To calculate isomers there are two methods: enumeration and counting. Finding the number of isomers in a given formula is called "counting." Finding the number of isomers, and all the individual elements of a formula is called "enumeration."
If you go to www.scctm.org./Awards/Ballard_Paper.pdf you will find a paper, which explains the calculation of numbers of isomers. Although, this article is chemistry specific, the first part of the article explains calculating the number of isomers.
On page 7 of the above paper you will find information about computerized calculations of isomers using enumeration. If you need to review isomers in general I suggest you go to www.chemistry-react.org/go/Tutorial/Tutorial_13027.html A site all about isomers. Just below the words, A creative look at Isomers, there is a green link. Click and you go to a new page Creative-Chemistry.org.uk Here you will find a simple definition of isomers, structural isomers, stereo isomers and more.