Teaching Perimeter, part 4

In elementary school, one of the biggest math concepts students will tackle is Area and Perimeter. For us teachers, it can feel like an overwhelming amount of important content. In this blog series, I’m breaking all of the area and perimeter standards down into more manageable chunks.

Part 1:Teaching Area Concepts

Part 2: Measuring Area & Multiplication

3: Composing & Decomposing Area and The Distributive Property (coming soon!)

This is part 4: Teaching Perimeter

Part 5: Area & Perimeter Relationships and Problem Solving

Standard:

Geometric measurement: recognize perimeter as an attribute of plane figures and distinguish between linear and area measures.

8. 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

I’ve broken perimeter down into two sections here. The first is understanding perimeter and solving problems with perimeter on its own. In part 5, we’ll tackle the relationship with area and perimeter. But it’s much easier to understand the relationship between area and perimeter if you’re familiar with both of those things first, of course!

Overwhelmed?? I’ve spent hours and hours thinking about this and creating a whole math unit so you don’t have to. Get my Teaching Perimeter Math Unit here!

Concepts: perimeter basic concepts

In my experience, kids pick up perimeter concepts pretty quickly, certainly much faster than area. Because of this, I usually teach area first, then perimeter (to give time for any review/ practice needed on area), and then the relationship between the two.

There are loads of tips and tricks out there for teaching perimeter. It’s a very TikTok/ Pinterest- friendly concept to teach. Kids can walk around the outside of shapes drawn or taped off on the floor, they can imagine a fence around the polygon, they can make shapes out of straws or string and then pull them apart and measure them in one line. For my kids that struggled, I had them color shapes on grid paper and then trace the perimeter with a crayon one square at a time while counting as they went. When they went all the way around, they knew they had the total perimeter.

Some notes on teaching perimeter in 3rd grade:

Perimeter is a linear measurement, and area is a measured in square units. If your kids are stuck and confusing the two, go back to part 1 of this series and review the three kinds of measurement and how they are different. It can be confusing for kids to switch back and forth between measuring lines and measuring flat spaces, but they eventually get the hang of it. And they’ll get more practice in part 5 as well, when they work on the relationship between area and perimeter.

Keep in mind that, while the area concepts in third grade only relate to rectangles, the perimeter concepts cover all sorts of polygons. This is a great chance to review the names of polygons and how many sides they each have. It’s also a great opportunity to review equal sides in the context of polygons. Of course, if you know one of the sides and you know the other side is (or 3 or 4 other sides are) equal, then it’s much easier to find the perimeter.

Concept: finding the perimeter

For kids who are still working on the basics of the concepts, counting the squares (or inches) as they go around the outside of the shape works just fine. Once they get the hang of that, I showed them how to mark one corner where they started, and then add as they go around the perimeter, until they get back to the corner they marked. For the kids that are good at mental math, they can add up all the numbers in their heads, although I did find that this way led to the most mistakes. I encouraged the kids to use their fingers to count the number of sides as they added, so they could at least check that they added the correct number of numbers (4 numbers for a rectangle, 6 for a hexagon, etc.)

Of course, this is perfect opportunity for lots of great math talks and letting students come up with and share their own strategies for finding the perimeter. I liked to name the strategy after the person who shared it. By the end of the unit, the kids were saying things like, ‘I used the Kelsey strategy to add all the sides together,’ or ‘I used the Marcus strategy to add the length and width and then double it.’ It definitely encouraged kids to come up with efficient and functional strategies that would help their classmates, and that the others would want to use.

For your advanced students, they can start writing equations or even formulas for finding the perimeter.

concept: finding an unknown side

This is where all those part-part-whole and fact families and addition/ subtraction relationship concepts come in. (You know, the ones parents love to make fun of as being ‘useless’ or ‘new math’ because they don’t get the underlying concepts.) If your kids have a solid foundation in understanding those basics, finding the missing piece should be… a piece of cake, lol. If not, well, maybe it’s a good time to stop and review some of those building blocks!

Concept: problem solving with perimeter

Once kids have internalized the concept of perimeter, it’s time to apply that learning. After all, if you can’t use what you’ve learned, then how helpful is it, really?

Most teachers have their own problem solving tips and tricks that they teach, so I’ll leave that up to you. I always liked having the kids write their own word problems for each other. If they leave out info or their problem doesn’t make sense, the other kids will let them know, don’t worry!!  My other favorite strategy was having the kids draw the problem, so if they didn’t get enough practice drawing arrays yet, then here’s their chance!

Overwhelmed?? I’ve spent hours and hours thinking about this and creating a whole math unit so you don’t have to. Get my Teaching Perimeter Math Unit here!