Area as Additive


With our Area as Additive lesson plan, students learn how to find the area of irregular shapes by breaking them into rectangles and using addition.

Included with this lesson are some adjustments or additions that you can make if you’d like, found in the “Options for Lesson” section of the Classroom Procedure page. One of the optional additions to this lesson is to have any more advanced students find the area of more advanced irregular shapes that need to be broken into several pieces

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What our Area as Additive lesson plan includes

Lesson Objectives and Overview: Area as Additive teaches students strategies for finding the area of rectangular figures that involve addition. At the end of the lesson, students will be able to find the area of rectangular figures by decomposing them into non-overlapping rectangles and then adding the parts. This lesson is for students in 3rd grade.

Classroom Procedure

Every lesson plan provides you with a classroom procedure page that outlines a step-by-step guide to follow. You do not have to follow the guide exactly. The guide helps you organize the lesson and details when to hand out worksheets. It also lists information in the blue box that you might find useful. You will find the lesson objectives, state standards, and number of class sessions the lesson should take to complete in this area. In addition, it describes the supplies you will need as well as what and how you need to prepare beforehand. For this lesson, the supplies you need are graph or grid paper, colored pencils or markers, and the handouts. To prepare for this lesson ahead of time, you can gather the materials and copy the handouts.

Options for Lesson

Included with this lesson is an “Options for Lesson” section that lists a number of suggestions for activities to add to the lesson or substitutions for the ones already in the lesson. One of the optional additions to this lesson is to have students create their own ideal park from their imagination, creating a key or labelling each part of their park. If you teach more advanced students, ask them to find the area of more advanced irregular shapes that they need to break into several pieces. If you’re looking for a hands-on activity, you could create irregular shapes, have students cut them out, and then cut out and measure the two rectangles to find the area.

Teacher Notes

The teacher notes page includes a paragraph with additional guidelines and things to think about as you begin to plan your lesson. It notes that students will need to be creative in thinking about these problems in a real-world context. This page also includes lines that you can use to add your own notes as you’re preparing for this lesson.


Area as Additive

The Area as Additive lesson plan includes four pages of content. The lesson begins by suggesting that you can find the area of an irregular shape by splitting it into two regular shapes using horizontal or vertical lines (not diagonal). The lesson gives several examples of ways you could split up a specific irregular shape and outlines which way is best and which ways wouldn’t work. If you break an irregular shape into squares and rectangles, it makes it very easy to find the total area. All you have to do is find the area of each square or rectangle and add them together! For example, if you had an irregular shape that could be split into one square and one rectangle, and the area of the square was 4 and the area of the rectangle was 10, you would simply add 4 + 10 for an area of 14.

The lesson then reiterates how to use this method. First, draw horizontal or vertical lines to split your irregular shape into two rectangles or squares. Next, find the angle of each rectangle. Finally, add the areas together. The lesson then includes two more in-depth examples to illustrate this method.


The Area as Additive lesson plan includes three worksheets: an activity worksheet, a practice worksheet, and a homework assignment. You can refer to the guide on the classroom procedure page to determine when to hand out each worksheet.


For the activity worksheet, students will design a park. They will use graph paper to design a local park using the provided sizes for various elements of the park. For example, a small tree is 4 square units, while a playground is 70 square units. Each student must use at least five items and seven trees in their design.


The practice worksheet asks students to complete two short exercises. For the first, they will add either a horizontal or a vertical line to split each irregular shape into two rectangles. For the second, they will find the area of each irregular shape.


Like the practice worksheet, the homework assignment asks students to split irregular shapes into two rectangles and find the area of various irregular shapes.

Worksheet Answer Keys

This lesson plan includes answer keys for the practice worksheet and the homework assignment. If you choose to administer the lesson pages to your students via PDF, you will need to save a new file that omits these pages. Otherwise, you can simply print out the applicable pages and keep these as reference for yourself when grading assignments.

Additional information


3rd Grade



State Educational Standards


Lessons are aligned to meet the education objectives and goals of most states. For more information on your state objectives, contact your local Board of Education or Department of Education in your state.