How to Find the Perimeter in Algebra

Perimeter of Squares, Rectangles and Parallelograms

• 1

  Determine the measurements for each side of the figure. Use a tape measure or count the units if you are working with a graphed object, if the measurements are not provided for you. For instance, a math problem may present a rectangle labeled as having two equal sides of 1 inch and two equal sides of 3 inches,.

• 2

  Add all the measurements to determine the perimeter. Alternatively, determine the formula to use and substitute the measurements in the formula and calculate the answer. While the sides of squares, rectangles and parallelograms may be represented by different letters in the formula, they all work by adding the sum of the sides. For instance, the formula for the perimeter of a square is P = 4a, while the perimeter of a rectangle and a parallelogram is P = 2a + 2b.

• 3

  Write the sum of the sides with the appropriate unit of measurement. For instance, the rectangle mentioned in Step 1 would have a perimeter of 8 inches:

  P = 2a + 2b

  P = 2 (1) + 2 (3)

  P = 2 + 6

  P = 8 inches

Perimeter of Circles

• 4

  Determine either the diameter (d) or the radius (r) of the circle. The diameter is a complete line, crossing the widest part of the circle, while the radius is half that measurement.

• 5

  Write down the formula for finding the circumference (c) of a circle. The formula depends on whether you have the diameter or radius of the circle:

  c = 3.14159 (2) (r) or

  c = 3.14159 (d)

• 6

  Substitute the appropriate measurements into the formula and solve. Label the answer with the indicated measurement. For example, given a circle with a radius of 2 feet:

  C = 3.14159 (2) (2)

  C = 3.14159 (4)

  C = 12.56636 feet

Perimeter of Triangles, Trapezoids and Other Objects

• 7

  Measure the sides of the objects involved, or determine the given measurements.

• 8

  Locate the formula necessary. The formula will always express the answer as the sum of sides. In an equilateral triangle, for instance, P = 3a. In a trapezoid, however, two sides are equal and two are irregular. The formula is:

  P = a + b + 2c.

• 9

  Substitute the measurements of the sides into the formula and solve. Label with the appropriate measurement. For example, a trapezoid with sides of 3 inches, a base 4 inches long and the other base 6 inches results in:

  P = a + b + 2c

  P = 4 + 6 + 2 (3)

  P = 10 + 6

  P = 16 inches