Calculate rectangle area, perimeter, and diagonal from length and width. Supports 8 units with automatic conversion between metric and imperial.
Both dimensions are converted to the same unit before calculating.
Area
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Perimeter
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Diagonal
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How It Works
The calculator uses three standard geometry formulas. Area equals length multiplied by width (A = L × W). Perimeter is twice the sum of both sides (P = 2 × (L + W)). Diagonal comes from the Pythagorean theorem (d = √(L² + W²)).
When the length and width units differ, the calculator converts both values to your chosen result unit before running the math. All conversions go through meters as a common base to keep rounding errors minimal.
Assumptions
This calculator assumes a perfect rectangle with four 90° angles. It treats all measurements as exact values — no measurement uncertainty is factored in. Unit conversions use standard definitions (1 inch = 25.4 mm, 1 foot = 0.3048 m, 1 yard = 0.9144 m, 1 mile = 1609.344 m).
When You Need This
Rectangular area calculations show up constantly: figuring out how much paint covers a wall, how many tiles fit a floor, how much sod covers a yard, or whether a piece of furniture fits a room. Builders use perimeter to estimate baseboard, fencing, or trim material. The diagonal matters when you need to check if a large item (a TV, a table, a sheet of plywood) can pass through a doorway at an angle.
Common Rectangle Sizes
Object
Typical Size
Area
Letter paper
8.5 × 11 in
93.5 in²
A4 paper
21 × 29.7 cm
623.7 cm²
Single car garage
12 × 20 ft
240 ft²
Tennis court
23.77 × 10.97 m
260.76 m²
Football pitch (FIFA)
105 × 68 m
7,140 m²
Frequently Asked Questions
What's the difference between area and perimeter?▼
Area measures the surface inside the rectangle — think of it as how much space the shape covers. Perimeter measures the total distance around the outside edge. Area is expressed in square units (cm², ft²), while perimeter uses linear units (cm, ft).
Does the order of length and width matter?▼
No. Multiplication is commutative, so 8 × 5 gives the same area as 5 × 8. By convention, the longer side is called the length, but swapping them has no effect on the results.
Can I use different units for length and width?▼
Yes. The calculator converts both values to the result unit you choose before multiplying. For example, you can enter length in feet and width in meters, then get the area in square centimeters.
How do I find a missing side if I know the area?▼
Divide the area by the known side. For example, if the area is 48 cm² and one side is 8 cm, the other side is 48 ÷ 8 = 6 cm.
Is a square also a rectangle?▼
Yes. A square meets every definition of a rectangle — four sides, four right angles, opposite sides equal. The only extra constraint is that all four sides are the same length. So every square is a rectangle, but not every rectangle is a square.
Why is the diagonal useful?▼
The diagonal tells you the longest straight-line distance inside the rectangle. It's handy for checking if oversized items (TVs, tables, plywood sheets) fit through openings at an angle. Screen sizes for TVs and monitors are also measured diagonally.
This calculator provides results based on the values you enter and standard geometric formulas. It is intended for educational and general-purpose use. For professional applications in construction, engineering, or surveying, confirm measurements with appropriate tools and standards.
