Rectangle Area and Perimeter Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/6/2026
The area of a rectangle is calculated as length × width, its perimeter as 2 × (length + width), and its semi-perimeter as length + width (half the perimeter). A rectangle measuring 5 m by 3 m therefore has an area of 15 m², a perimeter of 16 m, and a semi-perimeter of 8 m.
Explanation
Area and perimeter are the two basic measurements of any rectangular shape. Area (length × width) represents the surface covered by the rectangle, expressed in square units (m², cm²...) — useful for calculating how much paint, tile, or lawn to plan for. Perimeter (2 × (length + width), the sum of the four sides) represents the total distance around the shape, useful for example for calculating a length of fencing or baseboard. This calculator also gives the semi-perimeter (length + width, i.e. half the total perimeter): since a rectangle always has two pairs of identical sides, measuring or buying the length of just two adjacent sides is enough to know the full outline — a handy shortcut for quickly estimating the material needed without doubling it, or for recovering a rectangle's length and width from its area and semi-perimeter (the two dimensions are then the roots of a quadratic equation, the same logic used in our quadratic equation calculator). A square is just a special case of a rectangle where length and width are equal: this calculator therefore works just as well for a square, by entering the same value in both fields. To calculate the area of other geometric shapes, see also our triangle area calculator or our circle area and circumference calculator. For a surface to cover with tile or paint with a loss margin already built in, see the dedicated calculators in the home improvement category directly.
Example: a rectangle measuring 5 m by 3 m
Inputs
Length: 5 m. Width: 3 m.
Calculation
Area = 5 × 3 = 15 m². Perimeter = 2 × (5 + 3) = 2 × 8 = 16 m. Semi-perimeter = 5 + 3 = 8 m.
Result
This rectangle has an area of 15 m², a perimeter of 16 m, and a semi-perimeter of 8 m.
Frequently asked questions
What is the semi-perimeter actually used for?
Since a rectangle has two pairs of identical sides (two lengths and two widths), the semi-perimeter (length + width) equals the sum of just one side from each pair. It's a handy shortcut for quickly checking an order of material sold by the linear meter for only two adjacent sides, or for recovering a rectangle's dimensions when you only know its area and semi-perimeter.
How do I calculate the area of a square with this calculator?
Simply enter the same value in the length and width fields, since a square is a rectangle with all sides equal. A square with 4 m sides therefore has an area of 16 m², a perimeter of 16 m, and a semi-perimeter of 8 m.
What unit should the area result be expressed in?
Area is always expressed in the chosen length unit, squared: meters give m², centimeters give cm². The perimeter, on the other hand, stays in the same unit as the entered lengths, without squaring, since it's a distance rather than a surface.
Why isn't the perimeter enough to know a rectangle's area?
Because two rectangles can have exactly the same perimeter while having very different areas: a 1×9 m rectangle has the same perimeter (20 m) as a 5×5 m rectangle, but an area of 9 m² versus 25 m². You always need to know the length and width separately, not just their sum.