Unveiling the relationship between taxonomy and function of the microbiome is crucial to determine its contribution to ecosystem functioning. However, while there is a considerable amount of information on microbial taxonomic diversity, our understanding of its relationship to functional diversity is still scarce. Here we used a meta-analysis of 377 completely annotated and taxonomically different fungal genomes to predict the total fungal microbiome functionality on Earth with all known functions from level 3 of KEGG Orthology using both parametric and non-parametric estimations. The unsaturated model described the accumulation of functions with increasing species richness significantly better (P-value < 2.2e−16) than the saturated model suggesting the presence of widespread and rare functions. Consistent with the parametric approach, the non-parametric Chao-1 estimator that assumes a maximum functional richness did not reach a plateau. Based on previous estimates of fungal species richness on Earth, we propagated the unsaturated model to predict a total fungal microbiome functionality of 42.4 million. Of those, only 0.06% are known today since the vast majority belongs to yet unknown rare functions. Logically, our approach not only highlighted the presence of two types of functions but pointed towards the necessity of novel and more sophisticated methods to unveil the entirety of functions to fully understand the involvement of the fungal microbiome in ecosystem functioning.