The feasibility of adjusting individual composting processes to be able to produce the desired mass of compost of the required composition was evaluated. Data sets from experiments in tunnels were constructed and analyzed. Total mass and dry matter contents at the start and at the end of composting contained much statistical error. Error was propagated into the calculated central parameter of the process, the loss of dry matter. Water loss was estimated based on dry matter loss, heat generation and evaporation in a model. Estimated and actual losses from individual processes almost lacked correlation but the averages were rather similar. It is not the model but the error in input data that prevent the accurate prediction of the losses of water and of total matter. Moreover, error masked any correlation between the loss of dry matter and processing parameters. A model cannot be successfully applied to adjust an individual composting process. Compost producers should focus on getting the composition of the substrate constant at the start of processing. Adjusting an individual process is not a very reliable option.