Metagenomic studies have dramatically expanded our knowledge of the microbial world. Furthermore, the amount of sample for sequencing has significantly increased with the development of high-throughput sequencing technologies. However, fully capturing all DNA sequences carried by every microorganism in the environment is still impossible. Therefore, estimating a reasonable and practical amount for sequencing to achieve the objectives is particularly necessary. In the present study, we introduce a novel method for estimating the required minimum amount for metagenomic sequencing for a given goal. We also calculated the genomic proportion of each operational taxonomic unit and the detection efficiency of a specific gene (we have used SSU rRNA gene as an example) based on a given amount for random metagenomic sequencing. The reasonable and practical estimated amount for sequencing in metagenomic studies will provide good reference information when applying high-throughput sequencing for a given goal.