With the increasing globalization of drug development, it has become important that data from multi-regional clinical trials (MRCTs) can be accepted by regulatory agencies across regions as the primary source of evidence to support marketing approval of drugs. The Japanese MRCT guidance presented two criteria for evaluating consistency, which were generalized and evaluated using conditional, unconditional and joint approaches. For using the three approaches to evaluate regional consistency requirements, we propose a unify method in sample size and assurance probability calculation. The method involves simulating sufficient test statistic vectors across regions and its outputs are accurate in comparison with results from several published papers using complex analytical derivation and numerical integration. The method can be conveniently implemented by a statistical practitioner and applicable to more general consistency criteria across regions.