ARM Mortgage Calculator
Enter your loan details and ARM parameters to see your initial monthly payment, your maximum possible payment at the rate cap, total interest paid, and a full amortization schedule. Choose a preset ARM type (3/1, 5/1, 7/1 or 10/1) or set a custom adjustment schedule. The comparison chart shows how your ARM balance and payments stack up against an equivalent fixed-rate loan over the full term.
How an adjustable-rate mortgage works
An adjustable-rate mortgage (ARM) has two distinct phases. The introductory or "teaser" period locks your interest rate for a set number of years: 3, 5, 7 or 10 years depending on the loan type. After that, the rate resets on a schedule, typically once a year, based on a published index such as the Secured Overnight Financing Rate (SOFR) plus a fixed margin set by your lender. The result each period is capped by a rate cap structure so the payment cannot jump without limit. Three caps normally apply: the first-adjustment cap limits how far the rate moves at the very first reset, the subsequent-adjustment cap limits each later move, and the lifetime cap is the absolute maximum the rate can ever reach above its starting point. A common 5/1 ARM cap structure written as 2/2/5 means the rate can rise at most 2% at the first adjustment, at most 2% at each following adjustment, and at most 5% total over the life of the loan.
When an ARM makes financial sense
An ARM typically starts with a lower rate than a comparable fixed mortgage, which reduces your monthly payment and total interest during the introductory period. This advantage is most valuable when you know you will sell or refinance before the first rate reset, common for buyers who relocate every five to seven years. It is also useful when you expect interest rates to fall, since future adjustments could lower your payment rather than raise it. A physician or professional early in a career who plans to upsize in five years often captures the ARM savings without ever facing a rate adjustment. The break-even analysis in this calculator shows the year at which the ARM begins to cost more in cumulative interest than an equivalent fixed-rate loan, giving you a clear horizon to compare against your expected time in the home.
Payment shock and worst-case budgeting
The primary risk of an ARM is payment shock: the monthly payment at the lifetime rate cap can be substantially higher than the initial payment. With a starting rate of 5.5% and a 5% lifetime cap, the rate ceiling is 10.5%, which on a 30-year $350,000 loan increases the monthly principal and interest from about $1,987 to about $3,230, a 63% jump. Lenders and regulators require that borrowers qualify for an ARM based on the fully-indexed rate or the maximum rate reached within five years, whichever is higher, to ensure affordability. Before choosing an ARM, use the maximum payment figure in this calculator to verify that your household budget could absorb that payment without financial stress.
ARM index, margin and the fully-indexed rate
After the fixed period ends, your new interest rate is the sum of an index and a margin. The index fluctuates with market conditions and is most commonly the 30-day average SOFR, which replaced LIBOR as the standard US ARM benchmark. The margin is a fixed spread added by your lender, often 2.5% to 3.5%, and it stays constant for the life of the loan. So if SOFR is 4.8% and your margin is 2.75%, your fully-indexed rate would be 7.55%, subject to your cap. This calculator uses an expected rate change input rather than an explicit index forecast, which lets you model optimistic, base-case and pessimistic environments by adjusting that one field.
Common ARM rate cap structures
| Cap structure | First-adjustment cap | Subsequent cap | Lifetime cap | Typical for |
|---|---|---|---|---|
| 2/2/5 | 2% | 2% | 5% | 5/1 and 7/1 ARMs |
| 2/2/6 | 2% | 2% | 6% | 3/1 and 5/1 ARMs |
| 5/2/5 | 5% | 2% | 5% | Older and non-conforming loans |
| 1/1/5 | 1% | 1% | 5% | Some lender-specific products |
| 5/5 ARM | N/A | 5% | Varies | Adjusts every 5 years, single cap |
Standard cap structures in the US market. Always verify your specific loan documents for the exact cap structure that applies to your loan.
Frequently asked questions
What is the difference between a 5/1 ARM and a 7/1 ARM?
Both are hybrid ARMs with an introductory fixed period followed by annual adjustments. The first number indicates the fixed period: 5 years for a 5/1 ARM and 7 years for a 7/1 ARM. The "1" means the rate adjusts once per year after the fixed period ends. A 7/1 ARM offers rate certainty two years longer than a 5/1 ARM and is therefore useful for buyers who expect to stay 5 to 7 years. In exchange, the initial rate on a 7/1 ARM is usually a bit higher than on a 5/1.
Can my ARM rate ever go down?
Yes. If the underlying index falls, your rate at the next adjustment will decrease, subject to any floor and the same per-adjustment cap that limits increases. Use a negative value in the "expected rate change" field to model a falling-rate scenario. Many borrowers with ARMs originated at higher rates have seen their payments fall during periods of declining interest rates.
What is a rate cap and why does it matter?
A rate cap limits how much your interest rate can move. Three caps typically apply: the first-adjustment cap (how much the rate can change at the very first reset, commonly 2% or 5%), the subsequent-adjustment cap (how much it can change at each later reset, commonly 2%), and the lifetime cap (the maximum total increase from your starting rate, commonly 5% or 6%). Caps protect you from extreme payment shock but do not eliminate risk. Always budget based on the maximum payment at the lifetime cap.
How is an ARM monthly payment calculated?
During the fixed period, the payment uses the standard amortizing formula: P x [r(1+r)^n] / [(1+r)^n - 1], where P is the loan balance, r is the monthly interest rate (annual rate divided by 12), and n is the remaining number of months. After each rate adjustment, the lender recalculates the payment using the new rate, the remaining loan balance, and the remaining months left on the loan term. This recalculation is what causes the payment to change at each adjustment, even if the balance change is small.
Is an ARM or a fixed-rate mortgage better right now?
The right choice depends on how long you plan to stay in the home and the spread between ARM and fixed rates at the time you are borrowing. When ARM teaser rates are significantly below 30-year fixed rates, buyers who move or refinance within 5 to 7 years almost always come out ahead with an ARM. When the spread is small, the extra certainty of a fixed rate is usually worth it. This calculator's break-even chart shows the exact year that cumulative ARM interest overtakes fixed-rate interest, which you can compare directly with your planned time in the home.
What happens to my ARM when I refinance?
Refinancing replaces your existing ARM with a new loan, which can be either a new ARM or a fixed-rate mortgage. You will pay closing costs (typically 2% to 5% of the loan amount), so refinancing makes financial sense only when the new rate saves enough monthly interest to recoup those costs within your remaining time in the home. Many borrowers use the ARM's fixed period as a planned window before refinancing into a fixed-rate loan.