FinanceCalcExtra Payment & Bi-weekly Payoff Calculator
What paying a little extra actually saves — in interest and in years — with the baseline shown alongside so the difference is honest. Works on an existing loan, not just a new one, and handles both an extra monthly amount and a bi-weekly schedule.
Pay as scheduled
With extra
The accelerated schedule
How this is calculated
Each month is one step. Interest is charged on the balance you are actually carrying, the scheduled payment plus anything extra comes off it, and whatever remains becomes next month's balance:
principal = scheduled payment + extra − interest
balance = balance − principal
The scheduled payment itself comes from the standard amortizing-loan identity, with the rate per month r and the number of payments n:
And when you tell the tool you are already some months into the loan, the balance you start from is computed in closed form rather than by re-running the schedule:
- Closed form, not a loop — the balance after
kpayments has an exact formula, so nothing accumulates rounding error and the schedule ends at exactly zero rather than a few dollars short. - The last period clears exactly — the final payment is whatever is left, which is why it is usually smaller than the others.
- Extra payments hit principal only — the interest for the month is already fixed by the balance you were carrying, so everything above the scheduled payment reduces the balance one-for-one.
- Bi-weekly is modelled as its monthly equivalent — 26 half-payments is 13 full payments a year, so the tool adds one twelfth of a scheduled payment to each month. A real bi-weekly calendar moves those payments a few days earlier, which is worth a few dollars more, not a few years.
- Total paid is principal plus interest, so it always equals the starting balance plus the interest column.
Interest is carried at full precision and rounded only for display. A servicer that rounds to the cent every month, and charges interest to the day of payoff, will land a few dollars away from these figures — a difference in rounding and settlement date, not in the maths.
What paying extra actually does
Paying extra on a loan does not reduce the payment you owe next month. It reduces the balance the interest is charged on, and the saving shows up as a shorter term and a smaller total cost. Those are the only two places it can show up, and the calculator reports both — against what would have happened if you changed nothing.
Why an early extra payment beats a late one
Interest each month is the outstanding balance multiplied by the monthly rate. That balance is at its largest on day one and smallest at the end, so a dollar of extra principal applied in year one kills more future interest than the same dollar applied in year twenty. The effect is not subtle.
Worked example, $300,000 at 7% over 30 years. A single $5,000 lump sum in the first year saves about $33,250 in interest. The same $5,000 in year 21 saves about $4,930. In year 26 it saves about $2,030. Identical money, identical loan — only the date changes, and the date changes almost everything.
That is also why a small recurring amount punches above its weight. On the same loan, an extra $200 a month from the start saves roughly $116,600 and retires the loan about 7 years early. Starting the same $200 only in year 21 saves roughly $9,200. The first months of a mortgage are almost all interest — on this loan the very first payment is $1,995.91, of which $1,750 is interest and just $245.91 is principal — so that is exactly when extra principal is scarcest and most valuable.
Why bi-weekly works at all — and why it is not magic
There are 52 weeks in a year, so a bi-weekly schedule is 26 payments, each half the size of a monthly payment. Twenty-six halves make thirteen whole payments. You are therefore paying one extra full payment a year, and that is the entire mechanism — 13 payments instead of 12, paid in smaller instalments so it hardly registers.
On $300,000 at 7% over 30 years, that single extra payment a year is worth about $102,000 in interest and finishes the loan roughly 6 years earlier. It is a genuine result and no lender is doing you a favour: you are simply paying more, sooner. Which also means the "bi-weekly" label is not the point — paying a twelfth of your monthly payment extra each month does the same job, and the calculator shows both so you can see the arithmetic for yourself.
Extra money goes to principal, so the effect compounds
When you pay $1,995.91 on schedule, most of it is interest and only $245.91 touches the balance. An extra $200 skips the interest line entirely — that month's interest was already determined by the balance you started with — and comes straight off the principal.
The compounding then runs in your favour. A smaller balance means less interest next month, which means a bigger share of next month's scheduled payment goes to principal, which means a smaller balance the month after. Underpaying compounds against you the same way: this is the mirror image of the maths behind the compound interest calculator, running in reverse. It is also why paying extra on a high-rate balance is worth so much more than the same money on a low-rate one.
A lump sum early beats the same lump sum late
If you come into money, the arithmetic rewards applying it sooner rather than later — and it rewards applying it to the highest rate you carry first. A $5,000 lump sum paid against a 7% mortgage in the first year is worth about $33,250 in avoided interest. Left in a savings account at 4% for those years it would earn far less. Timing is the whole variable; the amount is fixed.
When not to do this
When the loan rate is lower than what the money would safely earn. Paying down a loan is equivalent to earning the loan's rate, tax-free and risk-free. If the loan is at 3% and a federally insured account pays 4% after tax, the arithmetic favours the account, not the loan. Run the numbers both ways before assuming the loan wins.
When it would leave you without an emergency fund. Money sent to a lender is very hard to get back. Being $2,000 ahead on the mortgage but forced to carry a balance on a 24% credit card because the car broke is a worse position than the one you started in. Fund the emergency reserve first, then accelerate. If you are already carrying card debt, the credit card payoff calculator will show you what that balance is really costing each month — it is usually the first thing to attack.
When there is a cheaper-to-avoid cost. Check for prepayment penalties or conditions on your loan, and check that your servicer actually applies extra money to principal rather than to next month's instalment. Neither is common, but both are worth one phone call before committing to a standing instruction.
This page is arithmetic, not advice. It computes what the schedule does; what you should do with your money depends on your whole position, not one loan.
Frequently asked questions
Does paying extra every month really shorten the loan?
Yes, by arithmetic rather than by favour. Every extra dollar reduces the balance immediately, and interest next month is charged on the smaller balance. On a $300,000 loan at 7% over 30 years, an extra $200 a month takes the loan from 360 payments to about 275 and cuts total interest from roughly $418,500 to roughly $301,900. The saving is interest you are never charged, not interest later refunded.
Is bi-weekly worth it, or is it just a marketing gimmick?
It is real but plain. There are 26 bi-weekly periods in a year, so 26 half-payments equal 13 full payments — exactly one extra full payment a year. Nothing else is happening. On $300,000 at 7% over 30 years the bi-weekly schedule alone finishes the loan about 6 years earlier and saves roughly $102,000 in interest. That is the same arithmetic as paying one twelfth of your payment extra each month.
Do extra payments go to principal or toward next month's payment?
On a standard amortizing loan, an extra payment goes entirely to principal — none of it is eaten by interest, because the interest for that month has already been charged on the old balance. That is why a $1,000 lump sum removes $1,000 of balance rather than $1,000 worth of payments. Some servicers apply extra money to next month's instalment unless you tell them not to, so check that it is landing on principal on your statement.
Is one big lump sum better than a smaller extra amount each month?
Not inherently — timing is what matters. A $5,000 lump sum paid in the first year of a $300,000 loan at 7% saves about $33,250 in interest. The same $5,000 paid in year 21 saves about $4,930, and in year 26 about $2,030. The earlier the money lands on the balance, the fewer months of interest are charged on it. Whether you can assemble the lump sum at all matters more than the choice itself.
Should I pay down my loan or invest the money instead?
This is a comparison, not a rule. Paying down a loan earns a guaranteed, tax-free return equal to the loan's interest rate. Putting the same money into a savings account earns its after-tax yield. When the loan rate is higher than the safe after-tax yield available to you, the arithmetic favours the loan. When it is lower — a 3% mortgage against a 4% savings account, for example — the arithmetic favours keeping the money. This page shows the loan side of that comparison; it is not a recommendation.
Why is my servicer's payoff figure slightly different from this one?
Two normal reasons. Servicers round the payment to the cent each month, while this calculator carries full precision and only rounds for display, so the two drift a few dollars apart over decades. And many servicers charge interest to the day of payoff rather than for a whole month, so the final figure depends on the exact date. Treat this as the arithmetic and the servicer's statement as the settlement number.