Area and perimeter: how to help your child practice

What is the difference between area and perimeter?

Perimeter measures the edge: if an ant walked all the way around a rectangle, perimeter is how far it walked. Area measures the inside: how many square tiles it takes to cover the shape with no gaps.

That's why they use different units. Perimeter is a length, like 20 cm. Area counts squares, so it's in square units, like 24 square cm (cm²).

In most US schools, 3rd graders find area by counting square tiles and then by multiplying, and find the perimeter of shapes by adding the sides. 4th graders use the formulas for rectangles and work backward to find a missing side. The math topic guide shows how measurement fits with the rest of elementary math, and the 4th grade guide covers what else comes up that year.

How do you find area and perimeter, step by step?

Start every problem by asking "Am I measuring the edge or the inside?" Then:

Perimeter of a rectangle (7 m by 3 m):

1. Write every side, not just the two you're given: 7, 3, 7, 3.
2. Add them: 7 + 3 + 7 + 3 = 20 m. The shortcut is 2 × length + 2 × width.

Area of a rectangle (7 m by 3 m):

1. Think of rows of squares: 3 rows of 7 squares.
2. Multiply length × width: 7 × 3 = 21 square meters.

Finding a missing side: if a rectangle's area is 48 square inches and one side is 6 inches, ask "6 times what is 48?" The other side is 8 inches.

L-shaped figures: split the shape into two rectangles, find each area, and add. A 5 by 2 rectangle joined to a 3 by 4 rectangle has an area of 10 + 12 = 22 square units.

What area and perimeter mistakes do kids make most?

Mixing up area and perimeter

Asked for the perimeter of a 6 cm by 4 cm rectangle, a child multiplies and answers 24. That's the area. The perimeter is 6 + 4 + 6 + 4 = 20 cm. Have your child trace the edge with a finger for perimeter and shade the inside for area.

Adding only two sides

A child finds the perimeter of the same rectangle as 6 + 4 = 10 cm. A rectangle has four sides. Have them label all four side lengths on the drawing before adding.

Leaving off square units

A child writes the area of a 7 by 5 rectangle as 35 feet. Area counts squares, so it's 35 square feet. Ask, "35 what?" each time, and have them draw a tiny square next to the unit if it helps.

How can you practice area and perimeter at home in 10 minutes?

Measuring real things makes the two ideas stick.

• Minutes 1–3: measure something. Use a ruler on a book or a placemat. Your child finds the perimeter by measuring all four sides, then the area by multiplying.
• Minutes 4–8: five mixed problems. Perimeter of a 5 by 3 rectangle (16), area of a 6 by 6 square (36), a missing side (area 40, one side 5), perimeter of a square with 7 cm sides (28 cm), and one "which is bigger" question.
• Minutes 9–10: same area, different shape. With 12 square tiles (or squares on graph paper), build a 3 by 4 rectangle and a 2 by 6 rectangle. Both have an area of 12, but their perimeters are 14 and 16. Ask your child to explain why.

Change the shapes and sizes each night.

How can Easy Twin help with area and perimeter?

Scan your child's geometry worksheet and Easy Twin makes a twin with the same kinds of questions and new numbers, so a 6 by 4 rectangle becomes a new rectangle with new side lengths. Every question comes with two hints and worked steps, visuals like tables are redrawn with new data, and your child can play the twin as a quest or you can print it with an answer key.

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