How to Determine the Diameter of a Circle
A circle is a shape in which all points are the same distance from the center. The diameter is the distance from one end of the circle to the other through the center point. You can figure out the diameter of any circle by measuring the distance across the circle or by using the circle's radius, circumference, or area. This article will explain each method.
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Instructions
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Draw three random dots on your circle. Connect the three dots to form a triangle. Using the ruler, find the midpoint of each side of the triangle. Using the protractor, draw lines at right angles from the midpoint on each side. Where the lines intersect is the center of the circle. Mark the center point with a dot.
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Draw a line across the circle through the dot marking the center point. Measure the line. This is the diameter.
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Calculate the diameter using the radius of the circle. The radius is the distance from the center to any point on the circle. Using the ruler, measure the distance from the center point to the edge of the circle. This is the radius. Multiply this distance by 2 and then you have the diameter, which is always twice the radius in any given circle.
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Determine the diameter using its circumference, which is the distance around the circle. The number pi, or 3.14, is always the ratio of the circumference compared to the diameter. The circumference is equal to the diameter multiplied by pi. If you know the circumference, divide the circumference by pi to find the diameter. For example, if a circle has a circumference of 15.7 centimeters, then the diameter is 5 centimeters, or 15.7 divided by 3.14.
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Figure out a circle's diameter using its area, or the number of square units in the circle. The area and diameter of a circle are not directly related, but you can find the radius from the area, then use that to find the diameter. The area of a circle is pi multiplied by the radius squared. So if you know the area, divide it by pi to find the radius squared. For example, if a circle has an area of 78.5 square meters, then the radius squared is 25 square meters, or 78.5 divided by 3.14. The radius would then be 5 meters (the square root of 25), making the diameter 10 meters, since the diameter is equal to the radius multiplied by 2.
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Tips & Warnings
Convert all measurements. If you have more than one measurement unit in the information you have, convert them to a common unit. For instance, if the known circumference of the circle is two feet, six inches, note that for calculation purposes, you need to use either 2.5 feet or 30 inches.
Be methodical in making your calculations so that you are less likely to make mistakes. Go over your calculations to confirm their accuracy.
