Extra principal on your mortgage — a calculator
Type your loan, rate, and term. See what a small extra principal payment does over three decades of amortization.
Nothing you type leaves the browser. No accounts, no sign-in, no saved history — a calculator that does not know who you are.
Standard payment (no extra)
With $100/mo extra principal
The extra $100 a month pays the loan off about 3 yrs 6 mo early and saves roughly $62,627 in interest over the life of the loan.
Why this is true
A mortgage payment is split into interest and principal each month. Interest is computed on the current balance, so an extra dollar of principal today lowers every future interest calculation. That is why a modest extra payment early in the loan outperforms a much larger one late.
The calculator runs a monthly amortization from the loan’s first month. The standard payment is fixed by the original term; the extra amount is added to principal on top. As soon as the balance would reach zero, the final month pays the remainder plus that month’s interest exactly.
Two honest caveats worth remembering. Money sent into a mortgage is illiquid — it comes back out only by selling or borrowing. A thin emergency fund or a card balance at 24% both have stronger claims on a spare hundred dollars than a mortgage at 6.5%. And the mechanics only work if the servicer applies the extra to principal, not to next month’s payment — usually a checkbox or a memo line, worth confirming once.
For a connected mortgage, Plumb shows the live balance and year-to-date interest, so the effect of extra payments appears in the actual numbers rather than a projection.
Plumb is financial education, not financial, investment, tax, or legal advice.