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Save time by using an online calculator. Find your interest rate due on each payment. Multiply your monthly percentage rate times your principal. Input this number into the monthly payment formula. Calculate the amount of principal paid each month. Subtract your principal paid each month from your monthly payment.

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There are many car loan payment calculators available for free online. Take advantage of these free services if you don't want to spend the time calculating your payments yourself. Search "Car loan payment calculator" and you will be provided with many options. If you still want to work it out by hand, continue to the next step. Start by converting your APR to a decimal by dividing it by 100. For example, if your APR is stated at 8.4%, 8.4/100 = 0.084. Next, find your monthly percentage rate by dividing your APR decimal by 12. So, 0.084/12 = 0.007. This is your monthly percentage rate expressed as a decimal. If, for example, your principal were $20,000 (if you borrowed $20,000 to buy your car), you would multiply this by 0.007 (from the previous step) and get 140. The formula is as follows: Monthly Payment  = (Interest rate due on each payment  x  principal)/ (1  –  (1  +  Interest rate due on each payment)^  -(Number of payments)) The top part of the equation (interest rate due on each payment x principal) is your number from the previous step. The rest can be calculated using a simple calculator.  The "^" indicates that the figure (-(Number of Payments)) is an exponent to the figure (1 + Interest rate due on each payment). On a calculator, this is entered by calculating 1 + interest rate due on each payment, hitting the button x^y, and then entering the number of payments. Keep in mind that the number of payments is made negative here (multiplied by negative one). In our example, the calculation would go as follows (assuming a loan duration of 5 years or 60 months):  Monthly Payment = (0.007 x $20000)/(1-(1+ 0.007)^-60 Monthly payment = $140/(1-(1.007)^-60) Monthly payment = $140/(1-0.658) Monthly payment = $140/0.342 Monthly payment = $409.36 (this number may be off by a few cents due to rounding) This is done by simply dividing your principal amount by the duration of your loan in months. For our example, this would be $20,000/60 months = $333.33/month In our example, this would be $409.36 - $333.33. This equals roughly $76. So, with this loan agreement, you would be spending $76 per month in interest payments alone.