Calculating the volume of a pyramid is quite simple, provided you are aware of the dimensions. Using the pyramid volume (V) formula, the only thing you have to do is find out the width, length and height of the pyramid.
Measure the width and length of the base. Make sure you use a common measure unit, centimeters (cm) for example.
Multiply the width by the length, to calculate the base area, which we will call "B." If, for example, the width and length are 6 and 7 cm, respectively, then the base area will be 42 cm^2.
Measure the pyramid's height (h). The height is the perpendicular distance between the apex of the pyramid (the tip) and the base. In other words, it's the line that forms a right angle with the base, while connecting top and bottom.
Use the Pythagorean Theorem to find out the pyramid's height, if you are not allowed to use a ruler, as part of an exercise. The theorem states that in any triangle, the square of the side opposite to a right angle, equals to the sum of the squares of the two remaining sides. For example, if the distance between the height axis and a pyramid's side is 3 cm and the side's length is 5 cm, then the height will be: 5^2=3^2 + h^2 or h^2=25-9=16, hence h=4 cm.
Apply the formula V=Bh/3. On our example, it would be V= (42x4)/3= 168/3= 56 cm^3.
Never forget to use squares units (square meters, for example) when you're referring to area and cubic units for volume.