How biweekly payments shorten a mortgage
A biweekly plan splits your monthly payment in half and pays that amount every two weeks. Because a year holds 26 two-week periods, you make 26 half payments — the equivalent of 13 monthly payments instead of 12.
That thirteenth payment goes entirely to principal, which is where the savings come from. On a 30-year mortgage it typically removes four to six years and a large share of the lifetime interest.
There is a second, smaller benefit: paying every two weeks reduces the average balance slightly during each month, so a little less interest accrues even before the extra payment counts.
Biweekly versus just paying extra
Mathematically, a biweekly schedule is nearly identical to adding one-twelfth of your payment to each monthly payment. If your servicer does not support true biweekly posting, the simpler route is to keep paying monthly and add that amount yourself — same result, no fees, and you can stop any month you need to.
Avoid third-party biweekly conversion services that charge setup and per-transaction fees. They provide no benefit you cannot get for free.
Check how your lender posts the payments
Some servicers hold each half payment in suspense and only apply the full amount at the monthly due date, which preserves the thirteenth-payment benefit but not the interest saved from paying earlier. Others do not accept partial payments at all.
Ask specifically whether biweekly payments are applied as received and whether the extra annual payment goes to principal.
Common mistakes
Confusing biweekly (26 payments) with semi-monthly (24 payments) — semi-monthly is just your normal 12 monthly payments split in two and saves almost nothing. Also, paying biweekly on a loan with a prepayment penalty can trigger fees, though this is rare on modern mortgages.