Foundations of Mathematical Thinking · Senior Infants · Long and Short , Measuring Length · Spring Term

Perimeter of 2D Shapes

Calculating the perimeter of various 2D shapes, including rectangles, squares, and composite shapes.

• NCCA Curriculum Specifications
• NCCA: Junior Cycle - Measurement - M.1

About This Topic

Perimeter measures the total distance around the outside of a 2D shape, such as rectangles, squares, and composite shapes made by joining simpler ones. In Senior Infants, students start by using everyday objects like cubes, straws, or strings to measure each side and add the lengths together. This builds on their prior experiences comparing lengths from the unit's key questions, like ordering sticks or finding how many cubes long a book is.

This topic fits within the NCCA measurement strand, fostering skills in direct comparison and non-standard units before formal rulers. It develops spatial reasoning as children visualise shapes and their boundaries, connecting to real-life contexts like outlining a playground or fencing a garden. Composite shapes introduce problem-solving by identifying outer edges only.

Active learning shines here because children manipulate physical shapes and units, turning abstract addition into concrete exploration. When they build, measure, and compare perimeters collaboratively, they notice patterns like a square's four equal sides, making the concept stick through trial and error.

Key Questions

1. Which pencil is longer , can you show me?
2. Can you put these sticks in order from shortest to longest?
3. How many cubes long is this book?

Learning Objectives

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

Before You Start

Comparing Lengths

Why: Students need to be able to compare lengths directly and understand concepts like 'longer' and 'shorter' before they can measure and sum lengths.

Counting and Addition

Why: Calculating perimeter requires counting units and adding these counts together, skills developed in earlier number work.

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.

Watch Out for These Misconceptions

Common Misconception

Perimeter means the space inside the shape.

What to Teach Instead

Children often confuse perimeter with area. Hands-on activities like walking string perimeters while contrasting with filling shapes with counters clarify the boundary focus. Peer sharing of measurements reinforces the distinction.

Common Misconception

All sides of any shape contribute equally to perimeter.

What to Teach Instead

For composites, students may double-count internal edges. Building with blocks and tracing only outer paths in groups helps them visualise and discuss shared sides, building accuracy through manipulation.

Common Misconception

Squares have different perimeter rules than rectangles.

What to Teach Instead

Young learners think squares are measured uniquely. Comparing both with same units side-by-side in pairs reveals the pattern of adding all sides, strengthening generalisation.

Active Learning Ideas

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.

30 min·Pairs

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.

45 min·Small Groups

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.

35 min·Whole Class

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.

25 min·Individual

Real-World Connections

• Construction workers use measuring tapes to find the perimeter of rooms or garden plots to determine how much baseboard or fencing material is needed.
• Designers of board games might calculate the perimeter of game boards to decide where to place special spaces or borders.
• Children can use their understanding of perimeter to measure the boundary of a play area in the park or to outline a picture frame.

Assessment Ideas

Quick Check

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

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

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.

Frequently Asked Questions

How to introduce perimeter to Senior Infants?

Start with familiar shapes like books or windows. Use non-standard units such as cubes or hand spans to measure sides concretely. Guide addition of side lengths through class counting chants, linking back to length comparison skills from the unit.

What are common perimeter misconceptions in early years?

Pupils mix up perimeter and area, or add internal edges in composites. Address by contrasting boundary tracing with inside filling, and using physical models to highlight outer paths only. Regular talk time lets children articulate and correct ideas.

How can active learning help teach perimeter?

Active methods like building shapes with blocks or string outlining engage multiple senses, making perimeter tangible. Children discover patterns through play, such as equal sides in squares, and collaborative measuring reduces errors while building talk skills. This approach suits Senior Infants' developmental stage, ensuring retention over rote learning.

Ideas for differentiating perimeter activities?

For advanced learners, introduce irregular shapes or standard units; for support, use larger objects and visuals. Pair mixed abilities for peer teaching, and provide templates for composites. Track progress with personal perimeter journals to tailor feedback.