How this calculator works
The calculator runs your loan twice and compares the two outcomes. The first run is your mortgage exactly as it stands: the scheduled payment, made every month, until the balance reaches zero. The second run adds whatever extra you entered, and applies any one-time payment in the month you chose.
Both runs use the standard amortization formula that lenders use. Your monthly rate is the annual rate divided by twelve, and the scheduled payment is the fixed amount that retires the balance in exactly the remaining term. Each month, interest is charged on the outstanding balance first; whatever is left of the payment reduces the principal. Extra payments go entirely to principal, which is what makes them so effective.
Two details worth knowing. The final payment is trimmed so you never overpay — if $412 is left, you pay $412 plus that month's interest, not another full installment. And every figure is rounded to the cent each month, the way a real servicer rounds it, so the totals here should line up with your statement rather than drifting a few dollars apart over 30 years.
How paying extra saves interest
Interest each month is charged on what you still owe. Nothing else. That single fact is why extra payments work so well, and why they work best early.
Take a $300,000 mortgage at 6% over 30 years. The scheduled payment is $1,798.65. In the first month, $1,500 of that goes to interest and only $298.65 touches the principal — you are paying the bank for the privilege of owing them $300,000, and barely denting the debt. Send an extra $200 that month, and the entire $200 comes off the balance. Every month afterward, interest is charged on a balance $200 smaller. That saving compounds, quietly, for the remaining 359 months.
The result is out of all proportion to the effort. On that loan, $200 a month — about $6.50 a day — retires the mortgage in 23 years and 3 months instead of 30, and saves roughly $91,000 in interest. You put in around $55,800 of extra payments and avoid $91,000 of interest. The gap is the interest that never gets a chance to accrue.
Timing matters more than most people expect. The same $200 a month, started in year fifteen rather than year one, saves a fraction as much, because by then most of each payment is already going to principal and there are far fewer months left for the saving to compound. If you are going to do this, the best month to start is this one.
Biweekly versus monthly extra payments
A biweekly plan means paying half your mortgage payment every two weeks instead of the full amount once a month. It sounds like a scheduling change. It is really a stealth extra payment: there are 26 fortnights in a year but only 12 months, so 26 half-payments equal 13 full payments — one more than you would otherwise make.
That is the entire trick, and it is a good one. On the $300,000 loan above, biweekly payments cut around five years off the term. But nothing magic is happening, and it is worth being clear-eyed about that, because servicers and third-party companies sometimes charge setup and per-transaction fees to "enroll" you in biweekly payments. You can produce the identical result for free by dividing your monthly payment by twelve and adding that amount to each monthly payment as principal — which is exactly what the biweekly toggle in this calculator models.
Biweekly is genuinely useful if you are paid fortnightly and the rhythm helps you stick with it. If you are paid monthly, a recurring extra principal payment is simpler, free, and entirely under your control — you can skip it in a bad month without calling anybody.
Should you pay off your mortgage early or invest instead?
Paying down a mortgage is a guaranteed, risk-free, tax-free return equal to your interest rate. Nothing else in personal finance is quite so certain. Investing usually offers a higher expected return, but "expected" is doing a lot of work in that sentence.
The arithmetic case is straightforward: if your mortgage is at 3% and a diversified portfolio returns 7% over decades, investing wins by a wide margin and it is not close. At 7%, the comparison narrows to roughly a coin flip against an uncertain market. Above that, paying down the loan starts to look like the better risk-adjusted deal. The rate on your loan is the number that decides this, not the size of the balance.
A few things should generally come first, whatever your rate:
- Any employer 401(k) match. That is an instant guaranteed return no mortgage rate can compete with.
- High-interest debt. A 22% credit card is costing you far more than a 6% mortgage is.
- An emergency fund. Money paid into a mortgage is very hard to get back out. You cannot eat home equity during a layoff.
The honest answer is that this is not purely a math question. A paid-off house is a real reduction in fixed costs and a real reduction in anxiety, and plenty of people who understand the spreadsheet perfectly well choose it anyway. That is a legitimate preference, not a mistake — just make it deliberately, with the numbers above in front of you, rather than by default.
Getting the most out of extra payments
- Tell your servicer the money is for principal. Otherwise it may be held and applied to next month's bill, which saves you almost nothing.
- Start early. An extra payment in year two is worth several times the same payment in year twenty.
- Round up rather than agonise. Rounding $1,798 up to $1,900 is easy to sustain and does most of the work.
- Check whether hitting 20% equity cancels your PMI. That is an extra saving this calculator does not include.