Area of a Rectangle Calculator
Enter the length and width of any rectangle to get its area, perimeter, and diagonal instantly. Or use reverse-solve mode: supply any two known values (for example, area and one side) and the calculator works out the missing dimension. Switch between millimetres, centimetres, metres, kilometres, inches, feet, yards, and miles. Results update as you type.
Formula
Worked example
A rectangle with length 10 m and width 6 m: Area = 10 x 6 = 60 m2, Perimeter = 2(10 + 6) = 32 m, Diagonal = sqrt(100 + 36) = sqrt(136) = approx 11.66 m. The aspect ratio is 10/6 = 1.67 : 1.
Rectangle area formula and how it works
The area of a rectangle is the product of its two side lengths: A = length x width. Because a rectangle has four right angles, every row of unit squares across the width fits exactly the same number of times down the length, so multiplication gives the total count. The unit of area is always the square of the length unit - square metres for metres, square feet for feet, square centimetres for centimetres, and so on. The perimeter is the total boundary length: P = 2 x (length + width), or equivalently 2 x length + 2 x width. The diagonal is the straight-line distance between opposite corners, found with the Pythagorean theorem: d = sqrt(length2 + width2).
Reverse-solve mode: find a missing dimension
If you already know the area and one side, you can find the other side with width = area / length. If you know the perimeter and one side, the missing side is width = (perimeter / 2) - length. If you know the diagonal and one side, use the Pythagorean theorem in reverse: width = sqrt(diagonal2 - length2). Select the appropriate mode from the "Solve mode" dropdown and the calculator rearranges the formula automatically.
Practical applications
Knowing the area of a rectangle is essential in many everyday situations. Flooring and tiling: multiply the floor length by width (in metres or feet) to find how many square metres or square feet of material to buy, then add 10 to 15 percent for cuts and waste. Painting: a room wall is roughly a rectangle; calculate each wall separately and sum the areas, then subtract windows and doors. Gardening: a rectangular lawn or garden bed area tells you how much turf, compost, or topsoil to order. Land surveys: a rectangular plot of land measured in metres gives an area in square metres (divide by 10,000 for hectares, or by 4046.86 for acres). Screen and paper sizes: aspect ratio matters as much as raw area when comparing displays or choosing paper sizes.
Unit conversions and tips
When inputs and outputs are in different unit systems, convert all measurements to a single unit before calculating. For example, a room measured as 3.5 m x 12 ft must be converted so both sides share one unit: 12 ft = 3.658 m, giving 3.5 x 3.658 = 12.80 m2. This calculator handles that by letting you pick a single consistent unit. A common mistake is mixing units - length in metres and width in centimetres would produce an answer in metre-centimetres, not square metres, so always check that both sides use the same scale. When buying materials such as tiles or carpet that are sold per square metre, add an extra 5 to 15 percent to the computed area to account for wastage, cuts at edges, and pattern matching.
Common rectangular dimensions and their areas
| Object | Typical dimensions | Approximate area |
|---|---|---|
| A4 paper | 21 cm x 29.7 cm | 623.7 cm2 |
| Letter paper (US) | 21.6 cm x 27.9 cm | 602.6 cm2 |
| Standard door | 0.91 m x 2.03 m | 1.85 m2 |
| King-size bed | 1.93 m x 2.03 m | 3.92 m2 |
| Parking space | 2.7 m x 5.5 m | 14.85 m2 |
| Tennis court | 10.97 m x 23.77 m | 260.7 m2 |
| Basketball court (NBA) | 15.24 m x 28.65 m | 436.6 m2 |
| Football pitch (FIFA) | 68 m x 105 m | 7140 m2 |
Quick reference for typical real-world rectangles. Dimensions are approximate.
Frequently asked questions
What is the formula for the area of a rectangle?
Area = length x width. If the length is 10 m and the width is 6 m, the area is 60 m2. The result is always in squared units: square metres (m2), square feet (ft2), square centimetres (cm2), and so on.
How do I find a missing side if I know the area?
Rearrange the area formula: width = area / length. For example, if the area is 60 m2 and the length is 10 m, the width is 60 / 10 = 6 m. Select "Area + Length (find Width)" in the solve mode dropdown and enter those two values.
How do I calculate the perimeter of a rectangle?
Perimeter = 2 x (length + width). A rectangle with length 10 m and width 6 m has a perimeter of 2 x (10 + 6) = 32 m. Use this when buying fencing, trim, or any material that runs around the outside.
How do I find the diagonal of a rectangle?
Use the Pythagorean theorem: diagonal = sqrt(length2 + width2). For a 10 m x 6 m rectangle, diagonal = sqrt(100 + 36) = sqrt(136) = approx 11.66 m. In construction this is often called a "3-4-5 check" or a "squareness check": if the measured diagonal matches the calculated value, the corners are square.
Is a square a rectangle?
Yes. A square is a special rectangle where length equals width. All four sides are equal and all angles are 90 degrees. This calculator works for squares too - just enter the same value for length and width.
What is the aspect ratio of a rectangle?
The aspect ratio compares the longer side to the shorter side, expressed as a ratio such as 16 : 9 or 1.78 : 1. Common aspect ratios include 1 : 1 (square), 4 : 3 (older TV screens), 16 : 9 (widescreen video), and 1.414 : 1 (ISO A-series paper like A4). The golden rectangle has an aspect ratio of about 1.618 : 1.
How do I convert square metres to square feet?
1 m2 = 10.764 ft2. Multiply the area in square metres by 10.764 to get square feet, or divide square feet by 10.764 to get square metres. For example, 60 m2 = 60 x 10.764 = 645.8 ft2. To avoid manual conversions, simply change the unit selector in this calculator to "ft" and re-enter your measurements.