Rectangle Calculator — Area, Perimeter & Diagonal
Find the area, perimeter, diagonal length and aspect ratio of a rectangle instantly. Works with any consistent unit — metres, feet, centimetres, inches or anything else.
Area = length × width (square units)
- 1
Sum of sides
8 + 5 = 13 - 2
Perimeter
2 × (8 + 5) = 26 - 3
Diagonal
√(8² + 5²) = 9.434 - 4
Area
8 × 5 = 40
How does this calculator work?
Area = length × width; perimeter = 2 × (length + width); diagonal = √(l² + w²). A rectangle 8 × 5 units has area 40 sq units, perimeter 26 units, and diagonal ≈ 9.434 units. Enter length and width in any consistent unit to get all three values instantly.
Formula
How this is calculated
A rectangle is a quadrilateral with four right angles, two pairs of equal parallel sides and two diagonals of equal length that bisect each other. Given the length (l) and width (w), three key measurements follow directly from the definitions of area, perimeter and the Pythagorean theorem.
Area is the amount of two-dimensional space enclosed by the rectangle: A = l × w. If your dimensions are in metres the area is in square metres; in feet, in square feet — the unit squares. Perimeter is the total distance around the boundary: P = l + w + l + w = 2(l + w). The diagonal connects opposite corners; because the diagonal, the length and the width form a right triangle, the Pythagorean theorem gives d = √(l² + w²). The aspect ratio l ÷ w describes the shape's proportions — a 16:9 screen has an aspect ratio of about 1.78, a square has exactly 1.
Frequently asked questions
No — the formulas are symmetric. Swapping length and width gives the same area, the same perimeter and the same diagonal. By convention "length" is usually the longer side, but the calculator is correct either way.
Any consistent unit you choose — metres, centimetres, feet, inches, millimetres. The output area is in square units of whatever you entered, and perimeter and diagonal are in the same linear unit. Do not mix units without converting first.
Divide both measurements by 12 to convert to feet first, then multiply: A (ft²) = (length in) ÷ 12 × (width in) ÷ 12. Alternatively, multiply the inch-based area by 1 ÷ 144, since 1 ft² = 144 in².
Also known as
TG we-Calculate Editorial Team. (2026). Rectangle Calculator — Area, Perimeter & Diagonal [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/generic-rectangle-calculator
TG we-Calculate Editorial Team. "Rectangle Calculator — Area, Perimeter & Diagonal." TG we-Calculate. 2026. https://we-calculate.com/calculator/generic-rectangle-calculator.
TG we-Calculate Editorial Team, "Rectangle Calculator — Area, Perimeter & Diagonal," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/generic-rectangle-calculator
@misc{wecalculate_generic_rectangle_calculator, title = {Rectangle Calculator — Area, Perimeter & Diagonal}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/generic-rectangle-calculator}}, year = {2026}, note = {TG we-Calculate} }
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