How to Calculate Principal & Interest Using a Rate Factor Sheet

Your monthly mortgage payment may also include a prorated property tax amount as well as a prorated homeowners insurance amount. If your mortgage payments are structured this way, you’re making PITI payments (principal, interest, taxes and insurance). Or you may be making only P&I (principal and interest) payments to your mortgage lender, being responsible for paying your taxes and insurance separately. Amortization schedules vary, depending on how each mortgagor makes payments; but if you want to know the amount of principal and interest payments that you’re paying each month for each $1,000 that you borrowed, as well as the total interest repayment, you can use a rate factor sheet.

An interest rate factor sheet, also called an interest rate factor chart, is a multicolumn table that displays various interest rates and loan terms. For each row that represents a different interest rate percentage, there’s a corresponding rate factor in the column that represents different loan terms. First you’ll find the interest rate you’re paying, typically by following the rates vertically down the left-hand side of the rate factor sheet, and then you’ll follow the row horizontally until you find where your rate intersects with the cell in the column under your loan term.

The beauty of using an interest rate factor sheet is that the interest rate factor formula is already calculated for you and built into the numbers that are listed on the sheet. By using this chart, you don’t have to make complicated math calculations to amortize your loan. The only math you’ll have to use involves a few simple calculations by using basic subtraction, multiplication and division.

Follow the interest rates down in the left-hand column on the rate factor chart until you find the interest rate for your mortgage. Follow this row to the right until you find the data cell underneath the term of your mortgage. For example, if your interest rate is 5 percent and the term of your loan is 15 years, the rate factor at the intersection of these two numbers is 7.91.

Divide the total amount of your mortgage by 1,000 to determine your monthly payment for each $1,000 you borrowed. If the amount you borrowed for your mortgage was $185,000, divide that number by 1,000 for a total of $185. Multiply $185 by the rate factor of 7.91 to calculate your monthly mortgage payment of $1,463.35.

Multiply the number of years in your mortgage by 12 to find the total number of your monthly payments. For the example above, you have a 15-year mortgage with a total number of 180 payments (15 years x 12 months/year). Multiply this total number of payments (180) by the monthly payment you just calculated above ($1,463.35) to figure the total amount of all your payments is $263,403.

Finally, subtract the original amount you borrowed ($185,000, the principal amount) from the total amount of all your payments over 15 years ($263,403) to calculate the amount of interest you’ll pay over the life of your mortgage, which is $78,403.

You can find a rate factor sheet from various sources, including your local Multiple Listing Service agent, your real estate attorney or online. If you search online, you can also find rate factor sheets (or rate factor charts) as tables that are listed as “monthly payments per $1,000 of mortgage.”