High Entropy Alloys (HEAs) are multicomponent systems incorporating several elements in a nearly equiatomic configuration. The content of each solute can typically vary between 5 and 35 at%. The high entropy associated to mixing several elements can inhibit the formation of intermetallic phases in favour of FCC or BCC solid solutions. Existing rules for predicting HEAs formation are at an incipient form, generally not offering information on the crystal structure. In this work, the interatomic spacing mismatch and bulk modulus mismatch across the lattice are considered for predicting the occurrence of HEAs. The work follows similar approaches to predict the formation of intermetallic phases and bulk metallic glasses, allowing the prescription of FCC or BCC HEAs occurrence. A statistical analysis on the reliability of the complete set of rules for predicting HEAs has been achieved by analysing approximately 400 different compositions.