Why extra payments punch above their weight
Every extra dollar goes 100% to principal, so it stops generating interest for every remaining month of the loan. On a $280,000 balance at 6.5%, an extra $200/month pays the loan off roughly 5 years early and saves about $70,000 — the earlier in the loan you start, the bigger the effect, because early payments face the most remaining months of compounding.
Extra monthly vs one-time lump sum
A lump sum today is equivalent to prepaying the loan's most expensive months. As a rule of thumb, a one-time payment saves roughly (amount × rate × years remaining) in simple terms, while a recurring extra payment compounds its advantage every month. If you get a bonus or tax refund, applying it to principal and then continuing a modest monthly extra is the strongest combination.
Should you prepay the mortgage at all?
Prepaying earns a guaranteed, tax-free return equal to your mortgage rate. If your rate is 6.5%, that's a strong risk-free return; if you locked 3% in 2021, investing the extra money will likely beat prepaying. Also confirm your servicer applies extra amounts to principal (not "next month's payment") and that there's no prepayment penalty — most US loans since 2014 have none.
How to use this calculator
Start with the current numbers from a recent mortgage statement: the outstanding balance (not the original loan amount), your interest rate, and your required principal-and-interest payment. Then add whatever extra you can commit — a recurring monthly amount, a one-time lump sum, or both. The results show your new payoff date, the interest saved, and the years shaved off versus your current schedule. Try a few scenarios: an extra $100/month, an extra $300/month, and a single $10,000 lump sum. Seeing the three side by side makes the trade-off concrete, and it usually reveals that even a modest, sustainable monthly extra beats waiting to make one big payment someday.