Optical packet switching (OPS) is being considered as one of the switching technologies for a future optical internet. For contention resolution in an optical packet switching (OPS) system, the wavelength dimension is generally used in combination with a fibre delay line (FDL) buffer. In this study, the authors propose to reduce the number of tunable wavelength converters (TWCs) by sharing TWCs for cost-effective design of an asynchronous OPS system with a shared or an output FDL buffer. Asynchronous and variable-length packets are considered in the OPS system design. To investigate the number of TWCs needed for the OPS system, an algorithm is proposed, which searches for an available TWC and an unused internal wavelength, as well as an outgoing channel. This algorithm is applied to an OPS system with a shared or an output FDL buffer. Also, the number of internal wavelengths (i.e. the conversion range of the TWC) needed for an asynchronous OPS system is presented for cost reduction of the OPS system.