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Area of a Rectangle Calculator

If you need to find the area and perimeter of a rectangle, this calculator is the handy tool you will need.

By simply inputting the length and width, this calculator will almost instantly find the perimeter (P) and the area (A).

If interested in calculators for a variety of other shapes, you can look at more of our handy calculators. But you can stick around here and learn more about finding the area of a rectangle.

A rectangle has four 90 degree angles. If the lengths of the sides are all the same, then the rectangle is also a square. The lengths of the sides will be given as a,b or you can use l and w for "length" and "width". The diagonal, which goes from one vertex to the opposite vertex cutting the rectangle into two squares, is called the diaganoal and noted as d.

Here are the basic formulas used by the calculator.

Area = a(b) and noted as (A)

Perimeter (distance around the outside of the rectangle) = a + a + b + b or 2a + 2b  and noted as (P)

Diagonal is d² = a² + b² which is the Pythagorean Theorem (see our Pythagorean Theorem calculator).  Note that to solve for d, take the square root of both sides of the =.

Example of calculating area of a rectangle.

Suppose the length is a = 6 inches

and the width is b = 4 inches

A = a*b , so A = 6(4) = 24 inches²

Using the same dimensions, we can calculate the perimeter.

The perimeter is 2a + 2b, so in this example the perimeter P = 2(6)+2(4) = 20 inches

Next to find the diagnoal using the same dimensions

d² = 6² + 4² = 36 + 16 = 52

Take the square root of both sides and the diagonal d is approximately 7.2 inches

These examples illustrate how to calculate manually, but if you prefer to use the calculator for quicker results or to merely check your work, then feel free to do so. A great feature of the calculator is that you can find either the length of the width if you know the perimeter and the one of the two dimensions.