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Perimeter is the distance around a shape. Measure each side and add them. A rectangle with sides 4 cm and 3 cm has perimeter 4 + 3 + 4 + 3 = 14 cm.
Measure each side with a ruler. Use string traced around the shape.
Confusing perimeter with area; forgetting sides; misaligning the measuring tool.
You've already learned what perimeter means — the total distance around a shape — and you've worked with standard measurement units like centimeters and inches. Finding perimeter is what happens when you combine those two skills: you measure each side with the right unit, then add the lengths together.
The procedure is straightforward, but the discipline matters. Start at any corner of the shape, measure that side, record the length, then move to the next side. Work your way all the way around until you're back where you started. Every side counts — for an irregular polygon with five sides, that means five measurements; for a rectangle, four (even though two pairs are equal). A common mistake is stopping after measuring the sides you can see in a diagram and forgetting sides that aren't labeled.
Rectangles offer a useful shortcut worth understanding: opposite sides are equal, so a rectangle with length 6 cm and width 4 cm has perimeter 6 + 4 + 6 + 4 = 20 cm. You can also compute this as 2 × (6 + 4) = 2 × 10 = 20 cm. This "double the sum of length and width" formula works because of the structure of rectangles — but for any other shape, just add all the sides individually.
The key distinction to keep sharp is perimeter vs. area. Perimeter is a length — measured in cm, inches, meters — because it's a distance along the boundary. Area is a surface — measured in square cm, square inches — because it covers a region. A wide, flat rectangle and a tall, narrow rectangle can have the same perimeter but very different areas. When a problem asks "how far around?", that's perimeter. When it asks "how much surface?", that's area. Labeling your answer with the correct unit (cm, not cm²) is an easy way to check which one you computed.