How this calculator works
Every extra dollar you put toward principal stops accruing interest for the rest of the loan, which both cuts your total interest paid and shortens your payoff time. This calculator builds two full amortization schedules — a baseline at your scheduled payment, and a second one with your extra payments applied to principal each period — and compares the two:
Interest saved = baseline total interest − total interest with extra payments
Biweekly payments are modeled as their standard financial equivalent: paying half your monthly principal & interest payment every two weeks works out to 26 half-payments a year — 13 full monthly payments instead of 12 — which this calculator applies as one-twelfth of your scheduled payment added as extra principal each month, a standard, accurate way to compare biweekly against monthly without needing a full semi-monthly payment calendar.
Worked example
On a $300,000 loan at 6.5% over 30 years, the baseline schedule accrues $382,633.47 in total interest over all 360 months. Adding just $200 extra to every monthly payment cuts total interest to $279,184.67 — a saving of $103,448.80 — and pays the loan off in 277 months instead of 360, 83 months (about 6.9 years) early. Switching to biweekly payments instead, with no other extra payments, brings total interest down to $295,377.32 (a saving of $87,256.15) and shortens the term to 290 months, 70 months (about 5.8 years) early — a smaller saving than the $200/month plan since it adds less extra principal per year, but requires no separate budgeting decision once it's set up.
What affects your result
- Size of the extra payment — larger and earlier extra payments save more, since they remove principal from the balance sooner, before as much interest has accrued on it.
- Interest rate — higher-rate loans benefit more from extra payments in absolute dollar terms, since more of every scheduled payment would otherwise go to interest.
- Timing of a one-time lump sum — the same lump sum applied in month 12 saves meaningfully more interest than the same amount applied in month 200.
- Biweekly vs a flat monthly extra — both add roughly the same "extra payment per year" concept, but a flat monthly extra of the equivalent size can outperform biweekly if you can sustain the higher monthly outlay.
A note on accuracy
This calculator computes on full, unrounded figures internally and rounds only for display, so results stay accurate across long amortization schedules. It assumes extra payments are applied directly to principal (check with your servicer that your payments are coded this way, rather than as an early next-month payment) and does not model potential prepayment penalties, which are rare on modern US mortgages but do still exist on some loan types. For general guidance on prepayment, see the Consumer Financial Protection Bureau.