Explanation: A Fermi is the earlier non–SI measurement unit of length, comparable to a femtometre. It is named after Italian physicist Enrico Fermi.
1 Fm = 10-15 m
In nuclear research, Fermi is used to measuring extremely short distances. A variety of different units are being used to evaluate such small lengths. SI units are international measurement systems. The SI units are expressed without the need for other fundamental variables such as height, pressure, etc.