CCSS.Math.Content.3.MD.D.8
The Standard
Solve real world and mathematical problems involving perimeters of polygons, including finding the perimeter given the side lengths, finding an unknown side length, and exhibiting rectangles with the same perimeter and different areas or with the same area and different perimeters.
Common Core State Standards for Mathematics · Measurement and Data
Teacher's field guide
What This Standard Means
What Students Need to Do
- Students find the distance around polygons by adding all side lengths. They use a known perimeter to find a missing side and compare rectangles by perimeter and area.
What Mastery Looks Like
- A student adds every side length with matching units and finds missing lengths from a known perimeter. The student can draw rectangles that share a perimeter but have different areas, or share an area but have different perimeters.
Common Misconceptions
- Students may add only the labeled sides or count square units instead of edge lengths. They may think equal perimeters always produce equal areas, or subtract the known sides incorrectly when finding a missing length.
How to Assess It
- Exit ticket: A rectangle has a perimeter of 24 cm, a length of 8 cm, and an unknown width. Find the width and show your equation.
Lesson moves
Ways to Teach It
Give pairs square tiles and string to build rectangles, then record each rectangle’s perimeter and area in a table.
Ask students to explain in writing why two rectangles can have the same perimeter but different areas.
Play Perimeter Match with cards showing polygons, side lengths, missing sides, and perimeter totals for students to pair.
Have students measure a desktop, book cover, or rug edge, then calculate how much trim would go around it.
Free download
Printable 3.MD.D.8 Worksheet
A ready-to-print activity worksheet aligned to CCSS.Math.Content.3.MD.D.8, with an answer key for the teacher on its own page. No account needed.
PDF, US Letter, prints cleanly in black and white.
Download the worksheetLearning progression
Where This Standard Sits
See the whole path on the Measurement & Data progression map.
Before This Standard
If students are struggling here, check these first.
- CCSS.Math.Content.2.MD.A.1
Measuring lengths with rulers supports finding polygon side lengths before adding them or using them to solve perimeter problems.
- CCSS.Math.Content.2.MD.B.5
Perimeter problems require adding side lengths and subtracting from a total when a side length is unknown.
- CCSS.Math.Content.3.MD.C.7
Area as multiplication supports comparing rectangles with fixed area while perimeter changes, but basic perimeter work can begin without it.
What This Unlocks
Mastery here sets students up for these next.
Keep exploring
Related Standards
- CCSS.Math.Content.6.G.A.1
Find the area of right triangles, other triangles, special quadrilaterals, and polygons by composing into rectangles or decomposing into triangles and other sha...
- CCSS.Math.Content.7.G.B.6
Solve real-world and mathematical problems involving area, volume and surface area of two- and three-dimensional objects composed of triangles, quadrilaterals, ...
- CCSS.Math.Content.3.MD.C.7b
Multiply side lengths to find areas of rectangles with whole-number side lengths in the context of solving real world and mathematical problems, and represent w...
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