Mortgage Recast Calculator

The decision a recast really asks you to make

A mortgage recast looks like a simple win: hand the lender a chunk of cash, and your monthly payment drops for the rest of the loan — same rate, same payoff date, no refinance, no credit check, no closing costs. For a few hundred dollars in admin fee, your required payment shrinks and stays shrunk. That is genuinely useful, and most recast calculators stop there: they show you the smaller payment and a tidy interest-saved figure and send you on your way.

But that smaller payment hides the real question. The moment you have a lump sum sitting in your account, you are not choosing between "recast" and "do nothing" — you are choosing between three very different futures for that money. You can recast and buy yourself a lower required payment. You can prepay the exact same amount and keep paying your old, higher payment, which pays the loan off years early and saves far more interest. Or you can skip the mortgage entirely and invest the cash. These paths can differ by tens of thousands of dollars over the life of the loan, and the difference depends on facts only you know: how tight your monthly budget is, how steady your income is, and what you can realistically earn elsewhere. This calculator was built to put all three futures on one screen so you can see the trade-off in dollars instead of guessing.

How to use the calculator

1. Enter your current loan balance — the principal you still owe today, not the original loan amount. Your latest statement shows it.
2. Enter your interest rate and the years remaining. The recast re-amortizes over the months you have left, so the remaining term matters as much as the rate.
3. Enter the lump sum you would put toward principal, and your lender's recast fee (often 150 to 500 dollars — leave the default if you are unsure).
4. Set your expected investment return — a realistic, after-tax annual figure you could earn if you invested the lump sum instead. This drives the invest-vs-pay-down comparison.
5. Read the result and the three-option table. The headline shows your new payment; the table shows recast, prepay-only and invest side by side, and the verdict tells you which fits your situation.

The math it does for you

Your monthly principal-and-interest payment is the amortization of your balance across your remaining months at your monthly rate. When you recast, the lender subtracts the lump sum from your balance and runs that same amortization on the smaller principal over the same remaining months. Because only the principal changes — not the rate or the number of payments — the new payment is simply your old payment scaled down by the smaller balance. Knock 50,000 dollars off a 300,000 balance and the payment falls by exactly one-sixth, to the cent.

Interest saved by the recast is the gap between the total interest you would have paid on the old schedule and the total interest on the new, smaller schedule over the same term. The calculator computes both precisely. It then does the calculation the others leave out: the prepay-only path. There it keeps your original, higher payment but starts from the smaller balance and solves for how many months it now takes to reach zero. Because the higher payment keeps hammering principal, the loan disappears years early and the interest saved is dramatically larger — the cost being that your required payment never drops. Finally, for the invest path, it compounds your lump sum forward at your expected return over the remaining term, so you can weigh a guaranteed mortgage-rate return against an uncertain market one.

A worked example

Take a 300,000 dollar balance at 6.5% with 25 years left, and a 50,000 dollar lump sum. Your current payment is about 2,026 dollars. Recast and it falls to about 1,688 — roughly 338 dollars a month back in your pocket — saving close to 51,000 dollars of interest over the remaining term, with the 250 dollar fee recovered in well under a month. Now look at prepay-only: keep paying 2,026 against the smaller balance and the loan is gone in about 17 years instead of 25, saving on the order of 144,000 dollars of interest — almost three times as much — but your monthly obligation never eases. That contrast is the whole decision: about 93,000 dollars of extra interest savings is the price of keeping your payment low. Whether that price is worth paying depends entirely on how much you value a guaranteed lower payment versus a faster, cheaper payoff.

The best-of-both move, in numbers

There is a fourth path almost nobody names, and it quietly beats the other three for disciplined homeowners: recast, then keep paying your old amount anyway. When you recast, your required payment drops — but nothing stops you from voluntarily paying more. So you lock in the lower minimum as a safety net, then keep sending the higher figure every month. The extra above the new minimum lands entirely on principal, so the loan clears on the same accelerated timeline as a plain prepayment, capturing the full interest saving — while you retain the right to fall back to the lower payment in any month that money is tight. In the worked example, that means the roughly 144,000 dollars of interest savings and the option to drop to 1,688 dollars whenever you need to. The table above shows this strategy on its own row (marked ★). The only thing it costs you is the small recast fee and the discipline to ignore the lower number on your statement. For anyone with steady income and a real emergency fund, it is usually the smartest use of the lump sum: you buy insurance against a bad month without giving up the faster, cheaper payoff.

How a recast actually gets processed

Knowing the mechanics helps you plan the timing of the lump sum. A recast is not instant, and the sequence matters because interest keeps accruing until the principal actually posts:

1. Confirm eligibility and the minimum. Call your servicer (not the original lender if your loan was sold) and ask whether your loan type allows a recast, what the minimum principal reduction is, what the fee is, and how many recasts they permit. Get the figures in writing.
2. Make the principal-only payment. Send the lump sum clearly marked "apply to principal" — many servicers have a separate field or address for principal-only payments. A payment dropped into the regular bucket can be applied to future installments instead of principal, which is not what you want.
3. Request the recast in writing. A principal payment alone does not trigger re-amortization; you must explicitly ask for the recast and usually sign a short form. Until you do, you have simply prepaid — your required payment stays the same.
4. Verify the new schedule. When the recast posts, confirm the new payment, that the rate and maturity date are unchanged, and that the fee was the amount quoted. Mistakes happen; a five-minute check protects the saving.

From request to a finalized new payment typically takes one to two billing cycles. Because the principal posts first, your interest savings begin the day the lump sum lands — not the day the paperwork finishes — so there is no penalty for the administrative lag.

A 30-second decision guide

Strip away the jargon and the choice usually resolves to a few honest questions about your own situation:

When recasting is the right move

When to prepay without recasting instead

If your budget comfortably absorbs your current payment, prepaying the lump sum and simply not recasting is usually the stronger play. You keep the higher payment, the extra principal compounds into an earlier payoff, and you capture the full interest saving the recast gives away. You also keep flexibility: nothing stops you from recasting later if your circumstances tighten. The only thing you lose is the lower required payment — which, if you never needed it, costs you nothing. Many homeowners default to recasting because it feels like the "official" option, when a plain prepayment would have served them better and for free.

When investing the cash may win

Paying down a mortgage is a guaranteed return equal to your rate — there is no safer "investment" with that yield, and it is effectively tax-free because you are avoiding interest rather than earning taxable income. The flip side is liquidity: money poured into home equity is hard to get back without borrowing against it. If your mortgage rate is low and you can reliably earn more after tax in a diversified portfolio over the same horizon, investing the lump sum can build more wealth and keep your cash accessible for emergencies or opportunities. If your rate is high, or you would lose sleep over market swings, the guaranteed return from recasting or prepaying is hard to beat. The calculator's projected invested value is there to make that comparison concrete, not to predict the market — treat the return you enter as a sober estimate, not a promise.

Recast vs refinance — a quick contrast

People often confuse the two because both can lower a payment, but they work in opposite ways. A refinance replaces your loan: you take out a new mortgage, ideally at a lower rate, pay closing costs of typically a few thousand dollars, and reset the term. It is the right tool when rates have dropped meaningfully below your current rate. A recast changes nothing about the loan except the payment — same rate, same term, same lender — and costs only a small admin fee. If your rate is already good and you simply want a lower payment after putting cash down, recasting is far cheaper and simpler. If chasing a lower rate is the goal, only a refinance can deliver it. Run your own numbers for both before deciding.

Common mistakes this prevents

Pro tips

• Confirm eligibility first. FHA, VA and USDA loans generally cannot be recast; ask your servicer about the minimum lump sum and any frequency limits before you plan around it.
• Recast and keep paying the old amount. This is the quiet best-of-both move: you get a lower required payment as a safety net, but by voluntarily paying the higher figure you also capture the faster payoff.
• Keep your emergency fund intact. A lower payment is small comfort if the lump sum that bought it was your last accessible cash. Liquidity first, optimisation second.
• Mind the timing of the fee. The break-even on the admin fee is usually trivial, but confirm the exact charge in writing so the saving is real from month one.
• Re-run the numbers at your real rate. A half-point difference in your rate or expected return can flip the invest-versus-pay-down verdict, so use your actual figures, not round defaults.

Where this fits with your other mortgage decisions

A recast is one lever among several, and the smartest homeowners compare them rather than grabbing the first one a lender offers. If you are weighing paying points to lower your rate, a temporary buydown, or simply shedding mortgage insurance once you have enough equity, each has its own break-even and its own best use. Use this recast calculator alongside the related tools below to see your whole picture — the payment, the interest, the payoff date, and the opportunity cost — before you commit a lump sum you cannot easily get back.

Frequently asked questions

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