Mortgage Recast Calculator
A recast puts a lump sum against the principal and re-amortises the remaining balance over the original term, lowering the payment without changing the rate or the end date.
| Item | Value |
|---|---|
| Balance before | $400,000 |
| Lump sum | $50,000 |
| Balance after | $350,000 |
| Payment before | $2,701 |
| Payment after | $2,363 |
| Months remaining | 300 |
M = (P − L)·i/(1 − (1+i)^−n) · Same rate and term, lower principal.
(M_before − M_after)·n · Total interest avoided.
A clear path from inputs to results.
A recast puts a lump sum against the principal and re-amortises the remaining balance over the original term, lowering the payment without changing the rate or the end date.
How to use
- Set current balance ($).
- Set interest rate (%).
- Set months remaining.
- Set lump sum ($).
- Read the headline metrics, then check the table for the detail.
The formula
Reference: Re-amortising the remaining balance over the original term (Broverman, Mathematics of Investment and Credit).
Worked example
At the default inputs
- Current balance ($): 400000; Interest rate (%): 6.5; Months remaining: 300; Lump sum ($): 50000
- 1
New payment: $2,363 · Old payment: $2,701 · Monthly saving: $337.60 · Interest saved: $101,281
New payment: $2,363 · Old payment: $2,701 · Monthly saving: $337.60
Interpreting the result
Paying $50,000 against $400,000 and re-amortising over 300 months cuts the payment by $337.60.
Assumptions & limitations
- The rate and the maturity date do not change.
- No recast fee, which some lenders charge.
Frequently asked questions
How is this different from refinancing?
A recast keeps the existing rate and term; a refinance replaces the loan and can change both.
Does a recast reduce total interest?
Yes, by the interest on the lump sum over the remaining term.
Methodology last reviewed 2026-10-22. Results are informational and not financial or engineering advice — see our disclaimer.