Write down the monthly payment of your anticipated loan.
Calculate your total gross monthly income. One way to do this is to take your gross annual salary and divide it by 12. The other method is to take your current gross year-to-date income and divide it by the ending pay period as represented in months. This information can be found on your latest paycheck stub. For example if your year-to-date income is $35,000 for the pay period ending July 15, 2010, the calculation would be $35,000 divided by 7.5 months, which equals $4,667.
Write your gross monthly income down next to the anticipated new payment.
Calculate the PTI by dividing the anticipated monthly payment by the gross monthly income. The result will be a decimal number less than 1. For example, if your gross monthly income is $5,000, and the anticipated new payment is $426, the PTI would be 0.09 or 9 percent.