When an accidental gas explosion occurs, a flame propagates spherically from the ignition point through the mixture of a combustible gas and air; the flame radius may reach 10 m or greater. Such a large-scale flame has a fractal structure induced by hydrodynamic effects, and the flame area, to which the flame speed is proportional, can be expressed in terms of the flame fractal dimension. Because the damage caused by an explosion is significantly influenced by the flame speed, knowledge of the fractal dimension can be used to estimate the consequences of the explosion. This paper discusses methods to predict the flame fractal dimension by computationally simulating the planar flame propagation. Three methods are tested and the results are compared. The first method, proposed in an earlier work, computes the fractal dimension from the dependence of the flame speed on the domain size; the second method is a Fourier analysis of the flame shape; and the third method is the widely used box-counting method. It is confirmed that these different methods yield consistent results.