This grade 3 perimeter lesson explains perimeter in one printable page, with definitions, a method and a worked example.
Perimeter is the distance around the outside of a shape. To find the perimeter of a polygon, add the lengths of all its sides. A triangle with sides 5 cm, 6 cm and 7 cm has a perimeter of 18 cm. Perimeter is measured in units of length, not square units.
Grade 3 Perimeter Lesson: key facts
- Perimeter is the total distance around a polygon.
- Add all the side lengths to find the perimeter.
- A rectangle’s perimeter is length + width + length + width.
- For an unknown side, subtract the known sides from the perimeter.
- Rectangles with the same perimeter can have different areas.
What is perimeter?
Perimeter is the distance around the outside of a shape. Imagine walking along every side of a park and stopping where you started. The length of that walk is the perimeter. To find it, add the lengths of all the sides.
The perimeter of a polygon is the sum of the lengths of its sides. It is measured in units of length, such as cm, inches, feet or meters, not in square units.
How do you find the perimeter of a polygon?
A polygon is a closed shape with straight sides. Add every side once. A trapezoid with sides 5 ft, 5 ft, 8 ft and 4 ft has a perimeter of \(5 + 5 + 8 + 4 = 22\) ft.
| Shape | Side lengths | Perimeter |
|---|---|---|
| Triangle | 5, 6, 7 cm | 18 cm |
| Quadrilateral | 5, 5, 8, 4 ft | 22 ft |
| Regular hexagon | six sides of 4 m | 24 m |
Is there a shortcut for rectangles?
A rectangle has two long sides and two short sides that are equal. A rectangle 8 inches long and 3 inches wide has a perimeter of \(8 + 3 + 8 + 3 = 22\) inches. You can also add the length and width and double it: \(2 \times (8 + 3) = 22\).
How do you find an unknown side length?
When you know the perimeter and all sides but one, work backward.
- Add the side lengths you know: \(8 + 9 = 17\).
- Subtract that sum from the perimeter: \(24 – 17 = 7\).
- Check by adding all sides: \(8 + 9 + 7 = 24\).
Can rectangles have the same perimeter but different areas?
Yes. All three rectangles below have a perimeter of 12 units, but their areas are 5, 8 and 9 square units. Shapes that are closer to a square cover more area with the same distance around.
Perimeter: frequently asked questions
What is the difference between perimeter and area?
Perimeter is the distance around a shape and is measured in units such as feet. Area is the amount of flat space inside a shape and is measured in square units such as square feet.
How do you find the perimeter of a rectangle?
Add the length and the width, then double the result, or add all four sides. A rectangle 9 feet long and 4 feet wide has a perimeter of 9 + 4 + 9 + 4 = 26 feet.
How do you find a missing side if you know the perimeter?
Add the sides you know and subtract that sum from the perimeter. If a triangle has a perimeter of 30 inches and two sides of 11 and 9 inches, the third side is 30 – 20 = 10 inches.
What is the perimeter of a square?
A square has four equal sides, so add the side length four times or multiply it by 4. A square with sides of 6 meters has a perimeter of 24 meters.
Can two shapes have the same perimeter but different areas?
Yes. A 5-by-1 rectangle and a 3-by-3 square both have a perimeter of 12 units, but their areas are 5 and 9 square units.
How to use this grade 3 perimeter lesson
Use this grade 3 perimeter lesson before the worksheet, then check what you learned with the quiz.
See every chapter on the Grade 3 math page.
