3rd Grade - Find Perimeter Of Shapes

Shapes and Geometry
Find Perimeter of Shapes
Calculate the total measurement around a multi-sided shape (polygon) whose side measurements are all expressed in whole numbers.
Measurement and Geometry: perimeter of polygons The ability to calculate the distance around a shape with multiple sides and know how to label that answer correctly.

Sample Problems


Define perimeter (the distance around the outside of a figure). Have the child find the word “rim” in the word perimeter to help him/her remember the definition. (A “rim” is the edge of something)


How do you find the perimeter?

Answer: To find the perimeter of a polygon, measure each side and add those measurements together. The answer label is the name of the unit of measure that was used. Common units of measure are inches, centimeters, feet, and yards. Of course, if the polygon’s sides are all the same length, your child may find it more efficient to multiply the number of sides times the length of each side (since all sides are the same length).


What is the perimeter of a square that is 3 inches on each side?

Answer: To find the perimeter of a square, multiply 4 times the length of one side, since all sides are the same length. Therefore, the square’s perimeter is 12 inches (4 x 3)


What is the perimeter of a box that is 2 inches wide and 6 inches long?

Answer: To find the perimeter of a rectangle, double the length and double the width and then find the sum of the two numbers. (Perimeter = 2 x Length + 2 x Width). Remind your child that a rectangle has 2 sides that are the same length and 2 sides that are the same width. Therefore, the box’s perimeter is 16 inches (2+6=8 and double that, which equals 16)


What is the perimeter of these triangles?

  1. An equilateral triangle whose sides measure 2 inches

  2. An isosceles triangle whose base is 2 inches and whose sides are 7 inches

  3. A triangle whose sides are 3 inches, 4 inches and 5 inches


  1. If all three sides are the same, multiply the number of sides (3) times the measurement of one of them (2); the result is 6 inches.

  2. Multiply the length of the 2 sides that are the same length by 2 (7 x 2, or 14, and add to this number the length of the base side (2) for a total of 16 inches.

  3. If all sides are different lengths, just add them all together: 3+4+5=12 inches.

Learning Tips


When completing calculations on perimeter problems the “answer label” is always the name of the unit of measurement that the measurements are using; i.e., inches, feet, centimeters, etc.


If one measurement is in a different unit than the other, all measurements must be converted to the same unit. In other words, if one measurement is in inches and one in feet, convert the number of feet to the number of inches (12 inches = 1 foot; 36 inches = 1 yard) and complete the solution. Express the answer in the unit that all measurements were converted to.


Children can use a piece of string to measure around their hand and then measure the piece of string on a ruler.

Children can also use a ruler to measure around a piece of paper or other household object that is easily measured. Once many objects are measured, children can draw objects on dot paper or centimeter paper and measure around them. Make sure children count the spaces between the dots when using dot paper to measure.


Mark off (with masking tape) a square section of your kitchen or living room floor. Ask the child to sit on the floor with a measuring tape or better yet, a package of rulers. Have the child measure around each side of the square area that you marked off and then add up all the sides.


Use candy bars as a special treat to calculate perimeter. Children can measure the width of the bar, and then the length. Allow children to measure to the closest inch unless they are experienced with fractions already, and remind them that measurements always start with the measuring tool’s “0” at one corner, or end, of the item to be measured.


It will be helpful to children to use the knowledge they gain in trying out these perimeter problems to develop a way (formula) to make the calculations quickly. Thus, while all perimeters can be calculated by adding the length of all sides together, for specific shapes there are quick formulas:

For a rectangle, width + length and multiply that number x 2 is often faster than adding all of the side measurements together. Another formula that is equally correct is “Double the length of one long side and double the length of one short side. Add those two numbers together.”

For any polygon whose sides are all of equal length, it is faster to multiply the number of sides by the length of one of them.

Extra Help Problems


How do you find the perimeter of a polygon? (Take the sum of the length of each side)


Define perimeter. (The distance around the outside of a figure.)


What is the formula for the perimeter of a square? (4s: 4 x the length of any side)


What is the formula for the perimeter of a rectangle? (P=2L + 2W)


What is the perimeter of a box that is 9 inches long and 2 inches wide? (22 inches)


A box is 5 inches long and 4 inches wide. What is the perimeter of this box? (18 inches)


Jose wants to fence in his carrot garden because he saw 3 wild rabbits nibbling in the garden yesterday. How many feet of fencing wire will he need to buy? His garden is 8 feet long and 2 feet wide. (8 + 2 + 8 + 2=20 feet; the “3” is not needed)


When Jose got to the garden center, he found that wire fencing is sold in rolls of 10 yards. How many rolls will he need to buy? (One; 10 yards = 30 feet; Jose needs 20 feet)


Jose found it very hard to cut his roll of fencing, so he decides to make the garden larger so that he can use up the whole roll. As you remember, Jose has 30 feet of fencing. How could Jose re-design his garden so that it will use up the 30 feet of fencing? It’s ok if the fencing overlaps a bit where it comes together but he doesn’t want to overlap the extra 10 feet that he has if he leaves his garden as it is. (Various answers; encourage your child to draw some rectangles to represent the garden and figure out how close to a perimeter of 30 feet can be obtained.)



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