Mortgage recast calculator.

How the numbers work

A recast re-amortizes the reduced balance (current balance minus your lump sum) at the same interest rate over the same remaining term, so the required monthly payment drops while the payoff date stays put. The new payment uses the standard fixed-rate formula P = B × r / (1 − (1 + r)⁻ⁿ), where r is the monthly rate and n the remaining months. The comparison column shows the alternative: apply the same lump sum but keep paying your current payment, which shortens the loan and usually saves more interest. Interest saved is measured against the schedule with no lump sum.

A recast is not automatic. Your servicer must agree to it, and eligibility, minimum lump sums, fees and loan types covered vary by servicer and investor, so confirm the terms in writing. For context on the federal rules covering mortgage payments and disclosures, see CFPB Regulation Z (12 CFR Part 1026); the amortization math here is standard and no rates or fees are assumed beyond what you enter.

This tool is a planning estimate, not financial advice. It assumes a fixed-rate loan with monthly payments and ignores taxes, insurance, escrow and mortgage insurance changes.

Worked example: a $50,000 lump sum on a $350,000 balance

A borrower owes $350,000 at 6.5% with 300 months left and pays a $50,000 lump sum against principal. The current payment is $2,363. Under a recast, the lender re-amortizes the $300,000 that is left over the same 300 months, so the payment falls to $2,026 a month, $338 lower. Under the alternative, the borrower keeps paying $2,363, and the loan is repaid in 18 yr 0 mo, 84 months sooner.

• Keeping the old payment saves more interest. The balance is cleared sooner, so far less interest accrues: $149,957 against $51,281.
• A recast buys cash flow, not savings. The payment drops by $338 a month and the payoff date does not move. A bigger lump sum scales it: with $100,000 the payment drop is $675 a month.

Next steps: compare an interest-rate change with the refinance break-even calculator, test a rate trade-off with the mortgage points calculator, or compare terms in the 15-year vs 30-year calculator.

How this is calculated

• New balance = current balance − lump sum.
• Recast payment = new balance × r ÷ (1 − (1 + r)⁻ⁿ), with r = rate ÷ 12 and n = remaining months. The rate and remaining term do not change.
• Keep-paying alternative: the current payment is applied to the reduced balance month by month until it is cleared.
• Interest saved = total interest on the original schedule − total interest under each option; the recast fee is then subtracted.

This is standard amortization arithmetic and no rates or fees are assumed beyond what you enter (formula and examples checked 1 October 2026). Whether your servicer offers a recast, the minimum lump sum, the fee and which loans qualify vary by servicer and investor, so confirm the terms in writing. The calculation covers principal and interest only and ignores taxes, insurance, escrow and mortgage insurance.

General information, not financial advice. Confirm every term with your servicer.