Vector selection requires one to ensure the library made is representative of the entire genome. Any insert of the genome derived from a restriction enzyme should have an equal chance of being in the library compared to any other insert. Furthermore, recombinant molecules should contain large enough inserts ensuring the library size is able to be handled conveniently. This is particularly determined by the number of clones needed to have in a library. The number of clones to get a sampling of all the genes is determined by the size of the organism's genome as well as the average insert size. This is represented by the formula (also known as the Carbon and Clarke formula):
where,
 is the necessary number of recombinants
 is the desired probability that any fragment in the genome will occur at least once in the library created
 is the fractional proportion of the genome in a single recombinant
 can be further shown to be:
where,
 is the insert size
 is the genome size
Thus, increasing the insert size (by choice of vector) would allow for fewer clones needed to represent a genome. The proportion of the insert size versus the genome size represents the proportion of the respective genome in a single clone. Here is the equation with all parts considered: