Lesson: Perimeter of Irregular Shapes

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Lesson Objective

SWBAT calculate the perimeter of a random shape on a grid.

Lesson Plan


State Standard:


Standard Name:

Area (parallelograms)


SWBAT calculate the perimeter of a random shape on a grid.

Essential Question:


Word Wall Words:


Do Now:



Ask students to recall information about polygons

You might ask students to consider how they might trace the perimeter of a polygon that is drawn on the board, or you may begin the day by running the perimeter of the school.

Direct Instruction:

You may choose to lead the students in a short discussion (see also: Perimeter Algorithm Discussion) about how to find the perimeter for random shapes. Explain to the students how to do the assignment. You should model or demonstrate it for the students, especially if they are not familiar with how to use our computer applets

1. Open your browser to Perimeter Explorer in order to demonstrate this activity to the students

2. Perimeter is the total length around the object. So imagine the grid lines are equal to one step. And imagine the outside edges of the figure are tight ropes. You want to see how many steps it will take you to get all the way around the edge. The number of steps would be the perimeter.

3. Once we have calculated the perimeter we will put our answer in the perimeter input box and click the check answer button. For now, ignore the area input box.

If you choose to, you may pass out the Worksheet to Accompany The "Perimeter" Applet



Guided Practice:

Try another example, letting the students direct your moves. Or, you may simply ask, "Can anyone describe the steps to use this activity?"

If your class seems to understand the process for doing this assignment, simply ask, "Can anyone tell me what you will do now?"

If your class seems to be having a little trouble with this process, do another example together, but let the students direct your actions: Can someone describe how I would find the perimeter of this shape?

Independent Practice:

Allow the students to work on their own and to complete the worksheet, should you choose to provide one. Monitor the room for questions and to be sure that the students are on the correct web site.

Math Journal:



Bring the class back together for a discussion of the findings. Once the students have been allowed to share what they found, summarize the results of the lesson.

You may wish to collect data from the students to demonstrate that there are even more than five possibilities for each of the areas.

Center Options:




Lesson Resources

Perimeter and Area of Irregular Figures Practice   Classwork


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