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Perimeter is the distance around a shape. For rectangles and squares, it's the sum of all side lengths. A rectangle with sides 3 and 5 has perimeter 3 + 5 + 3 + 5 = 16 units. For a square, perimeter = 4 × side length.
You've already worked with the general idea of perimeter — the total distance around the outside of a shape — by adding up all side lengths. Now you're focusing on two special shapes, rectangles and squares, where the side lengths follow predictable patterns that let you compute perimeter more efficiently.
For a rectangle with length 5 and width 3, the four sides are 5, 3, 5, and 3. Adding them: 5 + 3 + 5 + 3 = 16. Notice you're adding each dimension exactly twice. That pattern leads to a shortcut: perimeter of a rectangle = 2 × length + 2 × width, or equivalently 2 × (length + width). Both expressions are just compressed ways of adding all four sides. Use whichever feels more natural — they always give the same result.
A square is a rectangle where all four sides are equal. If one side is 6 units, perimeter = 6 + 6 + 6 + 6 = 24, or faster: 4 × side length. This formula isn't a rule to memorize blindly — it falls directly out of the fact that you're adding the same number four times, which is what multiplication means. You already know multiplication, so a formula that replaces four additions with one multiplication is just efficiency.
An important distinction to keep clear: perimeter is a linear measurement — it's a distance, measured in units like centimeters or feet. Area, which you'll study soon, measures how much space is enclosed inside a shape — and uses square units. The perimeter of a room is how much baseboard trim you'd need along the walls; the area is how much flooring you'd need to cover the floor. Two different questions, two different measurements, both about the same shape. Keeping this distinction sharp now will prevent a very common confusion that trips students up for years.