The rectangle area calculator instantly computes area, perimeter, and diagonal from length and width. Supports metric (cm/m) and imperial (in/ft/yd) units with a single toggle.
Dimensions
Results
Common Rectangle Dimensions
| Object | Dimensions | Area |
|---|---|---|
| US Letter paper | 8.5 × 11 in | 93.5 in² |
| A4 paper | 8.27 × 11.69 in | 96.7 in² |
| 1080p screen (16:9) | 23.5 × 13.2 in (typical) | 310 in² |
| Standard door | 32 × 80 in | 2,560 in² |
| Queen mattress | 60 × 80 in | 4,800 in² |
| Parking space | 8.5 × 18 ft | 153 ft² |
How to Calculate Rectangle Area, Perimeter, and Diagonal
A rectangle's basic properties can all be derived from just two measurements: length and width. Our rectangle area calculator handles all three formulas simultaneously, making quick work of geometry problems, room planning, material estimates, and more.
Area Formula
Area = length × width. For a room 12 ft wide and 15 ft long: Area = 12 × 15 = 180 sq ft. If you're buying flooring at $3.50 per sq ft, you'll need about 180 × $3.50 = $630 worth of material (plus waste factor).
Perimeter Formula
Perimeter = 2 × (length + width). For the same 12 × 15 room: P = 2 × (12 + 15) = 2 × 27 = 54 ft. This is the total baseboard or fencing needed to go around the rectangle's edge.
Diagonal Formula
Diagonal = √(length² + width²) — derived from the Pythagorean theorem. For 12 × 15: √(144 + 225) = √369 ≈ 19.21 ft. The diagonal is useful for checking if a rectangle is truly square (both diagonals should be equal) and for calculating cable runs across a space.
Metric vs Imperial Units
Toggle between metric (centimeters, meters) and imperial (inches, feet, yards) with the switch at the top. The unit label updates automatically so you always know what system you're working in. Common conversions: 1 ft = 30.48 cm, 1 in = 2.54 cm, 1 m = 3.281 ft.
Real-World Applications
Rectangle geometry is used constantly in construction and home improvement. Calculating flooring for a 10 × 12 ft bedroom: 120 sq ft. A garden bed 4 × 8 ft: 32 sq ft of soil needed. A storage room 6 × 9 ft: 54 sq ft footprint and 30 ft of shelving around the perimeter.
FAQ
What is the formula for rectangle area?
Area = length × width. For example, a rectangle that is 10 ft long and 5 ft wide has an area of 50 sq ft. This formula applies regardless of orientation — which side you call 'length' vs 'width' doesn't matter.
How do you calculate the perimeter of a rectangle?
Perimeter = 2 × (length + width). For a 10 × 5 rectangle: P = 2 × (10 + 5) = 2 × 15 = 30 units. The perimeter is the total distance around the outside of the rectangle.
How do you find the diagonal of a rectangle?
The diagonal is found using the Pythagorean theorem: Diagonal = √(length² + width²). For a 10 × 5 rectangle: √(100 + 25) = √125 ≈ 11.18. This is the length of a line drawn from one corner to the opposite corner.
Is this calculator free?
Yes, completely free with no signup required. All calculations run instantly in your browser.
Is my data private?
Yes. All calculations are performed locally in your browser. Nothing is sent to any server.
What units does this calculator support?
The calculator supports both imperial (inches, feet, yards) and metric (centimeters, meters) units. Toggle between systems using the metric/imperial switch.
What is the difference between a rectangle and a square?
A square is a special type of rectangle where all four sides are equal (length = width). Every square is a rectangle, but not every rectangle is a square. For a square, the area formula simplifies to side² and perimeter to 4 × side.