How to Calculate the Perimeter of a Place
A perimeter is the total length of the sides of a two-dimensional shape. For those places with corners and flat sides -- such as squares, rectangles, triangles, trapezoids and parallelograms -- calculating the perimeter is one of the simplest calculations you could hope to do. For circular places, though, the calculation becomes a little more involved. The perimeter of a circle is better known as the circumference.
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Instructions
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Squares and Other Flat-Sided Shapes
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Measure each side from corner to corner. Start at one corner and measure along the boundary to the next corner until you've measured all the sides.
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Multiply one side by four if the place's shape is a square or a rhombus, which is the name for four-sided shapes like diamond shapes, where the figure is closed and all sides are the same length. Technically, a square is also a rhombus. Alternately, add all four sides together.
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Multiply one of the short sides, if the place is rectangular, by two and one of the long sides by two, and add the resulting numbers together. You can also add the four sides together.
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Add together all the sides of other shapes. If the place you're trying to measure is big, measure as far as you can and mark the point at which you have to stop measuring and gather the tape before you move forward. Mark with a piece of tape or a toothpick. Ensure you start measuring from the exact point you stopped at before, and not an inch or two on either side of the point.
Circles
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Find the radius or the diameter of a circular or semicircular place by measuring from the center point to the edge for the radius, or across the entire circular area through the center point to get the diameter. If the shape is a semicircle, measure across the flat edge.
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Multiply the radius by two to get the diameter, if the radius is all you have.
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Multiply the diameter by pi. Pi is normally written with a Greek letter that looks like a horizontal line on top of two semi-diagonal lines; the character often looks like it's in italics, so you could think of the diagonal lines as italicized vertical lines. Pi's value is infinite -- Math Goodies notes computers have calculated it out to more than a trillion digits -- but if you don't have a calculator, use 3.14, the first three digits of pi.
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Divide the number by two to get 1/2 of the circumference for a semicircle, and add that to the diameter -- remember that for a semicircular area, the diameter forms one of the edges.
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Tips & Warnings
Break up irregular shapes that combine circles with other shapes when measuring perimeter. The Lonestar College System in Texas suggests breaking up shapes like a square with one end that's dome-shaped into the separate square and semicircle. Measure three sides of the square, find the circumference of a circle based on the diameter, divide the circumference by two and add to the three sides of the square.
For truly irregular shapes, such as curved areas that are too shallow to be full semicircles, or places with boundaries that loop and curve constantly, you may have to simply walk along, measuring small sections as you go.
