How to Find the Area of a Triangle

The area of a triangle is half the area of a related rectangle or parallelogram. That is why the formula includes one half.

Quick idea

Choose a base, find the height perpendicular to that base, then multiply by one half.

Steps

1. Choose the side you will use as the base.
2. Find the height that meets the base at a right angle.
3. Multiply base times height.
4. Divide by 2 and label with square units.

Worked example

Find the area of a triangle with base 10 and height 6.

Full solution

Step-by-step walkthrough

Step 1

The base is 10 and the height is 6.

Step 2

Ten times 6 is 60.

Step 3

Half of 60 is 30 square units.

Common mistake

The height must be perpendicular to the base. A slanted side is not always the height.

Practice problems

1. Find the area with base 8 and height 5.
2. Find the area with base 12 and height 3.
3. A triangle has area 24 and base 8. Find the height.

Answers

1. 20 square units
2. 18 square units
3. 6

Ask Pennpaper to explain it live

If the steps make sense but you still feel stuck, start a Pennpaper lesson and ask the tutor to draw the problem on the whiteboard. Seeing the symbols move step by step is often what makes the concept click.

Related guides

• Area and Perimeter Explained
• Pythagorean Theorem Explained
• Similar Triangles Explained