Optimum moisture content (MC) for three different compostable materials was estimated through the relationship 'MC vs. FAS', where FAS is the free air space, which is calculated from the values of both particle and bulk density (method 1). As the determination of particle and bulk density was complicated and tedious, two simplifications of the method 1 were tested to obtain an easier approach to establish the optimum MC. The first one (method 2) assumed that bulk density was a constant value (calculated for dry samples), independent of the moisture content of the sample. The second approach (method 3) was based on the measurement of the porosity of the materials. Independently of the method used for the estimation of FAS, values of MC and FAS were linearly correlated with high determination coefficients. Regression lines of methods 1 and 2 were comparable only for a fibrous material (cotton gin trash) because bulk density increased slightly when MC increased. For the other materials (grape marc and olive press cake) the simplification assumed by method 2 was not valid due to great increases of bulk density when MC increased. However, the comparison of the regression lines showed that methods 1 and 3 were not statistically different for the three materials tested. Thus, the optimum MC (FAS = 30-36%) of compostable materials can be estimated by a simple method that requires only easy measurements of porosity at several moisture contents.