The E value is computed using the following formula, in which l denotes the length of the query string, n denotes the number of strings stored in the database, and S is a score that measures the homology between nucleic acids or between amino acids. Note that k and m are positive constants.

E=k*l*n*exp^(－mS)

If the BLAST output results computed using this formula are displayed in the form 1E-X, this means that the quantity has the value 10^{-X}.