Perimeter of 2D Shapes

Hands-on activities work well for perimeter because young learners need to feel the boundary of shapes rather than just see static lines. Using everyday objects like blocks or string turns an abstract concept into something they can physically measure and discuss with peers, building confidence and accuracy.

• Senior Infants
• Foundations of Mathematical Thinking
• 4 activities
• 25 min–45 min

Learning Objectives

1. Calculate the perimeter of squares and rectangles by adding the lengths of all sides.
2. Identify the outer boundary of composite 2D shapes.
3. Measure the sides of 2D shapes using non-standard units like cubes or string.
4. Compare the perimeters of different 2D shapes.
5. Demonstrate how to find the perimeter of a shape by tracing its edge.

Experiential Learning

⏱ 30 min·Pairs

Block Building: Perimeter Paths

Provide multilink cubes for pairs to build rectangles and squares on mats. Children measure each side by lining up cubes, record the number per side, then add for total perimeter. Compare perimeters of different shapes they create.

Prepare & details

Which pencil is longer — can you show me?

Facilitation Tip: During Block Building: Perimeter Paths, encourage students to verbalize their measurements as they count cubes along each side to reinforce the idea of continuous measurement.

Setup: Varies; may include outdoor space, lab, or community setting

Materials: Experience setup materials, Reflection journal with prompts, Observation worksheet, Connection-to-content framework

• Apply
• Analyze
• Evaluate
• Self-Awareness
• Self-Management
• Social Awareness

Experiential Learning

⏱ 45 min·Small Groups

String Outline: Classroom Shapes

Cut yarn into lengths for small groups to outline pre-drawn shapes on the floor, like rectangles and L-shapes. Stretch string along each side, measure with cubes, and calculate perimeter. Discuss why composite shapes have longer paths.

Prepare & details

Can you put these sticks in order from shortest to longest?

Facilitation Tip: While guiding String Outline: Classroom Shapes, stand back and let groups troubleshoot their own string lengths before intervening to promote problem-solving skills.

Setup: Varies; may include outdoor space, lab, or community setting

Materials: Experience setup materials, Reflection journal with prompts, Observation worksheet, Connection-to-content framework

• Apply
• Analyze
• Evaluate
• Self-Awareness
• Self-Management
• Social Awareness

Experiential Learning

⏱ 35 min·Whole Class

Perimeter Hunt: Shape Scavenger

Give whole class cards with shape outlines. Students find classroom objects matching shapes, use paper strips to measure perimeters, and share findings on a class chart. Extend to composite shapes like tables.

Prepare & details

How many cubes long is this book?

Facilitation Tip: For Perimeter Hunt: Shape Scavenger, provide clipboards and pencils so students can record their findings and compare results with classmates afterward.

Setup: Varies; may include outdoor space, lab, or community setting

Materials: Experience setup materials, Reflection journal with prompts, Observation worksheet, Connection-to-content framework

• Apply
• Analyze
• Evaluate
• Self-Awareness
• Self-Management
• Social Awareness

Experiential Learning

⏱ 25 min·Individual

Playdough Perimeters: Custom Designs

Individuals roll playdough into sides of rectangles or squares. Measure each side with links, compute perimeter, then join shapes into composites and re-measure outer path only.

Prepare & details

Which pencil is longer — can you show me?

Facilitation Tip: When introducing Playdough Perimeters: Custom Designs, model how to press the playdough into a flat shape and trace the edge with a ruler to maintain focus on the boundary.

Setup: Varies; may include outdoor space, lab, or community setting

Materials: Experience setup materials, Reflection journal with prompts, Observation worksheet, Connection-to-content framework

• Apply
• Analyze
• Evaluate
• Self-Awareness
• Self-Management
• Social Awareness

Teaching This Topic

Teachers should always model measurement techniques using the same materials students will use, such as counting cubes aloud or stretching string tightly around a shape. Avoid pre-teaching formal rules; instead, let students discover that squares and rectangles follow the same addition rule by comparing their own measurements. Group work helps students articulate their thinking and correct each other’s misunderstandings in real time.

What to Expect

By the end of these activities, students will measure each side of a shape, add the lengths together, and explain why the total distance around the outside is called the perimeter. They will also begin to recognize that different shapes can have the same perimeter depending on their side lengths.

Watch Out for These Misconceptions

Common Misconception

During Block Building: Perimeter Paths, watch for students filling the inside of the shape with cubes instead of tracing the edges.

What to Teach Instead

Ask them to remove the cubes from the inside and focus only on the outer layer, then recount the cubes to reinforce the boundary concept.

Common Misconception

During String Outline: Classroom Shapes, watch for students measuring the string while it is still wrapped around the shape.

What to Teach Instead

Have them straighten the string first, then measure it against a ruler to emphasize the linear distance of the perimeter.

Common Misconception

During Perimeter Hunt: Shape Scavenger, watch for students assuming all sides of a composite shape contribute equally.

What to Teach Instead

Prompt them to build the shape with blocks first, then trace only the outer edges with their fingers to identify shared sides.

Common Misconception

During Playdough Perimeters: Custom Designs, watch for students treating squares as having a different perimeter rule than rectangles.

What to Teach Instead

Provide a square and rectangle side-by-side with the same side length, then have students measure both to see that adding all sides applies to both.

Assessment Ideas

Quick Check

After Block Building: Perimeter Paths, provide students with pre-cut squares and rectangles made from cardstock. Ask them to use unifix cubes to measure each side and then add the cube counts together to find the perimeter. Observe if they measure all sides and sum them correctly.

Exit Ticket

After String Outline: Classroom Shapes, give each student a drawing of a simple composite shape (e.g., an L-shape). Ask them to trace the perimeter with their finger and then write down how many unit cubes it would take to trace the outside edge. They can draw the cubes if needed.

Discussion Prompt

During Perimeter Hunt: Shape Scavenger, show students two different shapes, one larger square and one smaller rectangle. Ask: 'Which shape has a bigger perimeter? How do you know?' Encourage them to explain their reasoning using their measuring tools or by counting sides.

Extensions & Scaffolding

• Challenge students to create two shapes with the same perimeter but different side lengths, using playdough or straws, and explain their reasoning to a partner.
• For students who struggle, provide shapes with labeled side lengths so they can focus on adding rather than measuring.
• Deeper exploration: Introduce irregular shapes with curved sides, such as a half-circle, and discuss how to approximate their perimeters using straight lines or string.

Key Vocabulary

Perimeter	The total distance around the outside edge of a 2D shape. It is the length of the shape's boundary.
2D Shape	A flat shape that has length and width, but no depth. Examples include squares, rectangles, and triangles.
Composite Shape	A shape made by joining two or more simpler 2D shapes together. We only measure the outside edge.
Non-standard Unit	A unit of measurement that is not a formal unit like a ruler or meter stick. Examples include cubes, blocks, or string.