Pick one side of the triangle, and determine its length. This is the base.
Measure the perpendicular distance from the base to the vertex opposite it. This is the height.
Multiply the base of the triangle by its height.
Using the formula for finding the area of a triangle, divide the product of base and height by 2. If the base is 4 centimeters and the height is 5 centimeters, then the area is (4 x 5)/2, or 10 square centimeters.