Grade 3: Perimeter

Warm up with a few quick perimeter warm-ups on the board while the class settles, so every child starts thinking about the skill.

Perimeter is the total distance around the outside of a shape, measured in length units (cm, m). Area is the amount of surface a flat shape covers, measured in square units (square cm, square m). They are often confused because both use a shape's side lengths, so the key is keeping the two ideas, and their units, distinct. Model the method clearly, thinking aloud:

• Separate the two meanings first: perimeter is the fence around a garden, area is the grass inside it.
• Find perimeter by adding all the side lengths, and show the shortcut for a rectangle: 2 x (length + width).
• Build area concretely by covering a rectangle with unit squares and counting them, which reveals length x width.
• Stress the units: perimeter in cm, area in square cm, and never mix them.
• Explore that two shapes can share a perimeter but not an area (or the reverse), so one does not fix the other.

Work this through step by step on the board, then have the class talk you through a second one.

• Rectangle 5 cm by 3 cm:
• perimeter = 2 x (5 + 3) = 16 cm
• area = 5 x 3 = 15 square cm

Do the first few questions of the practice worksheet together, one child explaining each step. Check for understanding before releasing the class to work alone.

Students complete the practice worksheet independently while you circulate and support.

Circulate and look for these, they are the usual sticking points:

• Confusing the two, giving area when perimeter is asked for.
• Forgetting square units on area, or putting them on perimeter.
• Adding only two sides of a rectangle instead of all four for perimeter.
• Multiplying the wrong pair of lengths on a shape that is not a plain rectangle.

Pull the class back together. Ask one child to explain perimeter in their own words, pose a single check question everyone answers on a mini whiteboard, and name what you will build on next lesson.

Use the independent worksheet as the evidence. A child who can complete it accurately and explain one answer has met the objective; anyone who cannot needs the easier level and a short reteach next session.