Area and Perimeter of Rectangles (3rd Grade)

What is it?

Area and perimeter both describe a rectangle, but they measure different things. Area is how much flat space is inside, counted in square units, like tiles on a floor. Perimeter is the distance around the outside, counted in plain units, like a fence around a yard. For a rectangle, area is length times width, because the tiles fill equal rows. Perimeter is the sum of all four sides.

Step by step: a 6 by 4 rectangle

1. A rectangle is 6 units long and 4 units wide.
2. Area: cover it with 1-unit squares. There are 4 rows with 6 squares in each row.
3. 4 rows of 6 is 4 × 6 = 24, so the area is 24 square units.
4. Perimeter: walk around the edge and add every side: 6 + 4 + 6 + 4.
5. 6 + 4 + 6 + 4 = 20, so the perimeter is 20 units.

Common mistakes

• Mixing the two up. Area is the inside (multiply), and perimeter is the edge (add).
• Adding only two sides for the perimeter. A rectangle has four sides, and opposite sides match.
• Leaving out the units. Area is in square units, and perimeter is in units.

Your turn

Here is a problem to try. Pause and let your child work it out before you check.

A rectangle is 5 units long and 3 units wide. What are its area and its perimeter?

Answer: Area 15 square units (5 × 3 = 15). Perimeter 16 units (5 + 3 + 5 + 3 = 16).

Practice at home

Use sticky notes or paper squares to cover the top of a book and count the rows. Then trace around the edge with a piece of string, or measure each side, and add. Ask your child which one is the area and which is the perimeter, and how they can tell.