Lump Sum Mortgage Payment Calculator
Modeling a bonus, inheritance, or tax refund toward your mortgage? See exactly how much interest and time a one-time extra payment saves — and how timing changes the result.
Your results will appear here
Fill in your loan details and lump sum amount to see how much interest and time you could save.
Your savings summary
| Metric | Without lump sum | With lump sum |
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Amortization schedule
| Period | Payment | Principal | Interest | Extra | Balance |
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Why timing changes how much a lump sum saves
Interest is charged only on the balance you still owe. Applying a lump sum early removes that money from the interest calculation for every remaining month of the loan — so the same amount applied in year 1 saves more than in year 15, simply because it has more months left to "work." Use the "Applied in month #" field above to compare timing scenarios side by side.
Emergency fund first
Most advisors recommend keeping 3-6 months of expenses in savings before applying a large lump sum to your mortgage, since mortgage prepayments are not liquid.
Check for penalties
Confirm your lender's prepayment privilege limits — exceeding them can trigger a penalty that offsets some of your interest savings.
Consider recasting
Some US lenders offer "recasting" after a lump sum: your payment is recalculated lower on the same term, rather than shortening the term — ask if this option is available.