For shapes such as squares, rectangles and circles, you can use formulas to calculate the perimeter when you only know one or two dimensions. When you need to find the perimeter of a shape made up of a combination of other shapes, it may appear at first that you are not given enough dimensions. However, you can use the given dimensions to calculate the other necessary dimensions and then find the perimeter of the entire shape.
Make sure to add only the lengths along the outside of the whole combined or irregular shape -- don't use the entire perimeters of all the smaller shapes into which you divided the shape.
Draw straight lines to divide the combined or irregular shape into regular shapes that you know how to find the perimeter of, such as rectangles, right triangles and half circles.
Calculate missing perimeter measurements from the given measurements. If you have a shape composed of a rectangle and a half circle, for example, compute the perimeter of the circle based on the dimensions of the rectangle. The diameter of the circle equals the length of the side of the rectangle it attaches to, so if that length is 4 inches, for example, use the formula for the perimeter of a circle and divide by two to find the perimeter of the half circle -- 4 x pi / 2 = 6.28 inches. If your divided shape includes a right triangle and you know the lengths of two sides of the triangle, calculate the length of the third side with the Pythagorean theorem.
Add the lengths of all the segments along the outside of the shape to find the perimeter. For the rectangle and half circle shape, for example, add the lengths of the three sides of the rectangle and the perimeter of the half circle to find the total perimeter of the shape. If you cannot divide an irregular shape into regular shapes, you must know the length of each segment of the perimeter. Add all the lengths together to find the perimeter.
- Make sure to add only the lengths along the outside of the whole combined or irregular shape -- don't use the entire perimeters of all the smaller shapes into which you divided the shape.
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Petra Wakefield is a writing professional whose work appears on various websites, focusing primarily on topics about science, fitness and outdoor activities. She holds a Master of Science in agricultural engineering from Texas A&M University.