How to use the rectangle area calculator
- Under Solve for, pick Area if you know the length and width, Length if you know the area and width, or Width if you know the area and length.
- Type the numbers you know into Length, Width or Area. The field being solved is read-only and fills in for you.
- Choose the Unit, from inches to kilometers. Use one unit for both sides.
- Read the area, perimeter, diagonal and aspect ratio, plus the area in other units such as square meters and acres.
The formulas
For a rectangle with length L and width W, the area is A = L × W and the perimeter is P = 2 × (L + W). The diagonal splits the rectangle into two right triangles, so by the Pythagorean theorem it is d = √(L² + W²). To find a missing side, divide the area by the side you know: L = A ÷ W or W = A ÷ L. The aspect ratio is L:W reduced to lowest terms. See Wolfram MathWorld, Rectangle for the full set of properties.
Worked example with the default values
Solve for Area with Length = 10 ft and Width = 6 ft:
- Area = length × width = 10 × 6 = 60 ft²
- Perimeter = 2 × (length + width) = 2 × (10 + 6) = 32 ft
- Diagonal = √(length² + width²) = √(10² + 6²) = √136 = 2√34 ≈ 11.6619 ft
- Aspect ratio = 5:3
- Area in other units: 6.6667 yd², 5.5742 m² and 0.0013774 acres
For a room 12 ft by 14 ft, the area is 168 ft², which is 15.6077 m². To find a missing side, pick Length, then enter an area of 60 and a width of 6. The calculator returns 60 ÷ 6 = 10.
Limits and edge cases
- Mixed units. The Unit applies to both sides. For a wall 8 ft 6 in long, enter 8.5 feet. The area converter switches an existing area between units.
- Conversions. They use the international foot, exactly 0.3048 m (12 inches of exactly 25.4 mm, per NIST). One acre is 43,560 ft².
- Irregular rooms. Split an L-shaped space into rectangles, find each area, and add them.
- Inputs. Values must be positive. Commas such as 1,250.5 are accepted. Zero, negatives or text show a message instead of a result.
The diagonal uses the rule behind the Pythagorean theorem calculator. For a triangular space, try the triangle area calculator. To turn square footage into flooring to order, use the carpet calculator.
Frequently asked questions
How do I find the area of a rectangle in square feet?
Measure both sides in feet and multiply them. A room 12 ft long and 14 ft wide has an area of 12 × 14 = 168 square feet. Choose feet as the Unit and the area appears in ft².
How do I find the length if I know the area and the width?
Divide the area by the width: L = A ÷ W. Choose Length under Solve for, then enter the area and the width. For an area of 60 and a width of 6, the length is 60 ÷ 6 = 10.
What is the difference between area and perimeter?
Area measures the surface inside the rectangle and is given in square units. Perimeter is the distance around the edge and is given in plain units. A 10 ft by 6 ft rectangle has an area of 60 ft² and a perimeter of 32 ft.
What unit is the area in?
The square of the unit you choose. Feet in gives square feet out, and meters in gives square meters out. The calculator also shows the same area in other units, such as acres.
Sources: Wolfram MathWorld, Rectangle and NIST, SI units of length.