To calculate interest, multiply the periodic interest rate by the principle amount. For example, if you borrowed $1000 with an interest rate of 10 percent, in a year your interest paid is $100.Continue Reading
Find your interest rate in your financial records for the product for which you are calculating interest.
For example: $1000 in principle times 0.1 (10 percent periodic interest rate) equals $100.
Once you determine the amount of interest, check with your bank or other institution to confirm that your rate does not vary.