Extra Mortgage Payment Calculator
What paying extra is actually worth — the interest it removes, the years it saves, and how a monthly amount compares to a yearly lump or biweekly payments.
The address carries whatever you changed, so anyone opening it sees these numbers. Nothing is saved on our side.
Interest saved
$150,122
Paying $300 extra a month puts in $77,100 and removes $150,122 of interest — clearing the loan 8 years 8 months sooner.
- $2,205
- Principal and interest
- 21 years 5 months
- March 2048
- 30 years 1 month
- $150,122
- $77,100
- $303,765
What each extra amount is worth
Interest avoided, and how much sooner the loan ends. Every extra dollar goes straight against the principal, so it cancels all the future interest that dollar would have accrued.
| Extra each month | Interest saved | vs yours | |
|---|---|---|---|
| Nothing extraThe scheduled term | $0 | −$0 more in total | |
| $100/moPaid off 3 years 9 months sooner | $66,545 | −$0 more in total | |
| $200/moPaid off 6 years 6 months sooner | $114,053 | −$0 more in total | |
| $300/moyoursPaid off 8 years 8 months sooner | $150,122 | — | |
| $500/moPaid off 11 years 10 months sooner | $201,905 | −$0 more in total | |
| $1,000/moPaid off 16 years 7 months sooner | $275,021 | −$0 more in total |
Is biweekly actually a trick?
It is one extra payment a year, described more excitingly.
Paying half the mortgage every two weeks means 26 half-payments a year, which is 13 monthly payments rather than 12. That is the entire mechanism — there is no compounding advantage and no lender magic. This calculator models it as exactly that, so switching between “biweekly” and adding a thirteenth payment spread monthly gives the same answer.
It is a genuinely good idea, and worth knowing you can get the same result by paying $184 extra a month without changing anything with your servicer — and without the fee some of them charge to set it up.
How this calculator works
The same month-by-month simulation as the payoff calculator, with the saving as the headline instead of the date.
Two schedules are run and compared: the loan as it stands, and the same loan with your extra payments applied. The difference in total interest is the saving, and the difference in months is the time removed.
Because both come from the same simulation, the comparison is exact rather than an approximation — and the biweekly option resolves to precisely one extra payment a year, which is all it has ever been.
Before you start
- Tell your servicer the money is a principal-only payment. Unlabelled extra can be applied to next month instead, which achieves nothing.
- Check your note for a prepayment penalty. Rare, not extinct.
- Keep an emergency fund first — a prepayment cannot be withdrawn again.
A worked example
$340,000 at 6.75% with thirty years left, paying $300 extra every month.
| Scheduled payment | Principal and interest |
|---|---|
| Interest if nothing changes | Over 361 months |
| Interest paying $300 extra | Over 257 months |
| Extra paid in | $300 × 257 months |
| Loan ends sooner by | 104 months |
| Interest never paid | The whole point |
Just under two dollars of interest removed for every dollar put in. Compare that to biweekly payments on the same loan, which add $183.77 a month in effect and remove $107,258 — most of the benefit, from a change that costs nothing to arrange beyond telling your servicer.
Questions people ask
Because interest is charged on the balance. A dollar taken off the principal today removes every future interest charge that dollar would have accrued for the rest of the loan — nearly thirty years of it, on an early payment. On a $340,000 mortgage at 6.75%, $77,100 of extra payments removes $150,122 of interest. The money is not earning a return; it is cancelling a cost.
Monthly, slightly, because each payment starts working sooner — but the gap is smaller than people expect. On the example here, $300 a month beats $3,600 once a year by a meaningful but not decisive margin. If a bonus is how you actually have the money, use the bonus. The difference between the two is far smaller than the difference between doing either and doing nothing.
Keep an emergency fund first. A mortgage prepayment is irreversible — you cannot get it back out without refinancing or selling — so money you might need should not go there. Once you have a cushion, compare your mortgage rate to what you would earn after tax. At 6.75% the mortgage is a high, certain, tax-free hurdle to beat.
No, and this surprises people. It shortens the loan rather than reducing the payment: you pay the same amount for fewer months. Some lenders offer recasting, which recalculates the payment on the lower balance for a fee — worth asking about if lowering the monthly figure is the actual goal rather than clearing the debt.
The later in the loan you make it, the less it saves, because there is less remaining interest for it to cancel. The same $10,000 removes far more interest in year three than in year twenty-five. If you are within a few years of the end, the arithmetic argues weakly and the psychology may argue strongly — both are legitimate.
Related calculators
An estimate on principal and interest only. Confirm with your servicer that extra payments are applied to principal, and check your note for a prepayment penalty.