Surface Area of a Rectangle Calculator
Enter the length and width of a rectangle to instantly find its area, perimeter and diagonal length. Uses the fundamental formula A = l × w.
units
units
Area = length × width
How does this calculator work?
The area of a rectangle is A = l × w (length times width). The perimeter is P = 2(l + w) and the diagonal is d = √(l² + w²). Enter any positive length and width in consistent units to get the area in square units, perimeter and diagonal instantly.
Formula
How this is calculated
A rectangle is a four-sided polygon with four right angles, opposite sides equal and parallel. Its area is the product of its two distinct side lengths: A = l × w. This is the simplest area formula in geometry and follows directly from the definition of area as the number of unit squares that tile the shape.
The perimeter is the total length around the boundary: since opposite sides are equal, P = 2l + 2w = 2(l + w). The diagonal connects two opposite corners and can be found by the Pythagorean theorem: d = √(l² + w²), because the diagonal, length and width form a right triangle.
Inputs must be positive and share the same unit; the area result is in the square of that unit, the perimeter and diagonal in the same unit. A square is a special rectangle where l = w, in which case A = l².
Frequently asked questions
Area (A = l × w) measures the space enclosed inside the rectangle in square units — useful for flooring, painting or carpeting. Perimeter (P = 2(l + w)) measures the total boundary length — useful for fencing, framing or borders.
Area is in the square of whatever unit you enter. If length and width are in metres, the area is in m². Ensure both inputs share the same unit.
Yes. A square is a special case of a rectangle where all four sides are equal (l = w). The area formula A = l × w still applies and simplifies to A = l².
Also known as
TG we-Calculate Editorial Team. (2026). Surface Area of a Rectangle Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/surface-area-of-a-rectangle-calculator
TG we-Calculate Editorial Team. "Surface Area of a Rectangle Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/surface-area-of-a-rectangle-calculator.
TG we-Calculate Editorial Team, "Surface Area of a Rectangle Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/surface-area-of-a-rectangle-calculator
@misc{wecalculate_surface_area_of_a_rectangle_calculator, title = {Surface Area of a Rectangle Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/surface-area-of-a-rectangle-calculator}}, year = {2026}, note = {TG we-Calculate} }
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