Find the area of a three-sided pyramid's base by multiplying the length of one side of the base by the height of the triangle that forms the base, then divide the answer by two. For example, if the side of the triangle that forms a pyramid's base is 40 feet long and the height of the triangle is 34.64 feet, the area of the base equals (40 feet times 34.64 feet) / 2, or 692.8 square feet.
Use the following formula to find the area of the base of a three-sided pyramid when you do not know the height of the triangle that forms the base: Area = (length of side squared * square root of 3) / 4. The square root of 3 is 1.732. So if the sides of your base triangle are 40 feet long, the area of the base equals (40 feet times 40 feet times 1.732) / 4, or 692.8 square feet.
Calculate the area of a four-sided pyramid's square base by multiplying the length of the side times the length of the side. For example, if the length of the side is 40 feet, multiply 40 feet by 40 feet to get an area 1,600 square feet.