Measurement & Data
Finding Perimeter
Finding the perimeter of a polygon given its side lengths.
The Distance Around a Shape
Area measured the flat space inside a shape. Perimeter measures something completely different — the total distance around the outside edge of a shape, as if walking all the way around its border.
Definition — Perimeter
To find the perimeter of any polygon, add up the lengths of every side. For a rectangle, this simplifies further, since opposite sides are always equal: perimeter equals twice the length plus twice the width.
Worked Example — Finding the Perimeter of an Irregular Polygon
Worked Example — Finding the Perimeter of a Rectangle
Geometry Canvas
Construct
Objects
- 1.
P1: a free point, draggable on the plane
- 2.
P2: a free point, draggable on the plane
- 3.
P3: a free point, draggable on the plane
- 4.
poly1: the polygon through P1, P2, P3
Measurements
- poly1area = 15perimeter = 17.66
Tip
Common Mistakes
Multiplying the side lengths together instead of adding them, confusing perimeter with area.
Perimeter is a distance around the outside, found by addition — multiplying side lengths instead finds area, a different measurement entirely.
Forgetting that a rectangle has two pairs of equal sides, and only adding one length and one width instead of two of each.
A rectangle has 4 sides total: two lengths and two widths. Add all four, or use the shortcut of doubling the length and doubling the width before adding.
Key Takeaways
- Perimeter is the total distance around the outside of a shape, found by adding all of its side lengths.
- For a rectangle, perimeter equals twice the length plus twice the width.
- Area and perimeter measure different things and can vary independently of each other.
Summary
Perimeter measures the distance around a shape's border, using addition rather than the multiplication area uses. The next unit turns from measuring shapes to organizing and displaying numerical data.
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