Work it in either direction: enter a lump sum to see the new repayment, or name the repayment you want and see what it takes to get there. Then check whether paying down the loan actually beats investing the same money.
A specific, practical question: how much would you need to pay off to get the monthly repayment down to a figure you're comfortable with?
Retirement usually means a drop in income, and a mortgage repayment that was comfortable on a salary can look uncomfortable on a pension. The instinct is to clear the loan entirely — but often you don't need to. Bringing the repayment down to a manageable level takes considerably less money than clearing it, and leaves the rest available.
This calculator works in both directions. Tell it the repayment you want and it gives you the lump sum. Tell it the lump sum you have and it gives you the repayment.
Sofia owes $420,000 at 6.2% with 18 years to run, and her repayment is $3,232 a month. Retiring next year, she wants that down to $2,000 — a level her pension comfortably covers.
To get there she'd need to pay off $160,076. That's a lot, but it's far less than the $420,000 needed to clear the loan, and it takes her total interest bill from $278,000 down to $172,000.
There's a second option she hadn't considered. Paying the same $160,076 but keeping the repayment at $3,232 clears the loan in 8 years 8 months instead of 18, and saves $201,000 of interest rather than $106,000. Lower repayment or shorter loan — same lump sum, and the right answer depends entirely on whether she needs the monthly cashflow.
Check with your lender before assuming a recast is available. Many require a formal request to reduce the repayment after a lump sum, some charge for it, and some simply don't offer it — in which case a lump sum shortens the loan whether you wanted that or not. It's a five-minute phone call and worth making before you commit.
Don't decide this in isolation. The comparison at the bottom of the page shows whether the loan or an investment is the better home for the money — but there's more to it. Before writing a large cheque, it's worth checking the liquidity consequences, since money in a house is very hard to get back out, and the fuller payoff-versus-invest comparison, which handles tax properly.
General information only — not financial advice. Results are estimates based entirely on the assumptions you enter, and assume a fixed interest rate, a fixed investment return with no volatility, and no tax, fees, break costs or lender charges. Real loans and real markets behave differently. Check with your lender whether they will recast your repayment after a lump sum; many require a formal request and some charge for it.