How ARM Payments Are Calculated: Formulas, Index, Margin, Caps

The two formulas

Formula 1: The fully-indexed rate

Fully Indexed Rate = Index Value + Margin

The index value changes with the market (typically daily, for SOFR). The margin is fixed for the life of the loan. Your ARM's rate at any adjustment equals the current index value plus the margin, capped by your periodic and lifetime caps.

Formula 2: The amortization payment

M = P · r(1+r)^n / ((1+r)^n − 1)

This is the standard mortgage payment formula. M is monthly payment, P is the remaining loan balance, r is the monthly interest rate (annual rate ÷ 12), and n is the number of remaining months. After each ARM adjustment, the payment recalculates using this formula with the new rate and the remaining term.

Index reference table

Every ARM references exactly one index. Knowing the index tells you what your rate will track over the life of the loan.

Caps table: how the limits interact

Your caps determine the worst-case payment path. The naming convention is initial / periodic / lifetime, so "2/1/5" means 2 points at the first adjustment, 1 point at each later adjustment, and 5 points above the start rate ever. The last column applies each limit to the $400,000 loan used in the worked example below, which starts at 5.50% with a 2.75% margin.

One nuance the table cannot show: the caps apply in order. The lender computes index plus margin, rounds it, then checks the floor, the initial or periodic cap, and the lifetime cap. A number can clear the periodic cap and still fail the lifetime cap, which is exactly what happens in year 9 of the worked example below.

Worked example: 10-year payment path for a 5/1 ARM with 5/2/5 caps

Loan: $400,000. Start rate: 5.50%. Margin: 2.75%. Index: 30-day SOFR, which begins at 4.30% and rises in the years after the fixed period. Caps: 5/2/5, so the first adjustment may add up to 5 points, later adjustments 2 points, and the lifetime ceiling sits 5 points above the start rate. Rounding is to the nearest 0.125%.

What this example shows. Three different mechanisms drive the rate. Rounding does the work in year 6, taking 7.05% down to 7.00%. The index does the work in years 7 and 8, where 7.75% and 9.25% clear every cap untouched. The lifetime cap does the work in years 9 and 10, where the arithmetic asks for 10.75% and 11.75% and the loan answers 10.50%. The payment rises from $2,271.16 to $3,442.93, which is $1,171.77 more each month, or 51.6% above where it started.

Two comparisons make the caps concrete. Uncapped, year 10 would charge 11.75% and a payment near $3,745.37, so the lifetime cap is worth about $302 a month by year 10. And the same index path under 2/1/5 caps would run 8.75% in year 8 ($3,022.89), 9.75% in year 9 ($3,258.33), and 10.50% in year 10 ($3,434.67), for a ten-year total of $317,693.04 against $321,447.24 under 5/2/5. The tighter structure saves $3,754 over years 6 through 10 on an identical index.

The rate floor (often missed by borrowers)

Most ARMs have a floor equal to the margin. With a 2.75% margin, your rate cannot drop below 2.75% even if SOFR goes to zero. This is a feature for the lender and a quiet constraint for the borrower. In a deflationary scenario where the index actually went negative (unlikely but possible in some market structures), the floor protects the lender's economics but limits your rate-decline benefit.

How each adjustment happens: the adjustment cycle

The formulas describe the numbers. The cycle describes the calendar that delivers them, and every servicer follows the same four steps.

Step 1, the adjustment date. Your note sets the date, typically the anniversary of the first payment on a 5/1 or 7/1 ARM, and every year after it on a /1 product. A 5/6 ARM would run the same cycle twice a year.

Step 2, the lookback window. The index used is not today's value but the value published a set number of days before the adjustment, commonly 45. On a 60-day notice the number is already roughly six weeks old when you see it, which is why the payment you are quoted can differ from what you calculated with a live index quote.

Step 3, the computation and the notice. Index plus margin, rounded to the nearest 0.125%, floor checked, initial or periodic cap applied, lifetime cap applied. The servicer mails the new rate and payment, typically 45 to 60 days ahead of the change, and you do not have to do anything to accept it.

Step 4, the recast. The payment is rebuilt from your current balance over the remaining term at the new rate. After 60 payments at 5.50% the balance is $369,842.41 with 300 months left, so the year-6 recast runs off that number, not the original $400,000.

ARM eligibility and qualification requirements

Underwriting uses the same two formulas from the top of this page, pointed at a harder question: what payment could this borrower face? Overlays vary by lender, but the pattern is consistent.

• The qualifying rate. Most lenders test your debt-to-income at the greater of your note rate or the fully indexed rate. On this page's loan that is 7.00% and $2,661.21 over a full 30 years, not the 5.50% teaser and $2,271.16 you start with.
• The first-five-year maximum. The ability-to-repay rule requires considering the highest rate available in the first five years, which here is the 10.50% lifetime ceiling and $3,491.98 when recast from the year-5 balance. Some programs add about 2 points of cushion, putting the tested rate at 9.00% and $3,218.49.
• Credit. Best pricing starts at a 700 FICO, with 720 common for jumbo ARMs and 680 a practical conforming floor. Credit shows up a second time after closing because it helps set the margin you carry into every future adjustment.
• Reserves and equity. Six months of the post-adjustment payment, roughly $20,950 at this loan's ceiling, is a common overlay, and the sharpest pricing sits at 80% loan-to-value or below.

Tips for choosing an ARM in 2026

• Ask for index and margin before you ask for a rate. Two loans can show the same 5.50% start and reset to 6.55% and 7.30% when margins are 2.25% and 3.00% at SOFR 4.30%, which is $176.40 a month apart on the year-5 balance.
• Compute the fully indexed rate yourself. Index plus margin is public arithmetic. If the lender's number differs from yours, the difference is rounding, a different index, or a detail worth asking about before you sign.
• Prefer the tighter cap structure at equal pricing. 2/1/5 holds the first reset to 7.50% where 5/2/5 allows 10.50%, and the ten-year example above shows $3,754 of payment difference on the same index.
• Check the floor, not just the ceiling. At a 2.75% floor your rate stops falling even when the index collapses, which caps how much of any rate rally you keep.
• Budget for the recast, not the reset. The payment at 10.50% is not "your balance times 10.50%"; it is the balance over the remaining term. $369,842.41 over 300 months is $3,491.98, and the same balance over 252 months would be higher still.
• Take every quote on the same day. The index moves daily, so a fixed-rate quote from Monday and an ARM quote from Thursday are not comparable. Our mortgage rates page shows both sides together.

Common ARM mistakes

• Reading the teaser as the payment. Qualification runs at $2,661.21, budgeting should run at $2,733.10 to $3,491.98, and only $2,271.16 is what you pay in year 1.
• Treating the cap structure as decoration. On the same index, 2/1/5 and 5/2/5 differ by $3,754 over years 6 through 10. The cap columns are as negotiable as the rate columns at the quote stage.
• Forgetting that rounding is a real number. 7.05% becomes 7.00%, which is $11.81 a month on the year-5 balance in this example, and it happens without anyone approving it.
• Ignoring the floor. A 2.75% floor means part of any rate decline belongs to the lender, permanently.
• Assuming the payment is fixed between adjustments. It is, until the recast. The change arrives once a year with notice, and the only way to shrink it in advance is to shrink the balance.
• Skipping the lookback. The index in your notice is about six weeks old. Comparing it to a live quote and concluding the servicer erred is the most common false alarm ARM owners generate.

How to model your own ARM scenario

The ARM Calculator uses the formulas above to project payments year by year. Enter your loan amount, initial rate, margin, index value, and cap structure, and it shows your monthly P+I for each year of the loan. Compare that path against a fixed-rate alternative using the Fixed vs ARM Calculator to see which structure wins for your assumed hold period.