No, you have to earn more than 8.35% to come out ahead.

If you take the standard version, the fully amortizing monthly payment at 6.25% is $1847.16, and after 10 years, your balance would be $252,712. You would thus pay down the loan balance by $47,288 over the 10 years.

If you take the interest-only version, and don’t make any principal payments, the balance after 10 years remains at $300,000. The interest-only payment at 6.375% is $1593.75, or $253.41 less than the fully-amortizing payment of the standard mortgage. To break even, therefore, the monthly cash flow saving of $253.41 must accumulate to $47,288 over the 10 years.

The break-even rate of return is 8.35%. This is the rate that must be earned on the monthly investment of $253.41 to generate a fund of $47,288 in 10 years. It is higher than the rate on the interest-only because it must offset the interest rate difference between the interest-only and the fully-amortizing versions.

To come out ahead, you must earn a return greater than 8.35%. Few people can do that without taking a lot of risk.

Copyright Jack Guttentag 2006