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Perimeter of Polygons

A 45-minute Grade 3 lesson on the distance around a shape, taught alongside area so that the two are told apart deliberately rather than confused later.

Perimeter of Polygons is a free 45-minute Grade 3 maths lesson plan for Common Core standard 3.MD.D.8 — find the distance around a shape. It is timed across 5 sections, gives the exact wording to use where the wording carries the mathematics, names 3 misconceptions with the teaching move that addresses each, and ends with an exit ticket and a printable worksheet with its answer key.

Grade:
Grade 3
Length:
45 minutes
Standard:
3.MD.D.8

Grade 3 · Measurement & Data

3.MD.D.8

Solve real world and mathematical problems involving perimeters of polygons.

Objective

Students will be able to find the perimeter of a polygon, and find an unknown side length given the perimeter.

Students are successful when they can

  • Adds all side lengths to find a perimeter.
  • Finds a missing side when the perimeter is given.
  • Says how perimeter differs from area in one sentence.

What you need

  • String for tracing around shapes
  • Squared paper
  • Polygon cards with side lengths marked
  • The printable practice sheet linked at the end of this plan

The lesson

Warm-up — walk the edge

5 min
  1. Lay string around a paper rectangle, cut it to length, and straighten it out.
  2. That length is the perimeter. Nothing about the inside of the shape was used.

Teach — around, not across

14 min
  1. Find the perimeter of a 5 by 3 rectangle by adding all four sides: 5 + 3 + 5 + 3 = 16 units.
  2. Point out that the two hidden sides are equal to the two given ones, which is a property of rectangles rather than a rule about perimeter.
  3. Now find its area — 15 square units — and put the two answers side by side. Sixteen units and fifteen square units, different questions and different units.
  4. Do an irregular pentagon by adding all five sides, so the rectangle shortcut is not mistaken for the definition.
  5. Now reverse it: a rectangle with perimeter 20 and one side of 6. The two sixes take twelve, leaving eight for the other two sides, so each is four.
  6. Finish with two shapes that have the same perimeter and different areas, and let the class check both.
“Perimeter is the walk around the edge. Area is the paint on the inside. Same shape, two questions, and the units tell you which you answered.”

Guided practice — both measures, every shape

12 min
  1. Pairs find perimeter and area for the same set of rectangles and record both with units.
  2. Then they solve two missing-side problems from a given perimeter.
  3. Finally they draw two different shapes with a perimeter of 12.

Independent practice

11 min
  1. Students complete a perimeter page.
  2. Perimeters found by multiplying two sides are area answers to a perimeter question, and the unit gives it away.

Close

3 min
  1. Show two rectangles with the same perimeter and different areas and ask which has more area.
  2. The surprise that these can differ is the right thing to leave hanging.

What goes wrong, and why

Each of these is a reasoning error rather than carelessness. The fix addresses the reasoning.

Multiplies the two given sides of a rectangle to find the perimeter.

Why: The recently learned area formula is being applied because the shape and the numbers look identical. Only the question changed.

Fix: Trace the edge with string first, every time. The string is a length, not a covering, and the two actions stay separate.

Adds only the two labelled sides of a rectangle.

Why: Only two numbers are given, so only two get used. The unlabelled sides are assumed not to count.

Fix: Label all four sides before adding. A rectangle's opposite sides are equal, so the missing labels can be filled in and then nothing is forgotten.

Says two shapes with the same perimeter must have the same area.

Why: Both are measurements of size, so they are assumed to move together. Nothing so far has separated them.

Fix: Build a 1 by 6 and a 3 by 4 rectangle with tiles. Same perimeter of 14, areas of 6 and 12. One counterexample settles it.

If they are not there yet

  • Work with shapes on squared paper where every side length can be counted rather than read.
  • Use string for every shape until perimeter and area are physically distinct actions.

If they finish early

  • Find the perimeter of a compound L-shaped figure.
  • Find all rectangles with a perimeter of 16 and compare their areas.
  • Given a perimeter and an area, find the rectangle.

Exit ticket

A rectangle is 7 by 2. Find its perimeter and its area. A different rectangle has perimeter 18 and one side of 5 — what is the other?

What to look for: 18 units, 14 square units, and 4 units. Swapped answers on the first mean perimeter and area are still one idea with two names.

Printable practice for 3.MD.D.8

Free to print and copy, answer key included. Each sheet can generate a fresh set of problems for a retake.

Questions about Perimeter of Polygons

How long does the Perimeter of Polygons lesson take?

45 minutes, split across 5 timed sections: warm-up 5 min, teach 14 min, guided practice 12 min, independent practice 11 min, close 3 min. The minutes are stated per section and add up to the stated length, so the plan can be cut or extended at a section boundary rather than abandoned halfway.

Which standard does Perimeter of Polygons teach?

CCSS.MATH.CONTENT.3.MD.D.8 — 3.MD.D.8, Measurement & Data for Grade 3: “Solve real world and mathematical problems involving perimeters of polygons.” In plain terms, find the distance around a shape.

What do I need to teach this lesson?

String for tracing around shapes, squared paper, polygon cards with side lengths marked and the printable practice sheet linked at the end of this plan. Everything listed is either ordinary classroom equipment or a free printable from this site — there is nothing to buy and nothing to prepare beyond photocopying.

What mistakes should I expect in this lesson?

Three, each a reasoning error rather than carelessness: multiplies the two given sides of a rectangle to find the perimeter; adds only the two labelled sides of a rectangle and says two shapes with the same perimeter must have the same area. The plan gives the thinking behind each one and the teaching move that addresses the thinking rather than the symptom.

How do I know whether the lesson worked?

The exit ticket asks: “A rectangle is 7 by 2. Find its perimeter and its area. A different rectangle has perimeter 18 and one side of 5 — what is the other?.” 18 units, 14 square units, and 4 units. Swapped answers on the first mean perimeter and area are still one idea with two names. That tells you whether the class needs a reteach or a thirty-second conversation before you plan tomorrow.

Is there a worksheet to go with Perimeter of Polygons?

Yes — 2 printable 3.MD.D.8 worksheets, linked at the end of the plan and free like the rest of the site. Each one comes with an answer key and 25 versions, so the practice after the lesson can be a different paper on every desk.

Is Perimeter of Polygons free to use?

Yes. The plan, its printables and their answer keys are free to read, print and photocopy for your own class, with no account and no email capture. It was last reviewed on 5 September 2026. Please do not resell it or republish it as your own.

Last reviewed 2026-09-05. Aligned to 3.MD.D.8 of the Common Core State Standards for mathematics.