A 5-year adjustable rate mortgage is a home loan that locks your interest rate for the first five years and then lets it move up or down at set intervals for the rest of the term. The initial rate is typically lower than a 30-year fixed mortgage, so your monthly payment during those first five years is smaller. After that, the rate resets based on a market index, and your payment changes with it. You get a discount now in exchange for accepting uncertainty later.
The Two Phases of the Loan
Every 5-year ARM has a fixed phase and an adjustable phase. For the first 60 months, your interest rate stays the same no matter what financial markets do. Your payment is predictable, and you can budget around it. Lenders can offer the lower starting rate because once the fixed period ends, the interest-rate risk shifts to you.1U.S. Department of Housing and Urban Development. FHA Adjustable Rate Mortgage
After month 60, the loan enters its adjustable phase. The rate gets recalculated at regular intervals for the remainder of the term, and the monthly payment is reset each time to reflect the new rate. How often that recalculation happens depends on which version of the loan you have.
5/1 vs. 5/6 ARMs
The number after the slash tells you how often the rate resets during the adjustable phase. A 5/1 ARM adjusts once a year. A 5/6 ARM adjusts every six months. Both have the same five-year fixed period at the start.
Fannie Mae and Freddie Mac, the two government-sponsored enterprises that buy most conforming mortgages, now require SOFR-indexed ARMs with six-month adjustment periods rather than annual ones.2Freddie Mac. SOFR ARMs Fact Sheet If you’re taking out a conforming loan, you’re almost certainly looking at a 5/6 ARM. The 5/1 structure still exists in portfolio loans and some non-conforming products, but it’s no longer the standard.
With a 5/6, your payment can change twice a year once the fixed period ends. The rate caps account for the faster cycle, but it’s still more frequent variability than the old annual model.
How the New Rate Is Calculated
When the adjustable phase starts, your lender doesn’t pick a rate at will. The formula is written into your loan contract and has two pieces plus a timing rule.
The Index
The index is the variable piece. Nearly all new ARMs use the Secured Overnight Financing Rate, or SOFR, which reflects actual overnight lending transactions backed by Treasury securities.2Freddie Mac. SOFR ARMs Fact Sheet Conforming ARMs specifically use the 30-day average SOFR published by the Federal Reserve Bank of New York.
You may run across references to older indexes like the Constant Maturity Treasury (CMT) or LIBOR. Both are effectively gone for new originations. Fannie Mae stopped buying CMT-indexed ARM loans after mid-2021, and LIBOR was phased out entirely.3Fannie Mae. Lender Letter LL-2021-05 If one of those shows up in a conversation about a new loan, something is unusual.
The Margin
The margin is the fixed piece. Your lender sets it at origination and it never changes. It’s the lender’s markup over the cost of money. If the 30-day average SOFR is 3.00% and your margin is 2.50%, your fully indexed rate is 5.50%. That’s the rate your lender would charge for the next period, subject to the caps in your contract.
The Look-Back Period
The lender doesn’t use the index value from the day your rate resets. Your contract specifies a look-back period, which for conforming loans is 45 days. If your rate adjusts on August 1, the lender uses the 30-day average SOFR from around June 17. That buffer gives the lender time to calculate the new rate and payment and send you the required notice before the change takes effect.
Rate Caps and the Worst Case
Rate caps are the guardrails that keep your rate from spiking without limit. Your contract defines three of them.
- Initial adjustment cap. This limits the change at the first reset after the fixed period ends. On a typical 5-year SOFR ARM, it’s 2 percentage points. Start at 4.00% and your rate can’t exceed 6.00% at the first adjustment, even if the fully indexed rate would be higher.
- Periodic adjustment cap. This limits each subsequent reset. For conforming SOFR ARMs, it’s 1 percentage point per adjustment period. On a 5/6, that 1-point cap applies twice a year.2Freddie Mac. SOFR ARMs Fact Sheet
- Lifetime cap. This is the absolute ceiling for the entire loan term, commonly 5 percentage points above the starting rate. A 4.00% loan could never exceed 9.00%.
Caps are often written in shorthand. A “2/1/5” structure means a 2-point initial cap, 1-point periodic cap, and 5-point lifetime cap. That’s the standard for conforming 5-year ARMs.
Caps work in both directions. Periodic adjustment caps limit rate decreases as well as increases.4Fannie Mae. Adjustable-Rate Mortgages ARMs If SOFR falls sharply, your rate still can’t drop by more than the periodic cap in a single adjustment. Most loans also have a floor, often equal to the margin, so the rate will never fall below the lender’s markup no matter how low the index goes.1U.S. Department of Housing and Urban Development. FHA Adjustable Rate Mortgage
What Happens to Your Monthly Payment
When a new rate takes effect, the lender recalculates your monthly payment from three inputs: the adjusted interest rate, your remaining loan balance, and the number of months left on the loan. The new payment is sized so the loan still pays off by the end of the original term. There’s no balloon at the end, and standard ARMs don’t allow negative amortization.
The lender compares the fully indexed rate (index plus margin) against the applicable cap and uses whichever is lower. If the fully indexed rate would be 6.50% but the initial cap limits you to 6.00%, you pay 6.00% until the next adjustment date.
The jump from a low introductory rate to a higher adjusted rate is often called payment shock. Even with a 2-point initial cap, going from 4.00% to 6.00% on a $350,000 balance increases the monthly principal and interest payment by roughly $400. On a 5/6, where adjustments happen every six months, you have less time between changes to absorb each increase.
Payments can also go down. If market rates fall during the adjustable phase, your rate drops at the next reset (subject to the periodic cap and any floor) and your payment drops with it.
Advance Notice Before Rate Changes
Federal law requires your lender to warn you before any rate change, with different timelines for the first adjustment and later ones.
For the initial adjustment at the end of the fixed period, the lender must send a disclosure between 210 and 240 days before the first payment at the new rate is due.5eCFR. 12 CFR 1026.20 – Disclosure Requirements Regarding Post-Consummation Events That’s roughly seven to eight months of lead time, enough to refinance or sell if you don’t want to ride the adjustable phase.
For every adjustment after the first, the notice window is shorter: at least 60 days and no more than 120 days before the first payment at the adjusted level is due.5eCFR. 12 CFR 1026.20 – Disclosure Requirements Regarding Post-Consummation Events
Each notice must include your current rate, the new rate, your current payment, the new payment, and an explanation of how the new rate was calculated, including the index value and margin used.6Consumer Financial Protection Bureau. 12 CFR 1026.20 – Disclosure Requirements Regarding Post-Consummation Events It also has to spell out your caps so you can see where your rate sits relative to the lifetime maximum.
No Prepayment Penalty
If you want to refinance or pay off the loan before the adjustable phase begins, nothing in the loan can stop you. Federal regulation prohibits prepayment penalties on any loan where the interest rate can increase after closing.7eCFR. 12 CFR 1026.43 – Minimum Standards for Transactions Secured by a Dwelling Every ARM by definition has a rate that can rise, so prepayment penalties are off the table. You’ll still owe normal closing costs if you refinance, but the lender can’t charge a penalty just for paying off the loan early.
When a 5-Year ARM Makes Sense
The core trade-off is simple. A 5-year ARM almost always starts with a lower rate than a 30-year fixed mortgage. The spread is often roughly 0.75 to 1 percentage point, though it moves with market conditions. On a $400,000 loan, that spread saves a few hundred dollars a month during the fixed period.
A fixed-rate mortgage keeps the same payment from the first month to the last. You never wonder what the rate will be next year, and you never need an exit strategy. The longer you keep a fixed loan, the more that locked rate is worth.
The ARM makes financial sense in narrower circumstances. If you know you’ll sell within five years, you keep the savings and leave before any adjustment. If you expect rates to fall, the adjustable phase could work in your favor. And if the spread between fixed and adjustable rates is wide, the early savings may justify the risk even if your timeline stretches a bit.
Where borrowers get into trouble is the middle case: they planned to sell or refinance before year five and couldn’t. Maybe the market softened and they owe more than the home is worth. Maybe their credit dropped and they don’t qualify to refinance. Maybe rates rose and refinancing into a fixed loan would cost about the same as the adjusted ARM rate. Any of these can trap you in the adjustable phase without a clean way out. If your plan depends on refinancing before the fixed period ends, think through what you’ll do if that plan fails.