Lump-sum mortgage payment calculator
A bonus, an inheritance, a home-sale overlap — one-time money raises a specific question: what does putting it on the mortgage actually buy? The calculator below models a single lump-sum principal payment on any date in your schedule and answers with the two numbers that matter: interest saved over the life of the loan, and how far the payoff date moves. The full table shows the lump landing in its month, and the chart shows the balance curve stepping down.
Timing is the underrated variable. The same dollars cancel more future interest the earlier they land — the table below quantifies it on an example $320,000 loan at a 6.5% example rate.
Preset: example $320,000 loan with a $20,000 lump sum — set your own numbers and date
Payment-by-payment schedule
| # | Date | Payment | Principal | Interest | Extra | Balance |
|---|
Click a year to expand its payments. Exports and print always include every payment.
How it works
- Open this page — the calculator is already set up for "Lump-sum mortgage payment calculator". Swap in your own amount, rate and term.
- Read off the payment, the payoff date and total interest. The full payment-by-payment table is right below.
- Add extra payments or switch to biweekly to watch the payoff date move and the interest saved appear.
- Download the schedule as Excel, CSV or PDF — generated on your device; your loan details never leave your browser.
Lump sum, recast, or offset the payment?
A plain lump-sum principal payment shortens the loan but leaves the required monthly payment unchanged — that’s what this page models. If your goal is a lower monthly payment instead, the move is a recast: after a large principal payment, the servicer re-spreads the smaller balance over the remaining term for a small fee, and the payment drops while the payoff date stays. Same money, two different outcomes — decide which problem you’re solving before the money moves. The recast vs refinance guide covers when each wins.
And the boring-but-critical mechanics: send the lump as an explicitly-marked principal-only payment, separately from your regular payment, then verify the new balance on your next statement. Large unmarked payments are exactly the ones servicers park as prepaid future payments.
The same $20,000 lump sum, three different years (computed by this site’s engine)
| Item | Detail |
|---|---|
| $20,000 in year 1 | saves $98,543 interest · payoff moves up 4 yr 10 mo |
| $20,000 in year 5 | saves $74,584 interest · payoff moves up 3 yr 10 mo |
| $20,000 in year 15 | saves $32,425 interest · payoff moves up 2 yr 1 mo |
Example loan: $320,000 at 6.5% over 30 years, lump paid in the first month of the named year. Computed by the calculator’s own engine at build time — reproduce any row by entering the same numbers above.
Good to know
- Keep your emergency fund out of the lump — principal payments are a one-way door; you can’t withdraw them later without refinancing or selling.
- If your loan is young and the lump is large, compare the recast option: the fee is typically small and you keep flexibility by lowering the required payment.
- Mortgage prepayment penalties are essentially extinct on US loans closed since 2014 (CFPB rules) — but a note check costs nothing.
Frequently asked questions
Is one big lump sum better than the same money as monthly extras?
If the money already exists, the lump wins — it all starts canceling interest today, while spreading it out delays most of it. If the money arrives over time (from income), monthly extras are simply the honest description of what you have. Model both above with the same total and compare the interest-saved lines; the gap is the cost of waiting.
Will a lump-sum payment lower my monthly mortgage payment?
Not by itself — the contractual payment stays fixed; the loan just ends sooner. To convert a big principal payment into a lower monthly payment you ask the servicer for a recast (re-amortization) afterward. The calculator shows the shortened-loan outcome; a recast would instead keep the original payoff date with a smaller payment.
Should the lump sum go to the mortgage or to investments?
The mortgage pays a guaranteed, tax-free-ish return equal to your rate; investments offer higher expected but variable returns, plus liquidity. At mid-6% rates it’s genuinely close and depends on your risk seat — our “pay extra or invest?” guide walks the comparison without cheerleading either side. What’s rarely right: leaving the decision unmade in a checking account.