Oblong — Definition, Formula & Examples

An oblong is a rectangle where the length and width are different. In other words, it is a rectangle that is not a square.

An oblong is a quadrilateral with four right angles and two distinct pairs of parallel sides of unequal length. It is the subset of rectangles that excludes squares.

Key Formula

P = 2l + 2w \quad\text{and}\quad A = l \times w

Where:

$P$ = Perimeter of the oblong
$A$ = Area of the oblong
$l$ = Length (the longer side)
$w$ = Width (the shorter side), where w ≠ l

How It Works

To tell whether a shape is an oblong, check two things. First, it must be a rectangle — all four angles must be right angles (90°). Second, its length and width must not be equal. If all four sides are the same length, the shape is a square, not an oblong.

Worked Example

Problem: A classroom door is shaped like an oblong that is 3 feet wide and 7 feet tall. Find its perimeter and area.

Step 1: Identify the dimensions: length = 7 ft, width = 3 ft. Since 7 ≠ 3, this rectangle is an oblong.

Step 2: Calculate the perimeter.

P = 2(7) + 2(3) = 14 + 6 = 20 \text{ ft}

Step 3: Calculate the area.

A = 7 \times 3 = 21 \text{ ft}^2

Answer: The door has a perimeter of 20 ft and an area of 21 ft².

Why It Matters

Many everyday objects — books, doors, phone screens — are oblongs rather than perfect squares. Recognizing this distinction helps when sorting shapes in early geometry and when solving area and perimeter problems that specify a non-square rectangle.

Common Mistakes

Mistake: Saying a square is an oblong because it is also a rectangle.

Correction: A square has four equal sides, so it does not qualify as an oblong. An oblong specifically requires that the length and width be different.