Choosing Between Fixed & Adjustable Rate Mortgages
February 2, 2004, Revised November 14, 2008, Reviewed February 12, 2011

"I am trying to choose between a 3/1 ARM at 4.625% and a FRM at 5.875%, both 30 years. I don’t expect to be out of my house within 3 years. What is the best way to make this decision?"

Whether the adjustable rate mortgage (ARM) or fixed rate mortgage (FRM) turns out better depends on what happens to interest rates in the future, which no one knows. Shoppers faced with this decision should ask themselves "Is this a risk worth taking," and "can I afford to take it?"

Scenario Analysis of ARM Versus FRM


The best way I know to deal with these questions is by determining what will happen to the rate and payment on the ARM if market interest rates change in ways that you specify. This "scenario analysis" provides a measure of the risk if rates increase, and the benefit if they don’t. It also allows you to determine the extent to which you can reduce the risk on the ARM by making the larger payment that you would have made had you selected the FRM.

A side benefit is that you can’t do scenario analysis without knowing all the features of the ARM that affect future rates and payments. The information you are forced to compile for this purpose you should have anyway. Otherwise, you don’t know whether you have found the best deal on your ARM.

Example of a Scenario Analysis


For example, you told me that your 3/1 ARM had a rate of 4.625%, but that rate holds for only 3 years, after which the rate adjusts every year. You did not tell me what I needed to know to calculate the rate and payment after the 3 years. I found out that your ARM rate was tied to the one-year Treasury index, which had a recent value of 1.28%, and had a margin of 2.75%. After 3 years, your rate would equal the index at that time plus 2.75%, subject to an adjustment cap of 2% (no rate change can exceed 2%) and a maximum rate of 10.625%.

You need all that to do scenario analysis, but you also want it for shopping. If you could find the same 3/1 ARM with a 2.5% margin, you should grab it.

The numbers cited below all assume loan amounts of $100,000, and came from calculator 7b Monthly Payment Calculator: Adjustable-Rate Mortgages Without Negative Amortization.

Stable Rate Scenario


A stable-rate scenario provides the best measure of the potential benefit of the ARM. The payment would be $514.14 for the first 36 months, and $481.76 thereafter, as compared to $591.54 on the FRM. If you made the $591.54 payment on the ARM, you would pay it off in 257 months.

Rising Rate Scenarios


I used 4 rising rate scenarios of gradually increasing severity: 1. Small rate increase: after 2 years, the index increases by .5 % /year for 3 years. 2. Moderate rate increase: after 1 year, the rate index increases by .75%/year for 4 years. 3. Larger rate increase: starting immediately, the index increases by 1%/year for 5 years. 4. Worst case: the index rises to 100% in month 2.

With the small rate increase scenario, the payment remains lower on the ARM than on the FRM over the entire 30 years. If the borrower makes the FRM payment, he will pay off in 304 months. The borrower thus benefits if rates are stable or decline, or have a delayed rise of 1.5% over 3 years.

With the larger rate-increase scenarios, the benefits of the ARM over the first 3 or 4 years are followed by losses. Skipping to the worst case, the payment would rise from $514.14 to $630.64 in month 37, to $754.44 in month 49, and to $883.74 in month 61 where it would remain until payoff. It is useful to know whether you could deal with these increases, even though the likelihood of their occurring is very low.

These payment increases could be reduced by making the larger FRM payment in the first 3 years. If you paid $591.54 rather than $514.14 for 36 months, you would reduce the worst case payment in months 61-360 from $883.74 to $856.01. The complete results for all the scenarios are shown below.

Scenario analysis doesn’t provide definitive answers to the questions posed at the beginning of this article. However, it does allow you to make an informed judgment based on all available information. In the face of an uncertain future, that’s the best anyone can do.


                                                              ARM Features
Initial Interest Rate on ARM 4.625%
Initial Rate Period 3 Years
Subsequent Adjustment Period 1 Year
Most Recent Index Value 1.28%
Margin 2.75%
First Rate Adjustment Cap 2.000%
Later Adjustment Caps 2.000%
Maximum Interest Rate 10.625%
Loan Term (in years) 30
Rate on FRM Loan Used as Comparison 5.875%
FRM Payment $591.54


Interest Rates and Monthly Payments Under 5 Interest Rate Scenarios

  SCENARIO
Months No Change Small Increase Moderate Increase Large Increase Worst Case
Rate % Pmt $ Rate % Pmt $ Rate % Pmt $ Rate % Pmt $ Rate % Pmt $
1-36 4.625 514.14 4.625 514.14 4.625 514.14 4.625 514.14 4.625 514.14
37-48 4.03 481.76 4.53 508.90 5.53 565.46 6.625 630.64 6.625 630.64
49-60 4.03 481.76 5.03 536.01 6.28 608.58 8.03 716.66 8.625 754.44
61-360 4.03 481.76 5.53 563.01 7.03 651.97 9.03 779.13 10.625 883.74
Borrower Makes the FRM Payment So Long As It Is Larger Than the ARM Payment
1-36 4.625 591.54 4.625 591.54 4.625 591.54 4.625 591.54 4.625 591.54
37-48 4.03 591.54 4.53 591.54 5.53 591.54 6.625 610.85 6.625 610.85
49-60 4.03 591.54 5.03 591.54 6.28 591.54 8.03 694.17 8.625 730.76
61-360 4.03 591.541 5.53 591.542 7.03 627.26 9.03 754.67 10.625 856.01


1Pays off in 257 months
2Pays off in 304 months
Sign up to Receive New Articles
Print