Money math
What One Extra Mortgage Payment a Year Does
One extra mortgage payment a year — the whole secret behind biweekly plans — cuts years off a loan. Here is the math, and how to do it without any fees.
Somewhere between folklore and financial advice sits the claim that paying your mortgage biweekly knocks years off the loan. It is true, and the reason is so mundane that the companies charging enrollment fees for biweekly plans would rather you not look at it directly.
The entire mechanism is one extra mortgage payment a year. Everything else is packaging.
Where the extra mortgage payment comes from
A year holds 26 two-week periods. Pay half your monthly payment every two weeks and you make 26 half-payments — thirteen full payments instead of twelve. The calendar smuggles in the thirteenth; nothing about the two-week rhythm itself does anything.
Which means the effect is reproducible without any biweekly plan at all:
- Add one-twelfth of your payment as extra principal each month, or
- Make one extra full payment a year from a tax refund or bonus.
Identical math, zero fees. The biweekly mortgage calculator shows the monthly-extra equivalent for your numbers, precisely because the free version is the better version.
What it actually saves
Take a $350,000 balance at 6.5% with 28 years remaining. The monthly payment is about $2,265, so the biweekly schedule adds roughly $189 a month of principal. The result:
| Normal schedule | With the extra payment | |
|---|---|---|
| Payoff | 28 years | about 23 years |
| Total interest | ~$411,000 | ~$323,000 |
Five years sooner, roughly $88,000 saved — from one extra payment a year. The savings scale with the rate: at 3% the same trick saves far less, at 7.5% more. And they shrink as the loan ages, which brings us to why.
Why early dollars do the heavy lifting
An amortising loan charges interest on the outstanding balance every month, so a dollar of principal removed today stops earning interest against you for every remaining month. As how mortgage payments are calculated shows, early payments are nearly all interest precisely because the balance is at its peak — which makes early extra principal the most leveraged dollar available.
The same dollar in year 25 cancels only a few years of interest. The lesson is not "it is too late" — extra principal always saves its rate — but the payoff from starting now rather than someday is larger than intuition suggests. It is the mirror image of why starting retirement savings early beats saving more later, the flat-then-bending curve from how compound interest works run in reverse.
The servicer detail that decides everything
Money sent beyond the payment can be handled two ways, and only one helps you:
- Applied to principal: the balance drops now, and every later month's interest is computed on the smaller number. This is the entire benefit.
- Held against next month: the money sits in suspense, the balance is untouched, and you have made the servicer an interest-free loan.
Most servicers do the right thing with extra sent alongside a normal payment, but some require a memo, a checkbox, or a separate transaction. Confirm once in writing, then verify on the next statement that the balance actually moved by the extra amount. Also glance at the note for a prepayment penalty — rare on recent US mortgages, but the check costs one minute.
Third-party biweekly services deserve extra suspicion: they typically hold your half-payments and forward ordinary monthly payments plus the annual extra, charging a setup fee and per-transaction fees for arithmetic you can do with a standing order.
Is the mortgage the right target at all?
Extra principal earns exactly your mortgage rate, guaranteed. Before aiming money at it, three things reliably pay more:
- An unclaimed 401(k) match — an instant 50–100% return; the match calculator prices what skipping it costs.
- High-rate card debt — a 22% card beats a 6.5% mortgage as a target three times over; avalanche vs snowball covers the order of attack.
- A thin emergency fund — principal locked in the house cannot fix a transmission; the emergency fund calculator sizes the buffer that comes first.
Past those, extra principal versus investing the difference is a genuine judgment call — guaranteed 6.5% against expected-but-volatile market returns — and reasonable people land on both sides. What the arithmetic here settles is only the mortgage half of that comparison.
Two related maneuvers
Recasting. After a large lump-sum principal payment, many lenders will re-amortise the remaining balance over the remaining term for a small fee, lowering the required monthly payment. Useful when the goal is breathing room rather than an earlier payoff — extra payments alone never reduce the required monthly amount.
Refinancing. When rates drop, refinancing attacks the same interest bill from the other side, and the two strategies stack: refinance into a lower rate, keep paying the old payment amount, and the entire difference becomes automatic extra principal. The refinance calculator prices the break-even on the closing costs.
A practical note on consistency: the monthly-extra version survives real life better than the annual-lump version. One extra mortgage payment made "when the refund comes" has a way of becoming a vacation, while $189 added to an automated payment is invisible after the second month. Set it up once, verify the first statement, and let the amortisation table do the rest for two decades.
For servicer rules on payment application and what to do when extra payments are misapplied, the CFPB is the primary US source and takes complaints that servicers answer.