How to find the area of a rectangle
The area of a rectangle is length times width: A = l × w. For a rectangle 10 units long and 4 wide, the area is 10 × 4 = 40 square units. The perimeter is 2 × (l + w) = 28 units — you can also get it from the perimeter calculator.
The formula is really a count of unit squares. Lay 1×1 tiles across a 10 by 4 rectangle and you get 4 rows of 10 tiles, which is exactly 40. That is why area comes out in square units while perimeter, which measures the distance around the edge, stays in ordinary linear units.
How to use this calculator
Enter the length and width in the same unit. The area appears in square units (square inches, cm², m²) and the perimeter in linear units. For slanted shapes try the parallelogram area calculator.
- Convert before you multiply. A wall 300 cm by 2 m must become 3 m by 2 m, not 300 by 2.
- Order does not matter — 10 × 4 and 4 × 10 give the same area.
- A square is a rectangle with equal sides, so enter the same number twice.
Worked example
Say a room is 10 ft long and 4 ft wide. Area = 10 × 4 = 40 square feet. To trim the edges you'd need 2 × (10 + 4) = 28 feet of molding. Add a third dimension and the same footprint becomes a box, measured by the rectangular prism volume calculator.
Frequently asked questions
- What is the formula for the area of a rectangle?
- A = length × width. Multiply the two side lengths measured in the same unit.
- What is the area of a 10 by 4 rectangle?
- 40 square units (10 × 4).
- How do I find the perimeter too?
- Perimeter = 2 × (length + width). For 10 by 4 that is 2 × 14 = 28 units, shown in the badge.
- What units is the area in?
- Square units of whatever you measured the sides in — square inches, square cm, square metres, etc.
- How do I handle an L-shaped room?
- Split it into two rectangles, find each area separately, and add them. The same idea works in reverse for a shape with a cut-out: calculate the full rectangle, then subtract the missing piece.