Illustrative Mathematics Grade 3, Unit 7: Two-Dimensional Shapes and Perimeter
In this unit, your child describes and sorts shapes by their attributes and learns about perimeter, the distance all the way around a shape. They group quadrilaterals like rhombuses, rectangles, and squares, add side lengths, find missing sides, and compare rectangles with the same area but different perimeters. A review of the year's work comes next.
By Robert “Bobby” BarnettMath teacher, parent, and founder of Bobby’s Math Club
What is my child learning in Unit 7?
- Finding the perimeter of shapes by adding side lengths, solving for unknown sides, and comparing perimeter with area. Learn more: Perimeter: the distance around a shape
- Sorting triangles and quadrilaterals by sides and corners, and explaining why a square is also a rhombus. Learn more: Sorting four-sided shapes into families
Sample questions to try together
These are real questions from our practice. Tap the speaker to hear one read aloud, or the light bulb for a hint, and talk about the answer together.
Question 1 of 3
A rectangular garden has sides that are 5 feet long and 3 feet wide. What is the perimeter of the garden?
- 8 feet
- 15 feet
- 16 feet
How you can help
- Count steps while walking around the edge of a room or a playground, and talk about the total as the perimeter in steps.
- Ask your child to sort crackers, windows, and tiles they see by shape group, and explain the rule they used.
- Make a rectangle with 20 toothpicks or straws and ask what other rectangles use all 20, then compare how much space each covers.
Where kids can get stuck
- Calling a tilted square a diamond, as if turning a shape changes what it is.
- Finding a missing side by subtracting only one known side from the perimeter.
- Thinking rectangles with the same area must have the same perimeter.
Practice makes perfect
Bobby's Math Club has practice problems that adapt to your child’s level, each with a detailed explanation and an easy-to-follow video, so your child can build confidence in these skills one step at a time.
Words your child will hear
- perimeter
- The distance all the way around the outside of a shape.
- side length
- How long one side of a shape is.
- attribute
- A feature of a shape, such as its number of sides or its kind of corners.
- quadrilateral
- A flat shape with exactly 4 straight sides and 4 corners.
- rhombus
- A quadrilateral whose 4 sides are all the same length.
- rectangle
- A quadrilateral with 4 square corners.
- square
- A quadrilateral with 4 equal sides and 4 square corners.
- triangle
- A flat shape with 3 straight sides and 3 corners.
Want to help your child master perimeter and quadrilaterals?
In Bobby's Math Club, your child practices these skills at just the right level, with a hint when they need one, a clear explanation after every question, and kid-friendly videos. Unlimited practice is free.
Join the club
Written by Robert “Bobby” Barnett
Bobby is an award-winning math teacher, the author of Meet Every Learner’s Needs, and the father of two elementary-school math learners. He uses Bobby’s Math Club with his own kids a few times every week. Learn more or contact Bobby.
Which math standards does Illustrative Mathematics Grade 3 Unit 7 cover?
Illustrative Mathematics Grade 3 Unit 7 covers these math standards. Each one links to what it means, sample questions, and teaching strategies.
For teachers: Unit names and order vary between editions and districts, so your school’s own materials have the exact sequence. Learn how Bobby's Math Club can help your students grow.