Rectangle Area Calculator | With Unit Conversion
Calculate rectangle area, perimeter, and diagonal from length and width. Supports 8 units with automatic conversion between metric and imperial.
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.