Intrinsic stress in a film-substrate system can have deleterious effects. To facilitate an understanding of stress generation and control film quality, measuring film stress is essential. In recent years research laboratories and industry have increasingly adopted indirect methods, which are usually based on the measurement of substrate deformation. The film stress is calculated by equations relating the stress to the deformation, such as the well-known Stoney's equation. However, when the two principal stresses at each point in the film plane are not equal and their distribution is nonuniform, the local application of Stoney's equation does not provide correct stress results. A numerical technique is presented, which overcomes these limitations and makes accurate stress determination possible.