Mortgage Recast Payment Reduction Calculator

How this mortgage recast payment reduction calculator helps you

A mortgage recast payment reduction calculator is designed to answer a practical question: what happens to your monthly payment and total interest if you send a large lump-sum payment to your lender and ask them to recast (reamortize) the loan? This tool estimates your new principal-and-interest payment, the reduction in your total interest costs, and how the recast compares with simply investing the lump sum instead.

Unlike a refinance, a recast keeps your existing loan in place. The interest rate, remaining term, and other contract terms usually stay the same. Only the balance changes—because you apply a one-time lump sum—which allows your servicer to recompute a lower payment over the remaining months. This can free up cash flow while still reducing interest costs over time.

Because you are tying up cash in home equity instead of keeping it liquid or investing it, there is a trade-off. This calculator walks through that trade-off so you can see, in simple numbers, how much you might save in interest, how much your payment might drop, and how those benefits compare to a hypothetical investment return on the same lump sum.

Inputs you will need

To get accurate results, gather the following items—usually found on your latest mortgage statement or loan documents:

• Original loan amount (USD): The amount you initially borrowed when you took out the mortgage. This is used for context only and does not affect the recast math directly, but it helps frame how far you have already progressed in paying down the loan.
• Current principal balance (USD): The amount you still owe right now, excluding any interest or fees. On a monthly statement this is often labeled as “principal balance,” “outstanding principal,” or “unpaid principal.”
• Annual interest rate (%): Your mortgage note rate, not the APR. For example, if your rate is 5.25%, enter 5.25, not 0.0525.
• Months remaining on the loan: The number of scheduled payments left if you keep making the current required payment. You can estimate this from your original term and how many payments you have already made, or find it in an amortization schedule if your lender provides one.
• Lump sum applied to principal for recast (USD): The extra amount you plan to send to your servicer specifically for a recast—often from savings, a bonus, an inheritance, or the sale of another property.
• Servicer recast fee (USD): Many lenders charge a flat fee for processing a recast. Enter that here so the calculator can net it out when comparing benefits.
• Monthly escrow (USD): Optional. Enter your monthly property tax and insurance escrow if you want to compare full payment amounts (principal + interest + escrow) before and after the recast.
• Annual investment return (%): A hypothetical long-term annual return you think you might earn if, instead of recasting, you invested the lump sum. This is a simplified constant rate used only for comparison.

Formulas used to estimate your new payment

The calculator uses standard mortgage amortization formulas to estimate your current and post-recast payments. It assumes a fully amortizing, fixed-rate loan with level monthly payments.

Let:

• B = current principal balance before the recast
• L = lump sum you plan to apply at recast
• r = monthly interest rate (annual rate divided by 12)
• n = number of months remaining on the loan

The standard monthly principal-and-interest payment P on an amortizing loan is calculated as:

After the recast, the balance used for amortization becomes B – L, while the interest rate r and remaining term n stay the same. The new monthly payment P_new is therefore:

P_new = (B - L) × r / [1 - (1 + r)-n]

The calculator computes both the current scheduled payment and the new recast payment. It then derives several comparison metrics:

• Monthly payment change: New principal-and-interest payment versus current principal-and-interest payment, and optionally including escrow if you provided it.
• Total remaining interest without recast: Sum of all interest that would be paid if you keep making scheduled payments without applying the lump sum.
• Total remaining interest with recast: Sum of interest under the new lower payment schedule, after applying the lump sum.
• Interest saved by recasting: Difference between those two totals, minus any recast fee you entered.

How the investment comparison is modeled

In addition to the payment and interest numbers, the calculator lets you compare a recast to the hypothetical scenario where you invest the lump sum instead.

At a high level, the model considers:

• The interest savings from recasting (a reduction in what you pay the lender over the remaining life of the loan).
• The projected future value of investing the lump sum at a constant annual rate.

Using your chosen annual investment return R (for example, 5% per year), the calculator approximates how much the lump sum might grow over the same remaining loan term n. If it treated the lump sum as a single deposit held for the remaining term, the growth could be expressed as:

Future value ≈ Lump sum × (1 + R)years_remaining

For simplicity, the tool uses a fixed annual percentage rate converted into a periodic growth rate. It then compares the projected investment value to the interest you would save by recasting, giving you a rough sense of which use of cash looks more favorable on paper.

This is only a simplified, educational comparison. Real markets are volatile, and your actual investment performance could be higher or lower than the constant rate you enter.

How to read your results

When you click the calculate button, the results area (below the form) will typically show several key outputs. Here is how to interpret them:

• New monthly payment: If this is substantially lower than your current payment, the recast significantly improves your short-term cash flow. If the difference is small, the main benefit may be interest savings rather than budget relief.
• Payment reduction (dollars and percent): A large percentage reduction means the recast has a strong impact on your monthly budget. Compare this number to your other financial goals (such as saving for retirement or building an emergency fund).
• Total interest remaining without recast: This is the baseline cost of leaving the mortgage alone.
• Total interest remaining with recast: This is what you might pay after applying the lump sum and having the lender recast your loan.
• Net interest savings (after fee): If this number is positive and large, the recast is helping you reduce the long-run cost of your mortgage. The recast fee is included here so you see the true net effect.
• Investment comparison: The calculator may show a projected value of investing the lump sum versus the interest savings from recasting. If the projected investment value is higher than the interest savings, investing might look better financially on paper; if the interest savings is higher, recasting looks more compelling from a purely numerical standpoint.

Remember that the best choice is not always the one with the absolute highest projected dollar benefit. Liquidity, risk tolerance, peace of mind, and your broader financial plan also matter.

Worked example

To make the numbers more concrete, consider a simplified example. The figures below are illustrative only.

Suppose you have:

• Current principal balance: $300,000
• Annual interest rate: 5.00%
• Remaining term: 300 months (25 years)
• Planned lump sum for recast: $50,000
• Servicer recast fee: $300
• Escrow: $400 per month
• Hypothetical annual investment return: 6%

Step 1: Convert the annual interest rate to a monthly rate:

r = 5.00% / 12 ≈ 0.4167% per month (0.004167 in decimal form)

Step 2: Compute the current principal-and-interest payment using the amortization formula. With a balance of $300,000, a monthly rate of 0.004167, and 300 months remaining, the payment is approximately:

P_current ≈ $1,753 (principal and interest only)

Including $400 of escrow, your current total monthly payment would be around $2,153.

Step 3: Apply the recast. The new balance is $300,000 - $50,000 = $250,000. Keeping the same rate and term, the new principal-and-interest payment becomes:

P_new ≈ $1,461 (principal and interest only)

With the same $400 escrow, your new total monthly payment would be about $1,861. In this illustration, the recast reduces your principal-and-interest payment by roughly $292 per month, and your full payment (including escrow) by the same amount.

Step 4: Compare lifetime interest. Without a recast, the total remaining interest over the next 300 months might be roughly:

Total interest without recast ≈ $225,900

With the recast, the total remaining interest might fall to about:

Total interest with recast ≈ $188,250

That implies a gross interest saving of about $37,650. After subtracting a $300 recast fee, the net interest savings is around $37,350.

Step 5: Compare with investing. If you instead invested the $50,000 at a constant 6% annual return for 25 years, the simple future value would be approximately:

Future value ≈ $50,000 × (1.06)25 ≈ $214,000 (rounded)

On paper, that projected future investment value is much larger than the $37,350 of interest savings in this example. However, the investment involves market risk and does not improve your monthly payment or debt level today. The recast, by contrast, immediately lowers required payments and guarantees the interest savings if you hold the loan to term and follow the schedule.

This example shows how the calculator can help you weigh short-term cash flow, risk, and long-term financial impact side by side.

Summary comparison: recast vs invest the lump sum

Assumptions & limitations of this calculator

This tool can help you understand the basic math behind a mortgage recast, but it necessarily relies on simplifying assumptions. Keep these points in mind when reviewing your results:

• Fixed-rate loans only: The formulas assume a fixed interest rate for the remaining term. Adjustable-rate mortgages (ARMs) or loans with future rate changes are not modeled.
• Level monthly payments: The calculations assume you make the exact scheduled payment each month, on time, with no additional principal prepayments beyond the lump sum used for the recast.
• No changes to term or other loan features: The recast is modeled as a simple reamortization of the remaining balance over the existing remaining months. It does not shorten or extend the term unless you choose to pay extra on your own.
• Taxes and insurance: Escrow amounts you enter are included only for comparing the size of your total monthly payment. They are not part of the interest calculation and are assumed to stay constant.
• Investment returns are hypothetical: The alternative investment return is treated as a steady annual percentage rate for illustration. Real-world investments fluctuate and may perform better or worse than assumed. The calculator does not model taxes, fees, or inflation on investments.
• Lender policies vary: Some servicers do not offer recasts, require a minimum lump sum, or apply different rules than the simple model used here. Fees and eligibility criteria can vary widely.
• No tax advice: Potential tax effects of mortgage interest, investment income, or itemized deductions are not modeled. Your personal tax situation can materially change the comparison.

Important disclaimer: The outputs of this calculator are estimates for educational and planning purposes only. They are not financial, tax, or investment advice, and they are not a substitute for reviewing your actual loan documents or speaking with a qualified professional. Always confirm recast policies and payoff figures directly with your lender or servicer before making large payments.