How the Mortgage Acceleration Calculator works
This tool measures what happens when you pay more than your required mortgage payment every month. It first finds your standard fixed payment using the same amortization formula lenders use to build your payment schedule, then re-runs that schedule month by month with your extra payment added straight to principal, tracking the balance until it reaches zero.
The formula
For a loan balance L, a monthly interest rate c (the annual rate divided by 12), and n months remaining, the standard fixed payment is:
M = L × c(1 + c)n / [(1 + c)n − 1]
That payment is split each month between interest (balance × c) and principal (the rest). The calculator then adds your extra monthly payment entirely to the principal portion, which shrinks the balance faster and lowers every future month's interest charge, since interest is always computed on whatever balance remains. If the rate is 0%, the standard payment simplifies to M = L / n and every extra dollar shortens the loan by one dollar of remaining principal.
Worked example
Take a $300,000 balance at 6.5% with 30 years (360 months) remaining. The standard payment is about $1,896.20/month, which would total roughly $382,633 in interest over the full term. Adding $200/month extra pays the loan off in about 277 months (23 years, 1 month) — nearly 7 years sooner — for total interest of about $279,185, a savings of roughly $103,449.
Why extra payments work
- Timing matters: a dollar of extra principal paid in year one avoids interest on that dollar for every remaining month of the loan. The same dollar paid in the final year only avoids a few months of interest, so accelerating early has the largest effect.
- The effect compounds: a smaller balance means less interest charged next month, which means more of next month's payment reaches principal, which shrinks the balance further — the acceleration builds on itself.
- Small amounts still add up: because mortgages run for decades, even a modest recurring extra payment removes years from the term and tens of thousands of dollars in interest, as the worked example shows.
Extra payments versus refinancing
Paying extra keeps your original loan, rate, and required payment unchanged — the extra amount is voluntary, so you can pause it in a tight month without penalty. Refinancing replaces the loan with a new rate, term, and required payment, and usually involves closing costs; it can lower your rate but removes the flexibility of an optional extra payment. This calculator only models the extra-payment strategy on your existing loan terms, not a refinance.