This article was co-authored by Joseph Quinones. Joseph Quinones is a High School Physics Teacher working at South Bronx Community Charter High School. Joseph specializes in astronomy and astrophysics and is interested in science education and science outreach, currently practicing ways to make physics accessible to more students with the goal of bringing more students of color into the STEM fields. He has experience working on Astrophysics research projects at the Museum of Natural History (AMNH). Joseph recieved his Bachelor’s degree in Physics from Lehman College and his Masters in Physics Education from City College of New York (CCNY). He is also a member of a network called New York City Men Teach.

Need to find the diameter of a circle? Calculating diameter is easy—you just need the radius, the circumference, or the area. Even if you don’t have any of those dimensions, you can still find the diameter if you have a drawing of the circle. Keep reading to learn all of the different formulas you can use to find the diameter of a circle, examples included.

## Steps

### Calculation Help

Calculating the Diameter of a Circle Using the Radius, Circumference, or Area

1. 1If 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. X Research source

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. 2If 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. X Research source

Example

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

3. 3If 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. X Research source

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

1. 1Draw 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. 2Label the points where the line crosses the circle points “A” and “B.”
3. 3Draw 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. X Research source
4. 4Draw a vertical line through the two points at which the outer circles intersect. This line marks the diameter of the circle. X Research source
5. 5Measure 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?DonaganTop AnswererDivide the circumference by pi.
• QuestionThe radius of a dinner plate is 6 cm; what is the diameter?ChristopherTop 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.

## Tips

• 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. X Research sourceThanks
• 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.Thanks

## Things You’ll Need

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

## Expert Interview 