"We'd like to make extra principal payments on our home mortgage each month, but we can't do that and invest for retirement. Which should have the priority?" There's no one-size-fits-all answer  your age, your tax bracket, what you would do with the tax savings from your mortgage interest, how long you expect to live in your home, and your general attitude toward being debt-free all play significant roles.

Let's say that two investors are wrestling with this question. Rob wants to begin building his retirement funds immediately. Dan thinks being in debt is more of a concern and plans on using any monthly surplus to make additional principal payments on his mortgage.

For comparison purposes, let's make their two situations identical: They both have new $120,000, fifteen-year 6% fixed rate mortgages; both can set aside $1,100 out of each month's paycheck (their monthly mortgage payment is $1,013, leaving them each with an $87 surplus); both are in the 31% tax bracket (federal plus state), and both have the opportunity to contribute to a retirement plan at work that will earn 8%, which is 2% more than their mortgages are costing them.

When they make their first month's mortgage payment, $600 of it is tax deductible as interest expense. Assuming they itemize their deductions, this will lower each of their taxes by $186 a month (31% of $600). What they do with that $186 savings can make a big difference.

Rob and Dan would like to get their hands on that savings sooner rather than later. They would get it back when they filed their income tax returns anyway, so why wait? So, they both change the withholding instructions they give their employers so that about $186 less is withheld for income taxes each month. By adding that amount to the extra $87 left from their monthly surplus, they each now have an extra $273 to work with. Rob contributes his $273 into his company's 401(k) plan while Dan takes his $273 and makes an extra principal payment on his mortgage.

Now here's where it can get confusing. To construct an accurate picture, we have to recognize that Rob gets a second tax deduction  this time for putting money into the retirement plan. Rob's $273 contribution is worth another $85 tax savings, which he could then also put into his 401(k). But then that $85 contribution would save him an additional $26 in taxes, which he could also put into his 401(k). But then that $26 . . . well, you get the idea. If Rob took maximum advantage of this, he could ultimately put $396 into his company retirement plan that first month (his $87 monthly surplus plus the tax savings of $186 for mortgage interest plus another $123 in tax savings for contributing to the company 401(k)).

Assume that both men are able to take the maximum advantage of the available tax savings as the years pass. Dan pays down as much extra on his mortgage each month as he can and pays it off completely in a little over 11 years. At that point, he shifts all the money he formerly put toward his mortgage each month into his retirement plan. He also adjusts his withholdings to take maximum advantage of the tax savings his contributions create.