An extra mortgage payment does not usually reduce next month’s required payment. It reduces the balance, which reduces the interest charged in every following month, which shortens the term. Understanding that chain — balance, then interest, then term — is the whole subject of extra-payment planning.
Timing and allocation
Each month, interest is balance × monthly rate; the scheduled payment covers interest first and the rest reduces principal; extra principal then reduces the balance further. Earlier extras save more interest because they shrink the balance for more months. A one-time extra in month 1 therefore beats the same amount in month 60, and a recurring monthly extra beats a single extra of the same total made late. The Extra Mortgage Payment Calculator runs the schedule month by month so both effects are visible, with the final payment capped so the balance ends at exactly $0.
The servicer question
Confirm how your servicer applies extra payments: some require instructions to apply extras to principal, some hold partial extras, and some apply them differently. The calculator assumes extras are applied to principal in the month paid — if your servicer does otherwise, the schedule will differ, and the difference is worth a phone call before you start.
A clean timing example
$120,000 at 0% over 120 months with $200 extra monthly: scheduled $1,000, with extras $1,200, payoff in 100 months — 20 months sooner, $0 interest saved (there was no interest at 0%). With a positive rate, the same extras would show both months saved and interest saved.