🔢 MathGrade 3Lesson 23
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Measure Perimeter of Polygons

Lesson preview: the Mini Lesson and Vocabulary are open to everyone. Guided practice, exercises, the parent checklist and the printable PDF worksheet come with full access (free 7-day trial or membership).

What you will learn

  • 1Calculate perimeter by adding all side lengths in inches and in centimeters
  • 2Solve perimeter problems for squares, rectangles, and other polygons in US customary and metric units
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Mini Lesson

The perimeter of a shape is the total distance around its outside edge. To find the perimeter, add up all the side lengths. For regular shapes (where all sides are equal), you can multiply one side length by the number of sides. Sides can be measured in inches (in), feet (ft), or centimeters (cm) — the math is the same in every unit. The answer is in the same unit as the sides.

Perimeter of Common Shapes

  • Square: all 4 sides are equal. P = 4 × side, so side = P ÷ 4. Example: side 5 in, P = 20 in (20 ÷ 4 = 5).
  • Rectangle: opposite sides are equal. P = 2 × (length + width). Example: 6 cm × 3 cm, P = 2 × 9 = 18 cm.
  • Triangle: add all 3 sides. Example: 5 + 4 + 3 = 12 in.
  • Regular pentagon: 5 equal sides. P = 5 × side. Example: side 3 cm, P = 15 cm.
  • Regular hexagon: 6 equal sides. P = 6 × side. Example: side 5 in, P = 30 in.

Irregular Shapes

  • For irregular shapes, you must add every side separately.
  • Example: quadrilateral with sides 7, 5, 6, 4 cm. P = 7 + 5 + 6 + 4 = 22 cm.
  • Example: quadrilateral with sides 3, 4, 2, 5 in. P = 3 + 4 + 2 + 5 = 14 in.
  • Make sure to count all sides — do not skip any! Keep your units the same the whole way through.

Words to know

Perimeter

The total distance around the outside of a shape

e.g. Add all 4 sides to find the perimeter of a rectangle.

Polygon

A closed shape with straight sides

e.g. A pentagon is a polygon with 5 sides.

Side

One straight edge of a polygon

e.g. A rectangle has 4 sides.

Length

How long one side of a shape is, measured in inches or centimeters

e.g. The length of one side is 6 in (about 15 cm).

Inch

A US customary unit of length (in); 1 in ≈ 2.5 cm

e.g. A new crayon is about 3 in long.

Centimeter

A metric unit of length (cm); 100 cm = 1 m

e.g. A pencil is about 15 cm long.

Add

To find the total of two or more numbers

e.g. Add the side lengths to get the perimeter.

Rectangle 5 in by 3 in, perimeter 16 inA rectangle labeled 5 in wide and 3 in tall with the calculation P equals 5 plus 3 plus 5 plus 3 equals 16 in shown below.Perimeter of a Rectangle (in)5 in5 in3 in3 inP = 5 + 3 + 5 + 3 = 16 inOr: P = 2 × (5 + 3) = 2 × 8 = 16 in
Three shapes with labeled sides and perimetersThree shapes: a square with side 4, a regular pentagon with side 3, and a triangle with sides 5, 4, 3. Each has its perimeter labeled.Perimeter of Different Shapes (cm)4 cmP = 16 cmSquare3 cm eachP = 15 cmPentagon5 cm4 cm3 cmP = 12 cmTriangleAdd all sides to find the perimeter
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A shortcut for rectangles: add the length and width, then double it. P = 2 × (l + w). Use the unit the sides are in: inches in, inches out; centimeters in, centimeters out.

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← Lesson 22: Calculate Elapsed TimeLesson 24: Measure Area with Unit Squares →