This calculator is taken from the "How to Create a Monster" article found in Dragon 276 (and on this site). It is used to assign the CR to any "non-epic" monster. Epic monsters use a different system than the one presented (and I am working on posting this one as well).

To determine a monster's CR follow the steps below.

1. Take the monster's average hit points and divide by 4.5
2. For each Special Attack and Special Quality add +1 or +2. Here are some suggestions.

• +2 for damage reduction, spell-like abilities, spells, SR, regeneration, fast healing or any attack that deals permanent ability damage.
• +1 for all other Special Attacks or Special Qualities.

The above is just a guesstimator. The best way to find a creature's CR is through playtesting.