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Mortgage recast vs. extra principal: what does the same lump sum really do?
A recast can lower the required payment after a large principal payment. Paying the same lump sum without a recast can keep the old payment and shorten the payoff. This calculator puts both choices beside the do-nothing baseline.
Compare the four outcomes
Principal and interest only. Taxes, insurance, and escrow are not included.
| Outcome | Required payment | Payoff timing | Remaining interest | Interest through stay |
|---|---|---|---|---|
| Do nothing | … | … | … | … |
| Recast | …… | … | …… | … |
| Extra principal, no recast | …Old required payment | … | …… | … |
| Recast, then keep paying old payment | …Old payment after recast | … | …… | … |
The point of the last row: …
…
How the math works
Do nothing keeps the current balance, rate, payment, and term. Recast applies the lump sum, keeps the same rate and payoff date, and recalculates the required payment. Extra principal applies the same lump sum but keeps the old payment, which usually shortens the payoff. Row D shows recast followed by the old payment, so its net saving equals row C minus the recast fee.
Worked example
With a $300,000 balance at 3.50% and 25 years remaining, a $50,000 lump sum leaves $250,000. The original payment is about $1,502 per month. A recast lowers the required payment to about $1,252, freeing about $250 per month. A $500 fee reaches simple break-even in about 2 months.
Keeping the old payment after the lump sum pays the balance off earlier and saves at least as much interest as the recast before fees.
When each option can fit
A recast can fit when lowering the required payment is the main goal and the servicer offers the option. Extra principal can fit when you want to keep the current payment and shorten the payoff. Doing nothing preserves liquidity. Fannie Mae describes re-amortization after a substantial principal curtailment as a servicer process, so ask for written terms before sending a lump sum.
Frequently asked questions
Does a recast save more interest than extra principal?
For the same lump sum and rate, extra principal with the original payment saves at least as much interest as a recast because the higher payment reduces the balance faster.
Can every mortgage be recast?
No. Eligibility, minimum principal curtailment, fees, and servicer approval depend on the loan program and servicing agreement. Fannie Mae describes re-amortized mortgages.
Is the recast fee included?
The fee is an input. The calculator subtracts it from recast savings and estimates fee break-even from the lower required payment.
Related reading
Sources and limitations
Recast process reference: Fannie Mae’s re-amortized mortgage guide. Recast fees, minimum lump sums, eligibility, servicer approval, and payment timing vary by lender and program.
Not financial advice: This calculator is for education and planning. It excludes taxes, insurance, escrow, prepayment penalties, lender rules, payment posting dates, and servicing changes.
ProjectHomeWise Editorial Team · Last reviewed October 8, 2026