Perimeter is the total distance around the outside edge of a shape. If you walked along the fence surrounding a rectangular playground, the distance you traveled would be the perimeter. If you traced your finger along the edge of a triangle drawn on paper, you'd be following its perimeter.

To find the perimeter of any shape, you add up the lengths of all its sides. A square with sides of 5 inches each has a perimeter of 20 inches (5 + 5 + 5 + 5). A triangle with sides of 3, 4, and 5 feet has a perimeter of 12 feet (3 + 4 + 5).

Perimeter matters in real life constantly. A farmer calculating how much fencing to buy for a field needs the perimeter. A landscape designer planning a garden border works with perimeter. Even a parent figuring out how many lights to string around a rectangular window uses perimeter.

People sometimes confuse perimeter with area, but they measure different things. Area tells you how much space is inside a shape, while perimeter tells you the distance around it. A small square could have the same perimeter as a long, skinny rectangle, even though they contain very different amounts of space.