How to Calculate Area of an Object

By Nicole Harms; Updated April 24, 2017
Calculate Area of an Object

Finding the area of a shape or three-dimensional object is a skill that almost any math student must master. Not only is area important in math class, but it is also something that you will use regularly in real life. For instance, when you need to figure out how much paint to buy for your room, you will need to know the area of the wall. Finding area may be a foundational math concept, but many students struggle with it because they do not learn the basic formulas. If you know the formulas and can apply them, you can master finding area.

Decide what type of shape the object is. This will determine the area formula that you will use.

Find the area of a square or rectangle by multiplying the length times the width. This formula looks like l*w. If the length is 5 and the width is 2, the area is 10 square units.

Calculate the area of a four-sided shape that is not a rectangle by multiplying the base (one of the sides) by the height. The height is a line drawn from the top of the shape to the base forming a right angle with the base. If the base is 10 and the height is 4, the area is 40 square units.

Find the area of a triangle by multiplying the base times the height and then dividing it by two. The base can be any side of the triangle, and the height is the measurement from that base to the vertex above it. This formula looks like (b_h)/2 or ½ b_h and is derived from the fact that a triangle is half of a four-sided shape. If the base is 10 and the height is 4, the area is 20 square units.

Determine the area of a circle by squaring the radius and multiplying it by pi, or 3.14. This formula looks like pi*r^2. If the radius is 5, the area is 78.5 square units.

Find surface area of a three dimensional shape by finding the area of each face separately using the above formulas, and then adding these areas together.

Tip

When you find the area of a circle, it is approximate, because pi is a never-ending, never-repeating decimal, so 3.14 is simply an approximation for the actual ratio. Remember to double check that you counted all of the faces, or flat sides, on a three-dimensional shape when you are looking for the surface area.

Warning

Always add the units, such as square inches or square feet, to your answer. Many teachers will count the answer incorrect if it does not have the units.

About the Author

Nicole Harms has been writing professionally since 2006, specializing in real estate, finance and travel. When she's not writing, she enjoys traveling and has visited several countries, including Israel, Spain, France and Guam. Harms received a Bachelor of Science in Education from Maranatha Baptist Bible College.