Rectangle Area Calculator

This rectangle area calculator lets you find the area from length and width, but also offers additional functions. You can enter any two values, such as two sides, area and side, diagonal and side or perimeter and side, and the tool will calculate all other values.

After you enter two values and select the units, the area will be shown as the main result, with the perimeter, diagonal, and length of the missing side displayed next to it. Since a rectangle can be fully defined by any two measurements, there is no need to pre-select menus.

What is the area of a rectangle?

A rectangle is a quadrilateral with four right angles. The opposite sides of a rectangle have the same length and are parallel. The area of a shape measures the amount of space inside it. In other words, it is the number of unit squares that fit inside the shape.

Because the unit squares are arranged into rows and columns, their number can be calculated by multiplying the length by the width. The area is always expressed in square units, such as square meters or square feet. For land areas, hectares or acres are used.

Formula for calculating the area of a rectangle.

The basic formula multiplies the lengths of two sides.

In this formula A represents the area, l is the length and w is the width. If one of these sides is in meters then the resulting area will be in square meters. If one of these sides is in feet then the resulting area will be in square feet. So make sure that both sides are in the same units before multiplying them together.

If the area is known, then the same formula can be reversed to find the length of a missing side.

Perimeter and diagonal of a rectangle

The remaining two measurements can also be directly derived from the side lengths. The perimeter is the distance along the outer edge while the diagonal is a straight line that connects two opposite corners. These values can be calculated using the Pythagorean theorem.

This calculator allows you to work backwards in two cases. If you enter the perimeter and one side, it calculates the other sides. If you enter the diagonal and one side, it calculates the remaining values. This is also a way to calculate area from "diagonal and width".

Calculating area by diagonals and a side.

If only the diagonal and one side are known, first use the Pythagorean theorem to find the missing side, then multiply those two sides together. Let's call the known side "b" and the diagonal "d".

The diagonal is always the longest value that cuts through a rectangle so it must be longer than any of its sides. If the entered diagonal is shorter than one side then such a rectangle cannot exist and the calculator will display a warning.

Use of the calculation tool:

Enter any two measurements and our calculator will find out the rest. The most common combinations are:

Examples of calculations:

Consider a rectangle with length of 5 centimeters and width of 4 centimeters. The area can be easily calculated by multiplying the two sides together.

The perimeter of this rectangle is 18 centimeters, which is equal to 2 times the sum of its length and width (5 + 4). According to the Pythagorean theorem, the diagonal is approximately 6.4 centimeters long. If you enter 5 and 4 in the fields for length and width, these results will be displayed simultaneously.

Calculating length from area:

Suppose a room has an area of 20 square meters and is 4 meters wide. The length can be calculated by dividing the area by the width.

If you enter 20 into the field for area and 4 into the field for width and leave the length field blank, the calculator will provide a length of 5 meters and also show the perimeter and diagonal.

An overview of important information about rectangles.

Area units: From square feet to acres.

Since area is the product of two lengths, the units for area are square units of length. If the side length is given in meters, then the unit will be square meters; if it is given in feet, then the unit will be square feet. For larger areas acres or hectares are used.

When you change the unit on any column, the calculator automatically converts all results. So for example, if you enter the dimensions of a garden in feet, you can read off the area directly in square meters or acres without doing any manual conversions.

The golden rectangle

A rectangle is a golden rectangle if its longer side is approximately 1.618 times the length of its shorter side. This value is known as the Golden Ratio and this ratio appears again and again in art, architecture, book covers, and credit card shapes.

The golden rectangle has an interesting property: if you cut a square off one corner of it, the remaining rectangle is also a golden rectangle, although smaller. The aspect ratio map shows how close your rectangle comes to this famous ratio.

If the perimeter is constant, then a square has the maximum area.

Of all rectangles with a given perimeter, the square has the largest area. A fence of length 20 meters can enclose a square with sides of 5 meters and an area of 25 square meters. If you use the same fence to enclose a rectangle that is 9 meters long and 1 meter wide, the area will be only 9 square meters. The closer a rectangle is to being a square, the larger the area it can enclose with the same perimeter.

For this reason, animal enclosures, plots of land and packaging often have a shape that is close to a square, as the goal is to contain as much space as possible with the shortest perimeter.

Areas of application for rectangular surfaces:

Calculating the area of a rectangle is one of the most useful calculations in everyday life. It allows you to determine how much carpet, laminate or tiles you need for a floor and how much paint you will need for a wall or lawn seeds for your garden.

It can also be used to determine the size of a plot or the area occupied by a building, calculate how much fabric is needed for curtains or tablecloths, or work out the area that can be printed on one side of paper or plywood. The cost estimation function allows these areas to be converted directly into prices.

Tips for accurate results:

Measure both sides with the same unit before inputting values or use the conversion function to convert values into the calculator. If it is not a perfect rectangular space, divide it into several rectangular areas, calculate the area of each and then add them together.

When buying flooring or paint by the square foot, you should normally allow for a five to ten percent reserve to account for waste, pattern matching and mistakes. If possible, measure a diagonal and use this to check. Add together the diagonal and one side and make sure that the result is a rectangle.