ARM Calculator

What is an ARM mortgage?

An adjustable-rate mortgage is a home loan whose interest rate changes on a schedule set by the note. It opens with a fixed introductory rate — typically 5, 7, or 10 years — and then reprices at regular intervals for the rest of the term. The notation tells you both halves: a 5/1 ARM holds its rate for 60 months and adjusts every 12 months after that; a 7/1 holds for 84 months; a 5/6 adjusts every six months once the 60-month window closes. The conventional market is written almost entirely in those three shapes.

The introductory rate is almost always below the same-day fixed rate, because you are the one accepting the rate risk. A 30-year fixed benchmarked at 6.50% might be offered as a 5/1 at 5.25% — on a $400,000 loan that is $2,208.81 a month instead of $2,528.27, a $319.46 discount every month for five years. That discount is the entire pitch, and it lasts exactly as long as the fixed window does.

After the window, the rate becomes index plus margin and is bounded by caps. On a $400,000 loan the payment recasts on whatever balance is outstanding: at 7.05% with $368,598 still owed over 300 remaining months, the payment is $2,616.94, which is $408.13 above the starting rate and $88.67 above the 6.50% fixed benchmark. The caps convert that open-ended risk into a specific worst case — with a lifetime cap of 5 points, a 5.25% start can never exceed 10.25%, and the payment can never exceed $3,414.63 on that balance. Whether the trade is worth it depends entirely on how long you keep the loan, which is the question the ARM vs fixed-rate comparison works through.

How to use this calculator

1. Enter the home price and down payment; the loan amount is the difference and drives every result.
2. Select the loan term — 30 years or 15 years. A shorter term changes how much of the balance is exposed to later adjustments.
3. Choose the ARM type: 5/1, 7/1, or 10/1. This sets how many months pass before the first recast (60, 84, or 120).
4. Enter the initial interest rate — the teaser you would actually be quoted, typically 0.50% to 1.00% below the fixed benchmark.
5. Enter the current index rate and the margin. The calculator adds them into the fully indexed rate, which is the number you should budget against.
6. Enter the adjustment cap (the most the rate may move at one reset) and the lifetime cap (the most it may move overall). The lifetime cap sets your maximum rate.
7. Read the initial monthly payment, the fully indexed rate, the maximum rate possible, the estimated maximum payment, and total interest across the fixed and adjusted periods.

A realistic set of inputs: home price $500,000, down payment $100,000, loan amount $400,000, term 30 years, ARM type 5/1, initial rate 5.25%, index 4.30%, margin 2.75%, adjustment cap 2.0%, lifetime cap 5.0%. That returns an initial payment of $2,208.81, a fully indexed rate of 7.05%, an adjusted payment of $2,674.66, a maximum rate of 10.25% (5.25% + 5.00%), an estimated maximum payment of $3,209.59, and total interest of $534,927 across 60 months at the teaser and 300 months at the indexed rate. Compare the $2,208.81 starting payment against a 6.50% fixed at $2,528.27 — that $319.46 monthly gap is what you are being paid to accept the adjustment risk.

How an ARM payment is calculated

Two formulas run in sequence. The first covers the fixed window and is the same amortization formula used on any mortgage; the second governs every adjustment after it.

M = P × [ i (1 + i)ⁿ ] ÷ [ (1 + i)ⁿ − 1 ]

M is the monthly principal-and-interest payment, P the balance, i the annual rate divided by 12, and n the number of payments remaining. During the fixed window, P is the original $400,000, i = 5.25% ÷ 12 = 0.004375, n = 360, and M = $2,208.81. At each adjustment the payment is recalculated with the new rate, the current balance, and the months still owed.

The new rate itself is built in three steps:

Fully indexed rate = Index + Margin
Rounded rate = Round( Fully indexed rate ) to the nearest 0.125% (or 0.25%)
Applied rate = min( Rounded rate , initial cap ceiling , periodic cap ceiling , lifetime cap ceiling )

Worked example: 5/1 ARM at 5.25% with 2/1/5 caps

Step 1 — index plus margin. The index is 4.30% and the margin is 2.75%, so the fully indexed rate is 4.30% + 2.75% = 7.05%. Step 2 — rounding. Notes round to the nearest 0.125% or 0.25%; 7.05% sits between 7.000% and 7.125%, so at a 0.125% increment the applied rate becomes 7.000%, worth $13.45 a month less than the unrounded 7.05% on $400,000. Read your note's rounding clause — some lenders round in their own favor. Step 3 — caps. With a 5.25% start and 2/1/5 structure, the ceiling at the first adjustment is 5.25% + 2.00% = 7.25%, each later ceiling is the previous rate + 1.00%, and no rate may ever exceed 5.25% + 5.00% = 10.25%. Step 4 — apply. The rounded 7.000% is inside the 7.25% ceiling, so it applies; had the index run to 6.00% instead, the fully indexed 8.75% would have been cut to 7.25%. Step 5 — recast. At month 61 the balance is $368,598 with 300 payments left. At 7.05% that is $368,598 → $2,616.94 a month; at the capped 7.25% it is $2,664.25; at the uncapped 8.75% it would have been $3,030.40, so the initial cap alone holds back $366.15 a month. At the lifetime ceiling of 10.25% the payment reaches $3,414.63 — $1,205.82 above where it started.

Worked scenarios

Both tables use a $400,000 loan, a 5/1 structure starting at 5.25%, a 2.75% margin, and a 6.50% fixed benchmark of $2,528.27 for reference. Adjustment payments are recast on the balance outstanding at each reset, which is what your servicer actually does.

Monthly payment at each rate, $400,000 loan over 30 years

The ARM stops beating the 6.50% fixed benchmark the moment the fully indexed rate reaches 6.50%, and since that rate is index + 2.75%, the crossover happens with the index at 3.75%. With the index at 4.30%, the ARM is already $146.39 a month more expensive than the fixed rate from the first adjustment onward, and the five-year discount of $319.46 a month — $19,168 in total — is what has to cover it.

Adjustment schedule with 2/1/5 caps binding

Read the ceiling column against the applied column and you can see exactly where each cap does its work. The initial cap holds year 6 to 7.25% even though the index-plus-margin number is 8.75% — a $366.15 monthly saving that year. The 1-point periodic cap limits every later step. By year 9 the lifetime cap is the binding constraint and it stays binding for the rest of the loan: the rate stops at 10.25% no matter how far the index climbs, and the payment stops at $3,366.42 on that balance. Without the caps the year-6 payment would be $3,030.40 instead of $2,664.25.

How ARM terms work

• Introductory rate: The teaser that holds for 60, 84, or 120 months on a 5/1, 7/1, or 10/1. At 5.25% against a 6.50% fixed benchmark it saves $319.46 a month on $400,000 — $19,168 over the five-year window of a 5/1.
• Index: The published benchmark your rate floats with — SOFR, the prime rate, or a Treasury-derived index. It is observed on a stated cycle and is the half of the equation you cannot negotiate.
• Margin: The lender's fixed markup over the index, typically 2.25% to 3.00%, set at closing and unchanged for the life of the loan. This example uses 2.75%; a 0.75-point margin difference is worth about $200 a month on a $400,000 loan once it adjusts.
• Fully indexed rate: Index plus margin — 4.30% + 2.75% = 7.05%. Underwriters qualify you on this number rather than the teaser, because it is the rate that governs the last 300 payments.
• Rounding: The computed sum is rounded to the nearest 0.125% or 0.25% before it is applied. Each 0.125% increment is worth about $33 a month on $400,000, so the clause is worth reading in the note.
• Initial adjustment cap: The maximum change at the first reset — 2 points under a 2/1/5 structure, 5 points under 5/2/5. On a 5.25% start that is a 7.25% or a 10.25% ceiling in year 6, which on this balance is $2,664.25 versus $3,414.63 a month.
• Periodic cap: The maximum change at each later reset — 1 point under 2/1/5, 2 points under 5/2/5. It controls how quickly the rate can walk from the first adjusted rate to the lifetime ceiling.
• Lifetime cap: The absolute ceiling above the initial rate, commonly 5 points. A 5.25% start can never exceed 10.25%, capping the payment at $3,414.63 on a $368,598 balance with 300 months left.
• Floor: The opposite limit. Rates can fall at a reset as well as rise, and most notes set the floor at or near the margin, so the lowest achievable rate is not zero.
• ARM type and frequency: The number after the slash is the adjustment frequency. A 5/6 reprices twice a year, which means twice the opportunity for the rate to move — and twice the observation dates to track.

ARM strategy for 2026

1. Budget the fully indexed rate, spend the teaser. Build your monthly plan on 7.05%, not 5.25%. On $400,000 that is $2,674.66 rather than $2,208.81 — direct the $465.85 difference into a savings account for five years and the first adjustment becomes a non-event.
2. Match the fixed window to your real horizon, then add two years. A 5/1 assumes you exit inside 60 months. If your honest plan is 4 to 6 years, a 7/1 at 84 months gives you the buffer without changing the rate much.
3. Qualify yourself at the maximum rate. The calculator's estimated maximum payment is $3,209.59 on this example, and the true lifetime-cap payment on the recast balance is $3,414.63. Put both against your income — if neither fits your budget, you are not buying a low payment, you are financing a future obligation you have already signed.
4. Shop the margin, not the teaser rate. Teasers are marketing; margin is the contract. Margins run 2.25% to 3.00%, and the spread is worth roughly $200 a month on $400,000 once the loan reprices — far more than any closing-cost incentive.
5. Know your index and its observation dates. SOFR, prime, and Treasury indices reset on different cycles. Checking the current level of the index named in your note tells you whether your next adjustment is likely to land near 7.05% or near the cap.
6. Plan the exit before you need it. The ARM strategy is really an exit strategy: sell or refinance before the rate risk arrives. Closing costs run 2% to 5% of the loan, so hold that cash separately rather than counting on a favorable market in the exact year you need one.
7. Re-run this calculator every year. A revised stay estimate, a moved index, or a shift in fixed-rate pricing flips the answer. The rates page gives you fresh fixed and ARM quotes to feed it.

Common ARM mistakes

• Judging the loan by its teaser. A 5.25% ARM is not competing with a 6.50% fixed — it is competing with the 7.05% you will actually pay for 300 of its 360 payments. On $400,000 that is $146.39 a month more than the fixed rate once the window closes.
• Assuming the recast uses your original loan amount. The payment recalculates on the balance you owe over the months you have left: $368,598 at 7.05% over 300 months is $2,616.94, not the $2,674.66 a fresh $400,000 over 360 months would produce. Budget the higher number and treat the difference as a cushion.
• Skipping the cap structure. A 2/1/5 note and a 5/2/5 note with identical teasers are different loans. In year 6 the first caps the rate at 7.25% while the second permits 10.25% — $2,664.25 versus $3,414.63 on this balance, a $750.38 monthly gap for doing nothing differently.
• Ignoring the margin because the rate looked good. The teaser is a rate sheet; the margin is in the note and it never changes. Three lenders at 5.25% can differ by 0.75 of margin, which is about $200 a month at adjustment and roughly $60,000 over 300 payments.
• Planning to refinance before the first adjustment with no contingency. The plan depends on rates, credit, and home values cooperating at the same moment. Closing costs of 2% to 5% — $8,000 to $20,000 on a $400,000 loan — plus a soft market can consume the $19,168 you saved during the fixed window.
• Treating the payment as unpredictable. Caps make it predictable: 10.25% maximum on this note and $3,414.63 on this balance. "I don't know what it will become" is the reason to run the numbers, not the reason to skip them.

Frequently asked questions

What is an adjustable rate mortgage? A home loan whose rate resets on a published schedule. It typically holds a fixed rate for 5, 7, or 10 years, then reprices annually — or every six months on a /6 note. On a $400,000 loan a 5/1 starting at 5.25% pays $2,208.81 a month for 60 months before the first recast.

How is an ARM rate calculated? The lender takes the index plus your margin — 4.30% + 2.75% = 7.05% — rounds to the nearest 0.125% or 0.25%, then applies the caps. Whatever survives those three steps becomes the rate for the next adjustment period, and the payment recalculates on your current balance over the remaining months.

What do the numbers in 5/1 and 7/1 mean? The first number is the years at the fixed introductory rate and the second is the adjustment frequency afterward. A 5/1 holds for 60 months then reprices every 12 months; a 7/1 holds for 84 months; a 5/6 reprices every 6 months once its 60-month window closes.

What are ARM caps? Three limits on how far the rate can move: the initial adjustment cap, the periodic cap, and the lifetime cap. A 2/1/5 structure allows 2 points at the first reset, 1 point at each later one, and 5 points total. Starting at 5.25%, that means 7.25% in year 6 and 10.25% forever.

How much can my ARM payment go up? The caps decide. On a $400,000 loan starting at 5.25%, the payment begins at $2,208.81, reaches $2,664.25 at the 7.25% first-adjustment ceiling, and tops out at $3,414.63 if the rate hits the 10.25% lifetime cap with $368,598 still owed — a $1,205.82 increase over the starting payment.

Is a 6.50% fixed rate better than an ARM? Not automatically. At 6.50% the fixed payment is $2,528.27 on $400,000 and never changes; the 5/1 at 5.25% is $319.46 cheaper for five years but $146.39 more expensive at a 7.05% fully indexed rate. The answer turns on how long you keep the loan — see the fixed vs ARM calculator.

Should I pick a 5/1 or a 7/1 ARM? The 7/1 buys two more years of fixed rate — 84 months instead of 60 — for a modestly higher teaser. It costs more up front and protects you longer, which suits anyone whose honest exit plan is 5 to 7 years. The 5/1 vs 7/1 comparison prices the difference.

Related calculators

• Fixed vs ARM Calculator — run both structures side by side over your stay.
• Mortgage Calculator — full PITI on a fixed-rate baseline.
• Mortgage Rates — today's fixed and ARM pricing for your inputs.
• How ARM Payments Are Calculated — the index, margin, rounding, and caps in depth.
• What Is an ARM Mortgage? — plain-English guide to how these loans work.
• Refinance Calculator — price the exit from an ARM into a fixed loan.