 Average nucleotide identity, ANI, is a robust and useful measure to gauge genetic relatedness between two genomes. Here, the authors develop fast ANI, a method to compute ANI using alignment free approximate sequence mapping, and show 95% ANI is an accurate threshold for demarcating prokaryotic species by analyzing about 90,000 prokaryotic genomes. This article was offered by Chirabh Jain, Luis M. Rodriguez R., Adam M. Felipe, and others.