BLASTX is a computer program that calculates internal blast pressure histories in single and multi-room buildings considering a wide range of building geometries. The calculated blast pressure histories include reflections off adjacent surfaces, shock propagation from adjacent rooms, blast wave shadowing around corners and quasistatic blast pressure. The user can specify openings in walls allowing shock and quasistatic pressure to propagate between rooms, which can open only after specified failure criteria are satisfied. BLASTX is based on semi-empirical methods, including nonlinear addition laws for blast pressures from multiple reflecting surfaces based on computational fluid dynamics. BLASTX has wide ranging capabilities for calculating blast loads for many different high explosive charge and room geometries, but it is not validated for all of these cases.