Arm Vs Fixed Rate Calculator
Compare adjustable-rate and fixed-rate mortgages over different time horizons. Enter values for instant results with step-by-step formulas.
Formula
M = P[r(1+r)^n] / [(1+r)^n - 1]
Where M = monthly payment, P = loan principal, r = monthly interest rate (annual rate / 12), n = total number of payments. The ARM calculation applies the initial rate for the fixed period, then recalculates using the adjusted rate for the remaining term.
Worked Examples
Example 1: 5-Year Stay Comparison
Problem: Compare a $350,000 loan with a 6.5% fixed rate vs a 5/1 ARM at 5.5% initial rate (adjusting to 7.5%) if you plan to stay 5 years.
Solution: Fixed payment: $350,000 at 6.5% for 30 years = $2,212/month\nARM initial payment: $350,000 at 5.5% for 30 years = $1,987/month\nMonthly savings with ARM: $2,212 - $1,987 = $225/month\nTotal savings over 5 years: $225 x 60 = $13,500\nSince you leave before the ARM adjusts, you keep all savings.
Result: ARM saves $225/month and $13,500 total over 5 years with no adjustment risk
Example 2: 15-Year Stay Comparison
Problem: Same $350,000 loan but staying 15 years. ARM adjusts to 7.5% after year 5.
Solution: Years 1-5: ARM saves $225/month = $13,500 total savings\nYears 6-15: ARM payment rises to ~$2,390/month at 7.5%\nExtra cost years 6-15: ($2,390 - $2,212) x 120 = $21,360 extra\nNet result: $21,360 - $13,500 = $7,860 more with ARM\nFixed rate wins for a 15-year stay.
Result: Fixed rate saves approximately $7,860 over 15 years compared to ARM
Frequently Asked Questions
What is the difference between an ARM and a fixed-rate mortgage?
A fixed-rate mortgage locks in one interest rate for the entire loan term, providing predictable monthly payments that never change. An adjustable-rate mortgage (ARM) starts with a lower introductory rate for a set period, typically 3, 5, 7, or 10 years, and then adjusts periodically based on a benchmark index plus a margin. The initial ARM rate is almost always lower than the equivalent fixed rate, which is the trade-off for accepting future rate uncertainty. After the fixed period ends, the rate can increase or decrease at each adjustment interval, usually annually, subject to caps on how much it can change per adjustment and over the life of the loan.
How do ARM rate caps work and why do they matter?
ARM rate caps are protective limits built into the loan that restrict how much your interest rate can change. There are typically three types of caps: an initial adjustment cap that limits the first rate change after the introductory period ends, a periodic adjustment cap that limits each subsequent annual change, and a lifetime cap that sets the maximum rate over the entire loan. For example, a 5/1 ARM with 2/2/5 caps means the rate can increase by at most 2 percentage points at the first adjustment, 2 points at each subsequent adjustment, and 5 points total above the initial rate over the loan lifetime. Understanding these caps helps you calculate your worst-case monthly payment scenario.
When does choosing an ARM make more financial sense than a fixed rate?
An ARM typically makes more financial sense when you plan to sell or refinance before the initial fixed period ends. If you expect to move within five to seven years, a 5/1 or 7/1 ARM lets you benefit from the lower introductory rate without ever facing the adjusted higher rate. ARMs also make sense when interest rates are expected to fall, since your rate would adjust downward. First-time buyers who expect significant income growth may prefer ARMs because they can handle potential rate increases later. However, if you plan to stay in your home long term and value payment predictability, a fixed-rate mortgage is generally the safer and more financially prudent choice.
What does the ARM fixed period mean for my payments?
The ARM fixed period is the initial timeframe during which your interest rate remains constant at the lower introductory rate. In a 5/1 ARM, the fixed period is 5 years, and the 1 means the rate adjusts annually afterward. During this period, your monthly payment behaves exactly like a fixed-rate mortgage but at a lower rate. Common ARM structures include 3/1, 5/1, 7/1, and 10/1 configurations. A longer fixed period provides more initial stability but typically comes with a slightly higher introductory rate compared to shorter-period ARMs. The key planning question is whether you will sell, refinance, or stay past this period, since that determines whether you face rate adjustments.
How much can I save with an ARM during the introductory period?
The savings during the ARM introductory period depend on the rate difference between the ARM and fixed-rate options. For a $350,000 loan, even a 1 percentage point lower ARM rate saves approximately $200 to $230 per month. Over a 5-year fixed period, that translates to roughly $12,000 to $14,000 in total savings. These savings can be invested, used to pay down principal faster, or applied toward other financial goals. However, you must weigh these savings against the risk of higher payments after the adjustment period. If the adjusted rate exceeds the fixed rate significantly, the long-term cost could surpass what you would have paid with the fixed-rate mortgage from the start.
What index determines ARM rate adjustments?
Most ARMs are tied to a benchmark index such as the Secured Overnight Financing Rate (SOFR), which replaced the London Interbank Offered Rate (LIBOR) as the primary benchmark. The lender adds a fixed margin, typically 1.75 to 3.5 percentage points, to the index value to calculate your new rate at each adjustment. For example, if SOFR is 4.0% and your margin is 2.5%, your adjusted rate would be 6.5% before caps are applied. Some older ARMs may reference the Constant Maturity Treasury (CMT) rate or the Cost of Funds Index (COFI). Understanding which index your ARM uses helps you monitor potential rate changes and predict future payment adjustments more accurately.