Models of the dust grains in the planetary nebula NGC 3918 are presented. The models, which are calculated for four grain materials -graphite, amorphous carbon, silicate, and iron- have a size distribution of particles based on that found for the diffuse interstellar medium. The infrared spectrum of the nebula -described mainly by IRAS photometry corrected for line emission- can be matched either with graphite grains with a size range of 0.04 - 0.30 μm, or with amorphous carbon grains having a size range 0.0005 - 0.25 μm. The implied depletions of gas phase carbon are only 11% and 4%, respectively. It is shown that iron grains cannot be the dominant dust material.