The space holder technique was widely used to fabricate metal foams specially for titanium foam. However, how to obtain the desired porosities has been a big challenge for this technique, because they are always not equal to the expected ones. The results of the previous study (i.e., P=ax+b. where a=1/(1+δ), b=δ/(1+δ)) resulted in a very interesting conclusion, that is the rate change of pore volume (δ) is an indefinite mathematical constant. On the basis of research works, this paper found a new results by establishing mathematical model which can be shown as equation δ=φ-1. Here, φ is the length index product stands for the ratio between actual value of length and designed length for a sintered metal foam. It reveals that the length index product (φ) is also an indefinite mathematical constant which we can measure its value. Therefore, δ was solved, means both a and b are solved, so the porosity (P) of titanium foam can be predicted by the equation P=ax+b dependent on the spacer content (x). It indicates that in the absence of measurement of porosity, the macroscopic dimensions of the sintered metal foam can be measured to obtain the control equation of porosity. The research of this paper provides new ideas and methods for the structural control of metal foams prepared by space holder technique based on powder technology.