Before answering this question first we need to understand why do cats urinate outside their litter box? 

1. An illnesses can cause your cat uncontrolled urination

2. Maybe the Little box is very uncomfortable for the cat to pee in.

Ways to deter cat peeing on the Carpet:

1. There are some natural cat repellents like Vinegar, coffee, Lavender, Citrus that you can spary or place near the Carpet to keep the cats away.

2. Essential Oils like peppermint and eucalyptus are also a good way to keep you cat away from the Carpet area.

3. If the natural ways don't work, you can try out some chemically induced product that you can easy find on the internet.