Water for Schools
Menu

Lesson plan Grade 3 Math

Juniper Bend Water Tower Fence: A Grade 3 Perimeter Lesson

A 45-minute grade 3 lesson where students find the perimeter of rectangular water-tower lots and a missing side length for a fictional town's fence.

Objectives

  1. Students will find the perimeter of a rectangle by adding all four side lengths.
  2. Students will find a missing side length when the perimeter is known.

Vocabulary

Water tower
A water tower is a big tank raised high above town that stores drinking water. Its height is what pushes water through the pipes, keeping the pressure steady even when the power goes out.

Also: perimeter, rectangle, side length

Teacher overview

The town of Juniper Bend (a fictional town) is putting a safety fence around the rectangular lot that holds its water tower. To order the right amount of fence, the public works crew needs the perimeter, the distance all the way around the lot. This lesson uses that real task to teach grade 3 perimeter: adding side lengths, finding a missing side, and comparing two lots that share a perimeter but hold different amounts of room. A common misconception is that students add only two sides of a rectangle instead of all four, so the models and the exit ticket check for all four sides.

Procedure

Hook (5 min): Show a picture of a water tower behind a fence. Ask, "If we walk all the way around the fence and back to where we started, how far do we walk?" Introduce the word perimeter as the distance around the edge.

Direct instruction (12 min): On grid paper, draw a rectangle 8 meters long and 5 meters wide. Model adding all four sides: 8 + 5 + 8 + 5 = 26 meters. Circle each side as you add it so students see there are four. Then model a missing side: a lot has a perimeter of 20 meters and three sides of 6, 4, and 6 meters, so the fourth side is 20 − 16 = 4 meters.

Guided practice (13 min): In pairs, students find the perimeter of two lots you sketch on the board: a 7-meter by 3-meter lot and a 9-meter by 9-meter square lot. Circulate and check that both sides are doubled. Ask one pair to explain how they knew to add four numbers.

Independent practice (10 min): Students complete the Student Handout, questions 1 through 6, on their own. Each question gives its own side lengths or perimeter, so students work them without the modeled numbers.

Closure / exit ticket (5 min): Students answer one question on an index card: "A lot is 4 meters by 9 meters. What is the perimeter?" Collect and scan for 26 meters and for all four sides added before dismissal; reteach the four-side model next day if many add only two.

Juniper Bend Water Tower Fence: A Grade 3 Perimeter Lesson — Grade 3 Math
Name: ____________________________ Date: ______________

Student Handout

All lots are rectangles. All lengths are in meters.

  1. A lot is 9 meters long and 6 meters wide. What is the perimeter?
  2. A square lot has sides of 7 meters. What is the perimeter?
  3. A lot has a perimeter of 24 meters. Three sides are 5, 7, and 5 meters. What is the fourth side?
  4. A lot is 10 meters by 4 meters. How many meters of fence go all the way around it?
  5. Lot A is 8 meters by 2 meters. Lot B is 6 meters by 4 meters. Both have a perimeter of 20 meters. Which lot has more room inside, and how much more?
  6. A lot is 12 meters long. Its perimeter is 34 meters. What is the width?

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key
  1. Answer: 30 meters. 9 + 6 + 9 + 6 = 30 meters.
  2. Answer: 28 meters. A square has four equal sides: 7 + 7 + 7 + 7 = 28 meters.
  3. Answer: 7 meters. The three given sides are 5 + 7 + 5 = 17 meters, and 24 − 17 = 7 meters for the fourth side.
  4. Answer: 28 meters. 10 + 4 + 10 + 4 = 28 meters.
  5. Answer: Lot B, by 8 square meters. Lot A holds 8 × 2 = 16 square meters and Lot B holds 6 × 4 = 24 square meters, so Lot B has 24 − 16 = 8 square meters more room even though both fences are 20 meters.
  6. Answer: 5 meters. The two long sides are 12 + 12 = 24 meters, so the two short sides share 34 − 24 = 10 meters, and 10 ÷ 2 = 5 meters for the width.

Differentiation

Support

Give students grid paper and have them draw each lot, then count the unit lengths along all four sides so perimeter is something they trace before they add. Provide a rectangle template with all four sides already labeled for questions 1, 2, and 4.

Extension

Ask students to draw two different rectangles that both have a perimeter of 16 meters and to find the area of each, then explain why the same fence can hold different amounts of room. Have them design a lot whose perimeter is exactly 30 meters and label its sides.

ELL supports

Pre-teach "perimeter," "side," "width," and "around" by walking the class around the edge of a rug or table and counting the sides. Provide the sentence frame "The perimeter is ___ meters because I added ___, ___, ___, and ___." Allow labeled grid drawings as answers where the objective allows.