A worked example: 5m × 4m room
A room 5m long and 4m wide has a floor area of 20 m² and a perimeter of 18m — twice the sum of length and width (2 × (5+4) = 18).
The formulas: area and perimeter from the same two numbers
Area = length × width, the space the floor covers. Perimeter = 2 × (length + width), the total distance walked around all four walls once. Both come from the identical two measurements, answering two different practical questions.
A bigger room: 6m × 4.5m
A larger 6m by 4.5m room has a floor area of 27 m² and a perimeter of 21m — both figures scale up together, though not by the same proportion, since area grows with the product of the dimensions while perimeter grows with their sum.
What perimeter is actually used for
Beyond just describing the room's boundary, perimeter is the practical figure for estimating skirting board, cornice, or wall-base trim length needed around the room — and it's also the starting point for estimating the total wall area of the whole room, not just one wall.
What this doesn't handle: irregular rooms
This assumes a simple rectangular room. An L-shaped or otherwise irregular room needs to be split into rectangular sections, each calculated separately, with the areas added together for the total.