An adjustable-rate mortgage payment is calculated in three steps. The lender adds a fixed margin to an index, limits the result with the loan's caps, then re-amortizes the remaining balance over the years left on the loan. As of October 19, 2026, the 30-day average SOFR stood at 3.88518%, according to FRED. That is the index most conforming ARMs use.

The arithmetic matters more this fall because more borrowers are choosing ARMs. The 30-year fixed contract rate reached 7.50% in the week ending October 9, its highest since November 2023. The 5/1 ARM averaged 6.54% that week, and ARMs rose to 10.5% of applications, a one-year high, according to the Mortgage Bankers Association.

That spread of roughly 0.96 percentage point is the case for the ARM. The reset math shows what the ARM can cost later.

Step one: index plus margin

Fannie Mae requires conforming ARMs to use a 30-day average of SOFR published by the New York Fed, and it caps the lender's margin at 300 basis points, according to its Selling Guide. Freddie Mac allows margins from 1.00% to 3.00%.

The guide sets the formula. The new rate is the margin plus the most recent index value available 45 days before the interest change date, rounded to the nearest one-eighth of a percent. That rounded figure is the fully indexed rate.

Take a 2.75% margin, which Ralo describes as common for conforming SOFR ARMs. Add it to the October 19 index value and the result is 6.63518%. Rounded to the nearest eighth, that is 6.625%, and this is the figure before any cap applies.

The lookback date can move the result. The same index read 3.80031% on October 7, FRED data show. A reset date a few weeks earlier would have started from a lower number.

Step two: the caps

Caps are usually written as three numbers: first adjustment, each later adjustment, and lifetime. A 2/1/5 structure allows a 2-point move at the first reset, 1 point at each reset after that, and 5 points in total, according to Ralo. Some products are looser. Newcastle Loans describes a 7/6 SOFR ARM with a 5/1/5 structure, which allows a 5-point jump at the first reset.

Under Fannie Mae's rules, lifetime caps limit increases only. Periodic caps limit moves in both directions. The rate may never fall below the margin.

The rate a borrower actually pays is therefore the fully indexed rate, unless a cap or the floor intervenes. The cap applies when the fully indexed rate sits further from the current rate than the cap allows.

Step three: re-amortizing what is left

At each reset, the new payment is calculated on the remaining balance over the remaining term. The original 30-year payoff date stays fixed, Newcastle Loans notes. The worked example below uses a hypothetical 5/1 ARM, which adjusts once a year after five years. The figures use the standard amortization formula.

A hypothetical 5/1 ARM at its first reset

Say a borrower took a $400,000, 30-year ARM at 4.50% with 2/1/5 caps and a 2.75% margin. The first reset uses the October 19, 2026 index value.

Payment, years 1 to 5 (4.50%, 360 months)$2,026.74
Balance after 60 payments$364,632
Fully indexed rate (3.88518% + 2.75%, rounded)6.625%
First-adjustment cap (4.50% + 2 points)6.50%
New rate (lower of the two)6.50%
New payment (6.50%, 300 months remaining)$2,462.02
Payment without the cap (6.625%)$2,490.58

The payment rises about $435 a month, or 21%. The cap saves about $29 a month at this reset. At the next annual reset, the 1-point periodic cap would allow the rate to reach 6.625% if the index holds.

Multiplying the rate by the balance gives only the interest portion. At 6.50%, that is about $1,975 a month. The payment is higher because the principal must now be repaid over 25 years, not 30.

The shorter term carries its own cost. The same $364,632 at 6.50%, spread over a fresh 30 years, would cost about $2,305 a month. The five years already used up add roughly $157 to every payment. On loans that adjust every six months, such as Freddie Mac's 5/6 and 7/6 products, this recalculation repeats twice a year.

Which inputs does an ARM calculator need?

The calculator guide covers each field, from lookback period to lifetime cap, using current numbers.

Read the ARM calculator guide

Where the math has already landed

About 150,000 ARM borrowers face their first adjustment in 2026, according to ICE data reported by CNBC. Many hold seven-year ARMs originated in 2019. In that group, the median rate moves from 3% to 6.06% on a median balance of $548,000. The median payment rises by about $1,080.

The 2020 cohort comes next. Some 74,000 households resetting in 2027 go from 2.75% to an estimated 5.79%, a median increase of $1,066, or 36%, according to 24/7 Wall St. That adds about $12,800 a year.

A jump of about 3 points in a single reset is possible only where the first-adjustment cap allows it. A 2-point cap would not.

The math also runs the other way. CNBC reported that a small share of resetting borrowers will see payments fall. The 30-day average SOFR was about 44 basis points lower than a year earlier, FRED data show. Even so, Fannie Mae's floor stops any decline at the margin.

Not the rate on the quote

Initial rate Fully indexed rate
Set by Lender pricing at origination Index plus margin, rounded
Applies During the fixed period From the first reset, subject to caps
October 2026 figure 5/1 average of 6.54% (MBA, week ending Oct 9) 6.625% with a 2.75% margin (Oct 19 index)

The index in an ARM note is also not the overnight SOFR rate. Overnight SOFR printed 3.88% on October 16, according to FRED. Mortgage notes reference a compounded average, which moves more slowly than the daily print.

Checking a loan estimate

Before trusting any calculator, a borrower should get these figures from the loan estimate or note:

  • the exact index, for example the 30-day average SOFR
  • the margin
  • the lookback period
  • all three cap numbers
  • the rate floor

The three steps above can then be run against the quote itself. A margin of 2.00% and a margin of 3.00% are both permitted under Freddie Mac's range, and they produce fully indexed rates a full point apart.

Quoted starting rates also vary by source. Bankrate's survey put the 5/1 ARM at 6.82% on October 19, while MBA's average was 6.54% the week ending October 9. Comparing margins and caps across several lenders shows which ARM is actually cheaper after the reset.