How To Calculate Area

Examples of calculating the area of a rectangle

	Area = Length x Width Area = 3 x 2 = 6 square units
	Area = Length x Width Area = 8 x 6 = 48 square units
	Area = Length x Width Area = 9 x 5 = 45 square units

Units for measuring area

We measure area using squares. We use different sizes of squares depending on how big or small an area is.

Example	Length of side on Squares	Unit
Size of the nail on your thumb	Millimeter	mm2
Size of piece of paper	Centimeter	cm2
Size of a room	Meter	m2
Size of a town	Kilometer	km2

 

Don't forget the wee 2

We write square sizes using a small 2 next to the unit. We write mm2, cm2, m2 , km2, cm2 We can say "63 millimeters squared" or " 63 square millimeters"

We could use small squares to measure large areas. The only problem with this is that we would end up having to use very big numbers. For example, a field might be measured at 5,000,000,000 square millimeters when 5,000 square meters would be a much easier size to say, write, and visualize.

You will probably hear more units for measuring area; square inches, square feet, square yards, square miles, acres, hectares are all units used for measuring area.

More Examples of Calculating Area

	Area = Length x Width Area = 9 mm x 4 mm = 36 mm2
	Area = Length x Width Area = 7 cm x 6 cm = 42 cm2
	Area = Length x Width Area = 8 m x 2 m = 16 m2
	Area = Length x Width Area = 7 km x 5 km = 35 km2

Area of a Square

The length and width of a square are the same so we just need to multiply the length by the length.

Area = Length x Length Area = 6 cm x 6 cm = 36 cm2

Area of a Circle

The area of a circle = πr²
where r is the radius of the circle and π is the ratio of a circle's circumference to its diameter.

π (pronounced "pie" and often written "Pi") is an infinite decimal with a common approximation of 3.14159. You can find out more about Pi here

Example of calculating the area of a circle

Area = πr2 Area = 3.14159 x (4 cm)2 Area = 3.14159 x 16 cm2 Area = 50.27 cm2 Answer rounded to 2 decimal places

Explanation of the Area of a Circle Formula

Take a circle and divide it into equally sized sectors and rearrange these as shown below. Notice how, as the sectors become smaller, the shape becomes more like a rectangle. Note: There is no limit to how small these sectors could be and to how closely they could resemble a rectangle when arranged.

Assuming we know that the circumference of a circle is equal to 2πr we can add dimensions to the "rectangle" as shown below. Using the area of a rectangle area formula, area = width x height we can see how our circle, re-configured as a rectangle, can be shown to have an area that approximates to πr x r or πr²

Circle Sectors Rearranged

Circle Sectors Rearranged - Starting to Look Like a Rectangle

Area of Compound Shapes

There are many cases where the calculation of a total area requires more than one area to be calculated followed by either an addition, subtraction, or some other combination of operations to find the required area.

Note: In the examples below the units of measurement are not shown and answers and the value of π (Pi) have been rounded to the nearest hundredth.

Example: Simple Compound Shapes

The area calculation example below is relatively simple. The shape can be seen as a triangle combined with a rectangle.

Area of triangle part: ½ x base x height ½ x 9 x 4 = 18 Area of rectangle part: width x height 9 x 6 = 54 Total area = 18 + 54 = 72

 

The example above illustrates a common requirement when working with compound shapes - finding dimensions that are not shown. When tutoring your children, give help, when needed, to find these "missing" dimensions. There is another example below.

Finding the dimensions

What are the dimensions of the small rectangular part? Width? 12 - 7 - 2 = 3 Height? 8 - 6 = 2

 

Example: Subtracting one area from another

In the example below, the shape can be seen as a rectangle with a triangle cut out.

Area of rectangle part: width x height 5 x 6 = 30 Area of triangle part: ½ x base x height ½ x 3 x 3 = 4.50 Total area = 30 - 4.50 = 25.50

 

Example: Partial areas

The example below is similar to one above although, since we have a semi-circle we need to calculate a fraction (one-half) of the circle's area. Note in this example the diameter, and not the radius is shown.

Area of triangle part: ½ x base x height ½ x 6 x 6 = 18 Area of semi-circle part: ½ x (πr2) ½ x (3.14 x 1.52) = 3.53 Total area = 18 - 3.53 = 14.47

 

Example: Decisions! Combine? Subtract

It is common to have more than one way to calculate the final area. In the examples below the shape can be seen as two rectangles combined or as one large rectangle with a smaller rectangle "cut out" from the top right corner.

	There is no right or wrong method.
	We can calculate the areas of rectangles a and b and then add to get the area. area = (3 x 6) + (6 x 4) area = 18 + 24 = 42
	Or we can calculate an "imaginary" rectangle, c, and subtract rectangle d from it to get the area. area = (9 x 6) - (6 x 2) area = 54 - 12 = 42

Calculating Area Worksheets

Print out the worksheets listed below and use them for practice when tutoring your children.

• Calculating Areas - Rectangles and Squares
• Calculating Areas - Rectangles
• Calculating Areas - Rectangles
• Calculating Compound Areas e.g. with rectangular shapes
• Calculating Compound Areas e.g. with rectangles, triangles, and circles
• Calculating Areas e.g. of Triangles
• Area of a Circle
• Calculating Surface Areas e.g. of Rectangular Prisms

You will find more printable geometry worksheets here.