 Do this calculation FIRST as if you're NOT making extra payments.  

EXAMPLES:
 If you want to calculate how much a mortgage payment will be on a $200,000 mortgage at 4.25% interest for 360 months, you would enter:
 200,000 for Loan Amount
 360 for Months
 4.25 for Interest Rate (Compounded Monthly)
 Press the Payment button, and you'll see that your payment would be $983.88. You will pay about $154,196.69 in interest over the life of this loan. Make sure that the month and year of your first payment is reflected in the first payment due field (in this case June 2015).
Now, let's say you would like to make extra monthly principal payments of $116.12 (to round the payment to $1100) for the next 10 years starting in June of 2015. You'll enter:
 Monthly for how often extra principal payments will be made.
 116.12 for the extra payment amount
 Select June and 2015 as the first extra payment date
 Select June and 2025 as the date to which the payment will be made. (May will be the last extra payment.)
 Press the View Amortization Schedule button, and you'll see that your mortgage will be paid in 322 months (instead of 360 months) and you'll pay about $130,279.28 in interest (instead of $154,196.69).
 In this next example, let's say you took out a 30 year (360 months), $200,000 mortgage in May of 1996 (first payment due June 1996) at 7.5% interest. Your required payments are $1398.43, yet you have been making monthly payments of $1500 (which you intend to continue) with the excess going to principal. Recently, you have received an inheritance and have decided to apply an additional $25,000 to your mortgage principal in Jan 2014. When will the mortgage be paid in full?
Enter:
 200,000 for the Loan Amount
 Leave the Months field blank
 7.5 for Interest Rate (Compounded Monthly)
 1500 for Payment
 Select May 1996 for when the first payment was due.
 Press the Months button, and you'll see that you'll pay $1500 for 287.58 months before your mortgage will be paid in full. You'll pay about $231,365.95 in interest over the life of this loan.
Next...
 Select One Time Only
 25000 extra payment
 Select Jan and 2014 for when you'll make the extra payment
 Just change this field to any YEAR AFTER the One Time Only payment will be made.
 Now, press the View Amortization Schedule button. You'll see that your mortgage will be paid in just 263 months (instead of 288 months) and you'll pay just $219,436.93 interest (instead of $231,365.95). You'll make your final mortgage payment in March of 2018!  YEAH!!!

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