Mortgage Prepayment by Doubling Principal
20 September 2004, Reviewed August 28, 2011

"I have been advised that I can cut the life of my mortgage in half by making an extra payment to principal each month equal to that month’s principal payment. Is this true?"


I have been asked this question many times and have always answered it in the same way. "No, it isn’t true, because your extra payments are too small in the early years."

Only recently did I decide to actually test the idea that doubling the amortization would halve the life of a mortgage. Using an extra payment spreadsheet from my web site, I assumed a 15-year loan at 6%, and made extra principal payments equal to the principal portion of the monthly payment.

I quickly realized that there are two ways this can be done. One way is to make extra payments equal to the original schedule of principal payments. When I did it this way, the loan paid off in 100 months, not 90.

However, if I based the extra payments on an actual schedule that reflects the impact of prior extra payments, payoff occurred in 91 months – just a tad past the halfway mark.

Principal payments based on the original amortization schedule assume no extra payments. In months 1 and 2, they are $343.86 and $345.58. If I pay an extra $343.86 in month 1, however, the actual principal payment for month 2 would be $ 347.30 rather than $345.58. The additional payment of $343.86 in month 1 reduces the balance on which the interest for month 2 is calculated, resulting in less interest and more principal in month 2.

It thus appears that you can cut the life of your mortgage in half, or almost in half, if you make extra payments to principal, provided the extra payments equal the actual principal payments. While the required amount has to be recalculated every month, this is easy to do if you download the first of my extra payment spreadsheets, and update it every month.

The merit in this approach is that you don’t require anyone’s permission, and having a concrete goal such as cutting the life of the loan in half is one way to discipline yourself to save.

The downside of the scheme is that you must increase your savings every month over the previous month. For example, on my $100,000 loan at 6% for 15 years, the required savings would rise from $344 in month 1 to $833 in month 90. This might work well for some but for many if not most borrowers, it would not.

Borrowers who want to cut the life of their mortgage in half can do it in many ways. For example, the four savings plans shown below would all pay off my $100,000 15-year 6% mortgage in 90 months. They are thus alternatives to the double amortization plan with its rising extra payment.

*A flat additional monthly payment of $539 starting in month 1.

*A flat additional quarterly payment of $1624 starting in month 3.

*A flat additional annual payment of $7021starting in month 12.

*A combination of flat additional payments of $300 a month starting month 1, and $3,000 a year starting month 6.

The four plans were derived from my calculators 2a and 2c, which you can use to develop your own plan. It should meet your own goals, which might be more or less ambitious than shortening the mortgage term by half. And it should be based on a realistic appraisal of the amount and timing of the savings you will be able to generate.
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