Rectangle Area Calculator: Area, Perimeter and Diagonal

How to use the rectangle area calculator

1. Enter the length of the rectangle in the first field.
2. Enter the width in the second field.
3. The area, perimeter and diagonal update automatically below.
4. Change either value to recalculate instantly.

How the rectangle area is calculated

The area of a rectangle is found by multiplying its length by its width. The formula is straightforward:

Area = length x width

For example, if a garden bed measures 12 meters long and 5 meters wide, the calculation is 12 x 5 = 60, giving an area of 60 square meters.

The perimeter is the total distance around the outside edge. Because a rectangle has two pairs of equal sides, the formula adds the length and width together and doubles the result:

Perimeter = 2 x (length + width)

Using the same 12 by 5 garden bed: 2 x (12 + 5) = 2 x 17 = 34 meters of fencing would be needed.

Finding the diagonal

A rectangle's diagonal splits it into two right triangles. You can find the diagonal length using the Pythagorean theorem, where the length and width are the two shorter sides:

Diagonal = sqrt(length2 + width2)

For a rectangle that is 3 units long and 4 units wide, the math works out to sqrt(9 + 16) = sqrt(25) = 5. This 3-4-5 relationship is one of the most common right triangles in construction and carpentry because all three sides are whole numbers.

Common rectangle dimensions reference

This table shows the area, perimeter and diagonal for several frequently encountered rectangle sizes. Use it as a quick check alongside the rectangle area calculator above.

Practical uses for rectangle measurements

Knowing how to calculate rectangle area comes up constantly in everyday tasks. When buying flooring, carpet or tile, suppliers price materials by the square foot or square meter. Measuring the room's length and width and multiplying them tells you exactly how much material to order, plus a small overage for cuts and waste.

In landscaping, determining the area of a rectangular lawn helps you buy the right amount of seed, fertilizer or sod. For painting walls, calculating the area of each rectangular surface lets you estimate how many gallons of paint the job requires.

Fabric and sewing projects also rely on these calculations. If you need to cut a rectangular piece of cloth for curtains or upholstery, finding the area ensures you purchase enough yardage without overspending.

Rectangles versus squares

A square is simply a rectangle where the length and width are identical. All squares are rectangles, but not all rectangles are squares. The formulas used in this rectangle area calculator work perfectly for squares too. If you enter the same number for both fields, the area becomes the side squared, the perimeter becomes four times the side, and the diagonal becomes the side multiplied by the square root of 2.

If you need to find the area of a shape with three sides instead of four, use the triangle area calculator, which applies different geometric formulas depending on the information you have available.

Units and measurement tips

This tool does not lock you into specific units. Whether you measure in inches, feet, centimeters or meters, the math stays the same. The only rule is that both the length and width must be entered in the same unit. If you mix feet and inches, convert everything to one unit first before entering the values.

The resulting area will be in square units (for example, square feet if you entered feet). The perimeter and diagonal will remain in the original linear unit. Keeping track of units prevents costly ordering mistakes when purchasing building materials or fabric.

Worked examples

Each result below was produced by the same code the tool runs, so you can check your own numbers against them.