Adjustable Rate Mortgage Amortization Calculator With Caps and Resets

A normal amortization schedule rests on one payment, calculated once with M = P·r(1+r)ⁿ/((1+r)ⁿ−1), that clears the loan in exactly n months. The method is laid out step by step in our guide to how mortgage amortization is calculated. An ARM keeps that formula but uses it more than once. During the initial period the payment is computed as if the starting rate would last the whole term. At each reset the lender takes three inputs — the balance still owed, the months still remaining, and the new rate — and runs the formula again.

That has a consequence most people miss. The payment after a reset is not the old payment plus some percentage; it is a brand-new level payment for a shorter loan. Because the balance has been falling, a small rate increase can produce a smaller jump than you might expect, while a large one lands on a loan that has fewer months left to spread it over. The calculator above repeats this re-amortization at every adjustment date until the final payment.

The rate after the fixed period is not chosen by your lender on the day. According to the CFPB, it is the index plus the margin, subject to any rate caps. The index moves with the market; the margin is set in your loan agreement and does not change after closing. The CFPB adds that margins can vary a lot between lenders and can be negotiated, so the margin deserves as much attention as the starting rate.

For conventional loans sold to Fannie Mae, the Selling Guide, section B2-1.4-02 (read October 2026) requires the ARM to be tied to SOFR, specifically a 30-day average of SOFR published by the New York Fed. The same section caps the mortgage margin at 300 basis points, rounds the fully indexed rate to the nearest one-eighth of a percent, and says the rate may never fall below the margin regardless of any downward cap. For reference, the New York Fed reported SOFR at 3.88% for October 16, 2026, and the 30-day average at 3.88518% on October 19, 2026 (read October 2026).

Caps are the third input. The CFPB describes three: an initial adjustment cap on the first change, a subsequent cap on each later change, and a lifetime cap measured from the initial rate. All three are printed in the Adjustable Interest Rate (AIR) table on page 2 of your Loan Estimate, which is where the numbers for this calculator should come from.

Any ARM tool should be tested against a published answer. The Consumer Handbook on Adjustable-Rate Mortgages (CHARM booklet, read October 2026) prints a sample 5/1 ARM: $216,000 over 30 years at an initial 3%, first change at the beginning of the 61st month, changes every 12 months after that, a 2% limit on the first change and on subsequent changes, and a maximum rate of 8%. We fed those terms into the same routine the calculator uses, with the worst-case toggle on.

Inputs from the CFPB CHARM sample Loan Estimate; computed by the site's amortization formula on 2026-10-19.

Every figure matches the sample once it is rounded to the dollar, which confirms the method: the payment is re-amortized on the remaining balance and remaining term at each reset, and the rate climbs by the cap until it hits the ceiling.

Here is the same loan under six different futures. Every scenario uses a hypothetical 6.50% initial rate on $400,000 over 30 years with a five-year fixed period. The 6.625% “flat” case is the October 19 30-day average SOFR plus an example margin of 2.75%, rounded to the nearest eighth the way Fannie Mae's guide describes. The 2.75% margin and the cap structures are examples to show the mechanics, not quotes or market averages.

Inputs: $400,000, 30-year term, hypothetical 6.50% initial rate, 5-year fixed period; computed by the site's amortization formula on 2026-10-19.

The flat-index row is the most instructive. Even when the market does not move at all, the payment still rises by $29.33 a month, because the starting rate was below the fully indexed rate. The CHARM booklet calls this a teaser effect and warns that the rate rises when the discount ends even if the index is unchanged. At the other end, every worst-case row converges on 11.50% — the initial rate plus the 5-point lifetime cap — and the cap structure decides only how fast you get there.

Cap notation lists the initial cap, the subsequent cap and the lifetime cap in that order, in percentage points. The CFPB's cap page says the initial cap is commonly two or five points, the subsequent cap one or two points, and the lifetime cap most commonly five points, with some loans higher. Treat those as the CFPB's description of what exists, and read your own figures off your AIR table.

The rows above show why the first number matters so much. Under 2/2/5 the worst-case payment climbs in three steps — $3,015.12, then $3,523.38, then $3,782.41 — giving a household two years to adjust, refinance or sell. Under 5/2/5 the entire increase arrives in month 61: from $2,528.27 to $3,806.11, a jump of $1,277.84 in a single reset. The lifetime ceiling is the same; the shock is not. If two offers share an initial rate, the one with the smaller initial cap is the gentler loan to live with, and the CFPB tip is to ask each lender to calculate the highest payment you could ever owe.

A longer fixed period does two things at once: it delays the first reset, and it lets more principal be repaid before any reset happens, so later increases hit a smaller balance. To isolate those effects, this grid holds the initial rate at the same hypothetical 6.50% for all three. In real offers, the initial rate usually differs by fixed-period length, so enter each lender's actual rate in the calculator.

Inputs: $400,000, 30 years, hypothetical 6.50% initial rate, annual resets, example caps 2/2/5, rate rising by the full cap at each reset; computed by the site's amortization formula on 2026-10-19.

The worst-case ceiling payment falls by about $196 a month between the 5-year and 10-year versions, purely because the 11.50% maximum is applied to a smaller balance over fewer months. The ten-year loan also spends five extra years at 6.50%, which is why its worst-case lifetime interest is roughly $120,000 lower.

Most ARM decisions are really bets on how long you will keep the loan. As a reference, Freddie Mac's Primary Mortgage Market Survey put the average 30-year fixed rate at 7.40% for the week of October 15, 2026 (read October 2026). Freddie Mac no longer publishes an ARM average, so the ARM below again uses a hypothetical 6.50% initial rate, matched to an ARM whose fixed period lasts at least as long as you stay.

Inputs: $400,000, 30 years; ARM payment $2,528.27 vs. fixed payment $2,769.52 (a $241.25 monthly gap); computed by the site's amortization formula on 2026-10-19.

Within the fixed period the ARM wins on both counts: a lower payment and a lower balance, because a lower rate sends more of each payment to principal. The risk begins the month you stay past the fixed period. The CFPB's fixed-vs-ARM page warns against assuming you will be able to sell or refinance before the rate changes, since home values and personal finances can shift. Our 15 year vs 30 year mortgage calculator covers the fixed-rate side of the comparison in more depth.

If you hold the ARM to the end, the comparison becomes a single number: the fully indexed rate that would make its lifetime interest equal the fixed loan's. On $400,000 at 7.40% fixed, total interest is $597,027. Running the ARM above with example caps of 2/2/5 at different fully indexed rates gives:

• Fully indexed 7.00%: $545,644 total interest — ARM ahead by about $51,000.
• Fully indexed 7.50%: $581,829 — ARM ahead by about $15,000.
• Fully indexed 7.71%: break-even with the fixed loan.
• Fully indexed 8.00%: $618,702 — the fixed loan ahead by about $22,000.

So, under these example terms, the fixed loan only wins over 30 years if the index plus margin settles about 1.1 points above the October 2026 flat case of 6.625% and stays there. That is not a forecast; it turns a vague bet into a specific rate you can judge. Change the margin or caps in the calculator and the break-even moves with them.

A spreadsheet version needs only one change from a fixed-rate template: the rate becomes a column, and the payment is recalculated at each reset row. Put the month number in column A, the rate for that month in column B, the beginning balance in C, and compute the payment in D with =IF(reset_row, PMT(B2/12, months_remaining, -C2), D1). Interest is =C2*B2/12, principal is payment minus interest, and the next row's balance is this row's balance minus principal. Note that the remaining months, not the original 360, go in PMT's second argument.

The rate column is where caps live. A cap-respecting formula is =MIN(MAX(target, prior-cap), prior+cap, initial+lifetime), using the initial cap at the first reset and the periodic cap after that. Our mortgage amortization calculator in Excel walks through the fixed-rate template this extends; adding the rate column turns it into the variable-rate sheet many people search for.

The CHARM booklet explains the naming: in a 5/6m ARM the first number is the length of the initial period and “6m” means the rate may adjust every six months after that. More frequent resets mean the rate follows the index more closely, in both directions. With a worst-case path, frequency and caps interact in a way that is easy to misjudge. Comparing the $400,000 example with caps 2/2/5 on annual resets and caps 2/1/5 on six-month resets:

• Both reach 8.50% and $3,015.12 in month 61.
• The six-month loan reaches 9.50% and $3,268.55 in month 67, while the annual loan is still at $3,015.12.
• The six-month loan hits the 11.50% ceiling in month 79; the annual loan gets there in month 85.
• Across years 6 to 8, the six-month loan collects $127,059 of payments against $123,851 for the annual loan, about $3,208 more.

A one-point periodic cap sounds safer than two points, but twice as many resets can more than cancel it out. The calculator's “Adjusts every” field lets you test both.

An adjustable rate mortgage amortization calculator is only as good as its assumptions. This one makes four:

• One rate path. The fully indexed rate is held constant after the fixed period. Real indexes move; use the worst-case toggle and a few different assumptions to bracket the outcome.
• Principal and interest only. Property tax, insurance and mortgage insurance sit on top and can change too; the CHARM sample shows $341 a month of escrow on its loan. Add yours with our mortgage calculator with PMI, taxes and insurance.
• Fully amortizing loans. Interest-only and payment-option ARMs, which the CHARM booklet says can produce negative amortization, behave differently; see our interest only mortgage calculator for the first kind.
• Lender-specific rules. Rate floors, rounding of the fully indexed rate, conversion options and lookback periods are set by your note. Your AIR table and ARM program disclosure govern; this page does not.

For a straight fixed-rate schedule to compare against, start from the main mortgage amortization calculator.