How to use this calculator
Extra principal shortens a loan because interest is charged on a smaller balance each month. This tool runs the month-by-month schedule twice — with and without your extras — caps the final payment so the balance ends at exactly $0, and reports months saved and interest saved, not just a smaller payment.
Enter your own figures in the form above. A blank field is treated as missing — not as zero — so the tool will ask you to complete it. A genuine zero is accepted where it makes sense (for example, no HOA dues). Press Calculate for a breakdown, Load worked example to fill in the example below, or Reset to return to the defaults.
Formula and assumptions
Each month: interest = balance × annual rate ÷ 1200 (rounded to cents in the schedule); scheduled principal = scheduled payment − interest; then extra principal is added; the final payment is capped at balance + interest. Baseline and with-extras schedules use identical timing so the comparison is like-for-like.
Worked example
Worked example: $120,000 balance, 0% rate, 120 months, $200 extra per month from the first payment. Scheduled payment $1,000; with extras $1,200; payoff in 100 months — 20 months sooner; interest saved $0 (there was no interest to save at 0%, which is exactly why the example isolates the timing effect).
Use the Load worked example button above to reproduce this example in the form, then change one input at a time to see what drives the result.
Limitations and common mistakes
Extra payments are assumed to be applied to principal in the month paid. Some servicers handle partial or extra payments differently (holding them, applying to interest first, or requiring instructions) — confirm your servicer’s policy. Taxes, insurance and escrow are not in this schedule; only principal and interest are compared.
Frequently asked questions
Does an extra payment reduce next month’s required payment?
Normally no — it reduces the balance and future interest, and shortens the term. The scheduled payment usually stays the same until payoff.
One-time or monthly — which saves more?
It depends on timing: earlier extras save more interest because they shrink the balance for more months. Try both in this tool.
Why is interest saved $0 in the example?
Because the example rate is 0%. With a positive rate, the same extras would show both months saved and interest saved.
Sources
Estimates only. Results from this calculator are not a loan offer, appraisal, engineering design, equipment selection or contractor quote.