How to Measure a Circle

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Mathematical tools allow you to draw and measure circles accurately.

Craft, home improvement and shop projects sometimes involve measuring a circle. The circle might be many feet in size or just a few inches. An accurate measurement of the circular part of your project can make the rest go smoothly.


Step 1

Learn the definitions of mathematical terms used when discussing circles. The "circumference" is the distance around the outside edge of the circle. The "radius" is the distance from the outer edge to the center of thecircle. The "diameter" is twice the radius or the distance from the edge on one side through the middle to the edge on the other side,

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Step 2

Learn the formula for the circumference of a circle so you do not have to measure all the way around a circle. Measuring the distance around the outside of a circle can be difficult, especially if you don't have a flexible measuring tape. It is often easier to measure the diameter and multiply by 3.14. The formula is: circumference = pi times the diameter. For most projects, 3.14 can be used as the value for pi. Rewriting the equation, the circumference = 3.14 X diameter. For example, a circle with a diameter of 6 inches has an approximate circumference of 18.84 inches. If you only know the radius of the circle, the formula "2 times pi times the radius" can be used. A circle with a radius of 15 inches has a circumference of 2 X 3.14 X 15 inches = about 94.2 inches.


Step 3

Learn the formula for the area of a circle. The formula for the area of a circle is: pi times the radius times the radius (or, pi times the radius squared). A circle with a radius of 12 feet has an area of 3.14 X 12 X 12 or 452.16 square feet.



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