Perimeter (Basic Introduction)

Perimeter is the total distance all the way around the outside edge of a closed flat shape - literally the length of its boundary. If you walked right around the edge of a park and back to your starting point, the distance you covered is its perimeter. Because perimeter is itself a length, it is measured in length units (cm, m, km), and every side must be in the same unit before you add. For a few common shapes there are tidy formulas worth knowing cold. A rectangle has opposite sides equal, so its perimeter is 2 x (length + breadth), written 2(l + b) - for a 5 cm by 3 cm rectangle that is 2 x 8 = 16 cm. A square has all four sides equal, so its perimeter is 4 x side, or 4a - a square of side 4 cm has perimeter 16 cm. A triangle's perimeter is just the sum of its three sides, a + b + c, so sides of 3 cm, 4 cm and 5 cm give 12 cm; if it is equilateral (all sides equal) this shortcuts to 3 x side, or 3a, giving 18 cm for a side of 6 cm. For any irregular shape there is no shortcut - you simply add up every side. The misconception CTET targets is confusing perimeter (the distance around, a one-dimensional length) with area (the space inside). Running a piece of string along the boundary and then measuring the string makes the one-dimensional nature of perimeter concrete. Drawing shapes on grid paper, where one square-side equals one unit, lets children count the boundary before any formula is introduced - the formula should feel like a faster version of something they can already do by counting.

✅ Solved examples

✏️ Practice — try these, take hints as needed

📝 Topic test — 8 questions

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