Forecasting a ventilation process is based on two factors: using a validated software (Dziurzyński et al., 2011; Pritchard, 2010) and a properly prepared database encompassing the parameters describing the flow of air and gases, compatible with the adopted mathematical model of the VentGraph software (Dziurzyński, 2002). With a body of measurement data and a mathematical model for computer calculations and air flow simulation at our disposal, we proceed to develop a numerical model for a chosen network of mine headings. Preparing a numerical model of a ventilation network of a given mine requires providing a collection of data regarding the structure of the network and the physical properties of its elements, such as headings, fans, or stoppings. In the case of fire simulations, it is also necessary to specify the parameters describing the seat of a fire and the properties of the rocks of which the rock mass is comprised. The methods which are currently applied to this task involve manual ventilation measurements performed in headings; the results obtained in the course of these measurements constitute a basis for determining physical parameters, such as the aerodynamic resistance of a heading, density of the flow of air, or natural depression. Experience shows that - due to difficulties regarding accessibility of headings, as well as the considerable lengths of the latter - there are some nodes and headings in mines where such measurements are not performed. Thus, an attempt was made to develop a new methodology that would provide the missing data on the basis of some other available information concerning - for example - the air density, the geometry of headings and elevations. The adopted methodology suggests that one should start with balancing the air mass fluxes within the structure of a network of headings. The next step is to compile a database concerning the pressure values in the network nodes, based on the measurement results - and provide the missing pressure values on the basis of the available results of measurements carried out in adjacent nodes, as well as the pressure value calculated on the basis of the heading geometry and the given volumetric flow rate. The present paper discusses the methodology of compensating and balancing the volumetric air flow rates within a network of headings (Chapter 2) and the methodology of determining pressure values (Chapter 3) in the nodes of the network. The developed calculation algorithms - verified by means of sample calculations performed for a selected area of a mine ventilation network - were introduced into the VentGraph software system. The calculation results were presented in tabular form. The Summary section discusses the minuses and pluses of the adopted methodology.