Rectangle Calculator
Calculate area, perimeter, diagonal, and aspect ratio of a rectangle or square.
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How to use this calculator
Basic rectangle formulas derived from its length and width.
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Enter the length and width of the rectangle.
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For a square, enter the same value for both.
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The calculator shows area, perimeter, diagonal length, and aspect ratio.
Frequently asked questions
What is the diagonal of a rectangle?
The diagonal connects opposite corners and forms a right triangle with the length and width. By the Pythagorean theorem: diagonal = √(l² + w²). For a 12×8 rectangle: diagonal = √(144 + 64) = √208 ≈ 14.42.
How do I calculate the area of an L-shaped room?
Break the L-shape into two rectangles, calculate each area separately, and add them. Alternatively, calculate the area of the enclosing full rectangle and subtract the missing corner rectangle.
What is the golden rectangle?
The golden rectangle has an aspect ratio of φ : 1, where φ (the golden ratio) ≈ 1.618. It is considered aesthetically pleasing and appears in art (Parthenon, Da Vinci's works), nature, and design. Credit cards (85.6mm × 53.98mm ≈ 1.586:1) closely approximate it.
How much paint do I need for a rectangular wall?
Calculate wall area (height × width). Subtract window and door areas. Divide by coverage rate (typically 10–12 m² per litre or 100–130 sq ft per quart). Multiply by number of coats. Add 10% for waste.
Rectangle area, perimeter, and diagonal
Common units and unit conversion
Area is in square units: m², ft², cm². If you enter length in feet and width in feet, area is in ft². To convert: 1 m² = 10.764 ft²; 1 ft² = 929 cm². Use consistent units — do not mix metres and feet.
Rectangles in architecture and design
Most rooms, doors, windows, screens, and pages are rectangular. The ISO 216 paper standard (A4, A3, etc.) uses a rectangle with aspect ratio 1:√2 — so folding in half gives the same proportions. Screen aspect ratios (16:9, 4:3, 21:9) are fundamental to TV and monitor design.
Optimisation: minimum perimeter for a given area
For a fixed area, the shape with the minimum perimeter is a square. A 100 m² square (10×10) has a 40m perimeter; a 100 m² rectangle (2×50) has a 104m perimeter. This is why cells, bubbles, and packaging tend toward square forms when minimising material cost.
Learn more from an authoritative source:
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Results are estimates for informational purposes only and do not constitute professional financial, medical, legal, or technical advice. Read full disclaimer →