This bachelor’s thesis presents principles of calculation of natural circulation in BFB-boilers. The operating principle of such boilers is briefly described and calculation programs utilized in analysis of natural circulation researched. Natural circulation in water-steam- is possible due to hydrostatic pressure produced by the density difference between the riser and downcomer pipes. When calculating water/steam circulation, the characteristics of two-phase flow have to be known. Steam and water flow in pipes at different velocities and, therefore, when assessing pressure drop correlations have to be used in design of natural circulation loops. Three different equations are needed in calculation of two-phase flow: energy balance, mass balance and momentum balance. When designing natural circulation there are two main requirements. Firstly, it has to be ensured that the mass flow of circulation is adequate so that dryout and burnout of pipes can be avoided. Secondly, temperature differences and vibrations in tubes have to be prevented. In preliminary design of natural circulation knowhow is often used. Programs such as NOWA, PPSD and Apros are used for calculation of specific design elements. Calculation begins with evaluation of water-steam-mixture mass flow. Example calculations with NOWA and PPSD are presented in this thesis. The analysis shows that results depend on the calculation model.