Mortgage Recast Calculator
New monthly payment after lump sum paid to principal.
Calculate new monthly payment after a mortgage recast with lump sum principal reduction. Enter balance and rate for an instant result.
What this tool does
Enter current balance, rate, remaining term, and lump sum. The tool shows new monthly payment and lifetime interest saved.
Enter Values
Formula Used
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Disclaimer
Results are estimates for educational purposes only. They do not constitute financial advice. Consult a qualified professional before making financial decisions.
250,000 balance at 5% over 20 remaining years, 30,000 lump sum: new balance 220,000, monthly payment drops from 1,650 to 1,452. Recasts reduce payment without refinancing — typically 250-500 fee. Term stays the same.
Run it with sensible defaults
Using current balance of 250,000, annual rate of 5%, years remaining of 20, lump sum of 30,000, the calculation works out to 1,451.90. Nudge the inputs toward your own situation and the output recalculates instantly. The defaults are meant as a starting point, not a recommendation.
The levers in this calculation
The inputs — Current Balance, Annual Rate, Years Remaining, and Lump Sum — do not pull with equal force. Not every input has equal weight. Flip one at a time toward extreme values to feel which ones move the needle most for your situation.
How the math works
Amortisation on reduced principal. The working is transparent — you can verify every step yourself in the formula section below. No black box, no opaque "proprietary model".
What the headline rate hides
Lenders quote a rate; what you pay is a blend of that rate, fees, insurance, and any early-repayment penalty built into the product. The figure here isolates the core interest cost so you can compare like-for-like across deals — then add the other costs separately before signing anything.
What this doesn't capture
The figure excludes arrangement fees, valuation costs, legal fees, insurance, and any early-repayment charges — those can add several thousand to the headline cost. Rate changes at renewal for fixed-term deals will shift the picture further. Use this for the core interest/principal math and add the other costs on top.
Related calculations worth running
Plans get firmer when you triangulate. Alongside this one, the mortgage calculator, the mortgage overpayment savings calculator, and the mortgage payment calculator tend to come up in the same conversations. Running two or three together exposes inconsistencies in any single assumption — which is usually where the useful insight lives.
Mortgage recast produces a new payment based on the inputs provided.
Inputs
This example uses typical values for illustration. Adjust the inputs above to match a specific situation and see how the result changes.
Sources & Methodology
Methodology
Amortisation on reduced principal.
References
Frequently Asked Questions
Recast vs refinance?
Lender must allow?
Why not just overpay?
Tax impact?
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