Extra Mortgage Principal Payment Calculator

Directing extra money toward your mortgage principal each month is one of the most straightforward ways to reduce total borrowing costs and reach financial freedom earlier. Every dollar paid above the scheduled minimum reduces the principal balance immediately, which in turn shrinks the interest charge for every remaining month of the loan. This compounding effect means that even a relatively small extra payment creates outsized long-term savings — the earlier in the loan term you begin, the greater the impact. Homeowners who refinance into a lower interest rate but keep their monthly payment unchanged are effectively making extra principal payments with each installment. Others deliberately add a round number each month — $100, $200, $500 — to systematically accelerate payoff toward a target date like retirement or college tuition start. Before committing, check your loan documents for prepayment penalty clauses, which are uncommon in modern mortgages but do exist in some older products. This calculator simulates both scenarios month by month so you can see the precise interest saved and new payoff timeline.

The standard monthly payment is derived using the amortisation formula M = P × [r(1+r)^n] ÷ [(1+r)^n − 1], where P is the remaining balance, r is the monthly interest rate (annual rate ÷ 12), and n is the remaining months. The total interest under the standard scenario is then (M × n) − P. The accelerated scenario runs a month-by-month loop in which each month the interest on the current balance is computed, the combined payment of M plus the extra amount is applied, and the balance is reduced by the principal portion. The loop ends when the balance reaches zero. The difference in total interest paid between the two scenarios is the amount saved, and the difference in months is the time saved.