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How to Calculate the Diameter of a Circle

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The diameter of a circle is any straight line segment that passes through the center of a circle with its endpoints in the circle. You can easily find it with the help of a ruler or calculations if you know 1 of 3 parameters — the radius, the area, or the circumference.

5-Minute Crafts would like to show you 4 methods for calculating the diameter of a circle. You’ll need a calculator and a ruler.

How to calculate the diameter if you know the radius

radius of a circle is any of the line segments from its center to its perimeter. It is equal to half the diameter. So, to calculate the diameter, simply multiply the radius length by 2.

How to calculate the diameter if you know the area of a circle

If you know the area of a circle, you can calculate its diameter like this: multiply the area by 4, divide by Pi (which is approximately 3.14), and then find the root from the resulting number.

❗ Keep in mind that the result will not be 100% accurate as Pi has an approximate value.

Let’s say the area of our circle is 20 cm². 20×4 / 3.14 = 25.48. Let’s use a calculator and find the root of this number. The diameter of the circle is 5.05 cm.

How to calculate the diameter if you know the circumference

Finding the diameter of a circle is slightly easier if you know its circumference. You just need to divide the circumference by Pi.

How to calculate the diameter if other parameters are unknown

Finally, if you don’t know any parameters of the circle but you have its drawing in front of you, try measuring the diameter with a ruler. Mark the center of the circle, and draw a line through it so that its ends lie within the circle. Then measure the line’s length.

✅ The easiest way to find the center of a circle is to draw a square around it, and then draw diagonals that connect opposite corners of the square. The intersection of the diagonal lines will be the center of the circle.

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