommunity AnswerYou 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).Thanks!

QuestionHow would I determine the diameter of a circle made with a 40 inch line?wikiHow ContributorCommunity AnswerI assume you mean that the circumference of the circle is 40 inches. Since circumference (C) = π*d, you would need end up with the equation 40= 22/7*d, so the diameter would be 12.73 Inches.Thanks!

QuestionWhat is diference between radius and diameter?Top AnswererThe radius is half the diameter.Thanks!

QuestionHow can I find the coordinates of a diameter?wikiHow ContributorCommunity AnswerThe 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.Thanks!

How to find the diameter of the whole circle
Video
Quick Summary
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 radiusorthe 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.
 Familiarize yourself with the use of a compass (the drawing tool, not the directionfinder). 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.
 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.
Things You'll Need

Calculator

Pencil with an eraser

Compass

Ruler

Digital calipers(optional)
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Video: How to find (diameter) of a Circle
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