Unit Plan 16 (Grade 3 Math): Perimeter—Walk the Edge
Measure and add side lengths to find perimeter, solve missing-side problems with equations, and clearly distinguish perimeter (linear units) from area (square units) in real-world contexts.
Focus: Measure and add side lengths to find perimeter; solve missing-side problems; clearly distinguish area vs. perimeter in real contexts.
Grade Level: 3
Subject Area: Mathematics (Measurement & Data • Geometry connection)
Total Unit Duration: 5 sessions (one week), 45–60 minutes per session
I. Introduction
Students develop a concrete sense of perimeter as the distance around a 2-D figure. They measure and sum side lengths, tackle missing-side problems using equations, and practice precision with units. Frequent contrasts between area (covering with square units) and perimeter (tracing with linear units) help prevent confusion.
Essential Questions
- What is perimeter, and how is it different from area?
- How do I find a missing side when I know the total perimeter?
- Why are units and reasonableness checks important when measuring and adding side lengths?
II. Objectives and Standards
Learning Objectives — Students will be able to:
- Define and find perimeter by adding side lengths of polygons using standard units (cm, in).
- Solve missing-side problems by writing simple equations and using subtraction/addition.
- Choose and use appropriate tools (ruler, grid) and attend to precision in measurement and labeling.
- Explain the difference between perimeter (linear) and area (square units) with examples.
- Justify answers with diagrams, equations, and unit labels; check for reasonableness.
Standards Alignment — CCSS Grade 3
- 3.MD.8: Solve perimeter problems (find perimeter of polygons; find unknown side lengths; represent same perimeter with different rectangles and same area with different perimeters).
- Mathematical Practices: MP.2 (Reason abstractly and quantitatively), MP.6 (Attend to precision). (Connections: MP.4 Modeling appears throughout.)
Success Criteria — Student Language
- I can find perimeter by adding side lengths and writing the units.
- I can find a missing side using an equation and check if my answer is reasonable.
- I can tell whether a problem is about perimeter (around) or area (covering with square units).