This may surprise you: you should worry more about saving for retirement than paying off your student loan. Yes, there are many bloggers testifying to how they paid off their loans, and many trying to tell you that you need to too. The frequency of such posts does not mean that they are right, or that they have factored in all that you need to for a “best use of cash flow over time” plan. In fact, paying off student loans that have relatively low interest rather than investing will be a costly mistake.

Compare student loan payoff blog posts to the many advisors who encourage paying off your mortgage. The urgency to retire your mortgage belongs to the Silent Generation, who survived the Great Depression and are risk adverse. Paying off a mortgage is usually not the way to maximize your net worth. (Watch for a post on “rent vs. buy” discussing investing and use of mortgage debt.) Furthermore, tying up so much capital in a house lacks for diversification and liquidity – you cannot sell your daughter’s room to cover tuition when she goes off to college.

How do you decide what loan to pay off when? Start with this rule:

Whenever the interest rate on debt is less than the annualized return on investments, only pay the minimum on the loans

because the investments will grow faster than using the same cash to pay off debts

The term “annualized” returns is key here, as one good year is not a good measure, nor is a recent bad year; you want the 5 or better 10 year average return.

Next, when applying this general rule to yourself, be sure to use after-tax values. You can deduct home mortgage interest in most cases, a portion of student loans in some cases, and credit card debt in almost no cases, while you can deduct your investment in your retirement plan and what you invest grows tax-free until withdrawn. Roth IRAs do not give you a deduction now, but do grow tax free and withdrawals are tax-free.

Here is a quick example: say your retirement plan is all in ETFs, so it grows at about 7% per year over time, say you have a student loan with an interest rate of 3%, because you consolidated all your undergrad loans, and it has a minimum payment of $500 per month, and say you have $1,000 per month for which you want to devise the best plan. Apply no more than the required minimum to student loan and invest all the rest, maxing out your 401(k) or 403(b) plan first, and then investing in a Roth IRA next. In 10 years, you will be so much better off than the person who used the full $1,000 to pay her student loan. What if you had a loan with an interest rate of 8%? Tough call. However, because the loan is compounding over time at a higher rate, that persuades me to apply more to the loan, provided that doing so did not give up an employer match on a 401(k) plan.

These examples are overly simple, I realize. Many of you face the quandary of student loan vs. retirement funding vs. rent or buy a home and more. I am working on that …. (Watch for a post on integrating decisions on buying a home, paying off student loans, investing for retirement and all the other issues you are likely to face.)