PPC (pay per click) advertising is a highly targeted marketing tool that's incredibly effective. However you need to understand what works for you. Thankfully there are certain metrics you need to track, as well as formulas you need to know in order to calculate the true value of your PPC efforts. The two most important formulas you need are Cost Per Click and Return On Investment. There are two related formulas that will give you a better understanding of your pay per click performance.