Questions on returning security deposit

[Jeff B.]

I have a tennant that is scheduled to move out at the end of September. They have been in the house for 16 months and part of their rent has been a $50 monthly charge for 2 pets ($25 each). When they move out, I will likely need to replace the carpet that I put in 16 months ago due to their cat urinating on the carpet.

Question is, how should I calculate the charge for replacing the carpet? They have paid $800 total additional for the pets. Does the replacement come out of the security deposit or is the additional charge to be applied to the replacement carpet only.

Thanks for your help!!