Mortgage Re Amortization Calculator: Lump Sums, Forbearance and Loan Modifications

Every fixed-rate payment comes from three inputs: the balance that still bears interest, the monthly rate (the note rate divided by 12), and the number of payments left. The formula is M = P·r(1+r)ⁿ / ((1+r)ⁿ − 1), the same one walked through on our mortgage amortization formula page. At closing, P is the amount borrowed and n is usually 360. Re-amortizing means plugging in today's values instead.

Change any one of the three and the payment changes:

• Lower P, same n: the payment falls. This is the lump-sum case.
• Higher P, same n: the payment rises. This is what happens when unpaid interest is capitalized into the balance.
• Same P, longer n: the payment falls, and total interest rises because the balance is outstanding for longer.
• Lower rate: the payment falls. Outside a refinance, this only happens in a modification.

Nothing else about the method changes. The new payment is still split each month into interest on the current balance and principal for the remainder, exactly as described in how mortgage amortization is calculated. What makes re-amortization worth understanding is that the two main situations move P in opposite directions, so the same word can describe both a smaller bill and a bigger one.

Fannie Mae's own servicing instructions show how literally servicers apply the "today's values" idea. Before working out a modification, a servicer handling a loan where the borrower has made extra principal payments must "first calculate the remaining mortgage loan term based on the interest-bearing portion of the borrower's pre-modification UPB and the contractual P&I payment" (Fannie Mae Servicing Guide F-1-27, read October 2026). In other words, prepaying shortened the loan; the servicer measures how much shorter before it re-amortizes anything.

Find the balance you will owe after the event (after the lump sum, or after arrears are added) and read across to the months that will remain. Every cell is principal and interest only; taxes, insurance and any mortgage insurance sit on top.

Monthly P&I at a 6.50% note rate. Computed with this site's amortization formula on 2026-10-19.

Three patterns stand out. First, the columns are perfectly proportional to balance: $300,000 always costs exactly twice what $150,000 costs at the same rate and term, so you can scale any cell for a balance that is not listed. Second, stretching the months has sharply diminishing returns. Going from 120 to 180 months cuts the payment on $300,000 by $793.12; going from 360 to 480 months cuts it by only $139.83. Third, the 480-month column is not hypothetical. It is the longest term Fannie Mae allows a servicer to extend to in its Flex Modification (covered below), and it shows why extending alone often cannot reduce a payment very much.

The rate here is fixed at 6.50% because re-amortization almost always keeps your existing note rate. If your loan carries a different rate, run it in the calculator above. For context, Freddie Mac's survey for the week of October 15, 2026 put the average new 30-year fixed rate at 7.40% and the 15-year at 6.71% (Freddie Mac PMMS, read October 2026); those numbers matter for a refinance, not for re-amortizing a loan you already hold.

1. You pay down principal and ask for a lower payment. This is a recast: the balance drops by your lump sum, the rate and maturity stay put, and the payment is recomputed over the same remaining months. Eligibility, fees and minimums are set by your servicer, and our mortgage recast calculator page covers that process in depth. The point to carry forward here is narrower: it is an optional step. The lump sum reduces your balance whether or not you ask for it.

2. You come out of forbearance. The CFPB describes forbearance as an arrangement where your servicer lets you temporarily pause or reduce payments, and is explicit that it "does not erase or decrease the amount you owe." The paused amounts have to be repaid, and the CFPB lists the routes servicers use: repay everything at once when payments restart; pause payments and add them at the end of the loan or put them into a separate loan; or spread the reduced amounts over a set number of months by raising your payment for that period (CFPB, What is mortgage forbearance?, read October 2026). It also warns that interest on paused amounts could keep adding up until you repay them. None of those three routes is a full re-amortization on its own, but each changes either what you owe at the end or what you owe each month, which is why it is worth computing before you agree to one.

3. Your loan is modified. Here the servicer genuinely re-amortizes. For conventional loans owned by Fannie Mae, the Flex Modification steps are spelled out: capitalize eligible arrearages (accrued interest and certain escrow and servicing advances paid to third parties; late charges may not be capitalized), set a fixed rate, possibly reduce the rate in 0.125% steps, then extend the term month by month, stopping at whichever comes first of a P&I payment reduction above 20% or a term of 480 months from the modification date. If those steps still fall short, a portion of principal may be set aside as non-interest-bearing forbearance (Fannie Mae Servicing Guide F-1-27 and D2-3.2-06, read October 2026).

The contrast case: a payment deferral is not a re-amortization. Fannie Mae's payment deferral moves between 2 and 6 months of past-due P&I into a non-interest-bearing balance due at maturity, sale, refinance or payoff, with no more than 12 months deferred over the life of the loan, and states that "all other terms of the mortgage loan must remain unchanged" (Fannie Mae Servicing Guide D2-3.2-04, read October 2026). Your monthly payment and payoff date stay as they were; the missed payments wait at the end.

This is the choice almost every borrower with spare cash actually faces, and the numbers below show how far apart the two outcomes are. The loan: $320,000 borrowed at 6.875% over 30 years, P&I of $2,102.17. After 84 payments the balance is $291,100.31 with 276 months to go. Without any extra payment, the remaining interest is $289,099.

Inputs as stated above; fees excluded. Computed with this site's amortization formula on 2026-10-19.

With $50,000, keeping the payment avoids $86,710 more interest than re-amortizing and ends the loan 88 months (seven years and four months) earlier. Re-amortizing frees $361.07 a month instead. Neither is wrong: the gap is the price of cash flow, and it is large because the freed-up $361.07 is no longer attacking a 6.875% balance.

There is a third path the table hides, because its numbers are identical to the "keep paying" rows. If you re-amortize and then voluntarily keep sending $2,102.17, the extra $361.07 each month goes to principal and the loan still finishes in 188 months with $152,733 of interest (assuming your servicer applies the excess to principal, which our extra payments and lump sum calculator lets you model month by month). You end up with the fast payoff and a lower contractual floor you can fall back to in a bad month. What that costs you is the servicer's fee for the recast, if it charges one. The comparison against a new loan altogether is on our recast vs refinance page.

Re-amortization after a hardship starts from a larger balance, so the first result is a higher payment, not a lower one. To see the size of the effect, take the same $320,000 loan from the previous section and suppose the borrower misses six payments starting with payment 85. Six months of interest at 6.875% on $291,100.31 is $10,006.57. Six missed P&I payments total $12,613.03. The following illustration applies the Flex Modification order to that loan using only the steps that need no outside data; it assumes the rate steps leave 6.875% in place and it does not model principal forbearance, which depends on the property's current value. It is an illustration of the arithmetic, not a servicer quote.

$320,000 at 6.875%, 30 years, six missed payments from payment 85, simple interest accrued on the unpaid balance. Computed with this site's amortization formula on 2026-10-19.

Two things follow. Capitalizing interest and simply re-amortizing to the original maturity raises the payment by $92.20, which is the wrong direction for someone recovering from a hardship. That is why Fannie Mae's process does not stop there. And even a 480-month term only gets this loan to a 12.29% reduction, short of the 20% target, which is exactly the gap the later rate and principal-forbearance steps exist to close. Extending also roughly doubles the interest still to be paid, from $291,372 to $583,963, because a $301,106.88 balance stays outstanding for 40 years instead of 22½.

If you are being offered a modification, ask the servicer for the new balance, rate, term and any non-interest-bearing amount in writing, then check the payment against the formula. You can find a HUD-approved housing counselor to go through the offer with you using the CFPB's housing counselor search (read October 2026).

The mortgage re amortization calculator above was built for the voluntary case, but its inputs are general enough to cover the other two once you know which number goes where.

For a lump sum: enter today's balance, your note rate and the years remaining, then the lump sum. It shows your current payment struck through, the new payment, the monthly saving, the interest the lump sum avoids, the new balance, and a schedule you can toggle between the re-amortized loan and the original. If your servicer charges a recast fee, enter it and the calculator shows how many months of savings it takes to repay it.

For the keep-the-payment comparison: enter the difference between your old and new payment in the "Extra Monthly After Recast" field. The schedule then shows how many payments the loan takes, which should match the payoff you would get by never re-amortizing at all.

For a modification offer: set the lump sum to $0, enter the post-capitalization balance the servicer quotes as the balance, the modified rate, and the new term in years (decimals work: 22.5 years is 270 months). The "new payment" figure is then the re-amortized payment you should see on the agreement. If it does not match, ask what is different; a non-interest-bearing amount, an escrow change or a step rate are the usual places to look.

For anything involving a new rate, use the refinance amortization calculator instead, and if your goal is the payoff date rather than the payment, the early mortgage payoff calculator is set up for that question. You can also start from the full schedule on the mortgage amortization calculator homepage.