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26 half-payments = 13 full ones. The calendar pays your 13th.
A $320,000 loan at 6.5% costs $408,142 in interest on a monthly schedule — and about $94,000 less on biweekly, paying off in 24 years instead of 30. The trick is pure calendar: 26 half-payments add up to 13 full payments a year, and that 13th goes straight to principal.
Homeowners use it to find the free money: 26 half-payments is a 13th payment a year — $94k saved and 6 years gone on $320k at 6.5%.
On $320,000 at 6.5% for 30 years: about $94,000 in interest and roughly 6 years of payments — the loan dies in ~24 years. The savings scale with rate: at 4% the same loan saves ~$48,000 and ~4 years; at 7.5% it approaches $130,000. Two mechanisms stack: the 13th payment (the big one — principal reduction) and slightly faster amortization from more frequent payments (minor). The catch nobody mentions: results depend on the servicer APPLYING each half-payment when received — some batch both halves and apply monthly, which keeps only the 13th-payment effect and quietly kills the rest.
Mathematically identical when done right — adding 1/12 of your payment to each monthly check delivers the same 13th payment. The real differences are behavioral and contractual: biweekly matches paycheck timing for many workers (painless budgeting), while manual extra payments require discipline but stay flexible in lean months. Warning on third-party plans: some servicers and outside companies charge $200–$400 setup plus per-draft fees for biweekly programs that do exactly what you can do free by paying extra principal. If the servicer offers true biweekly (applied on receipt) with no fee, take it; otherwise DIY the 1/12 method and mark the extra as "principal only" so it doesn't get credited as a future payment.
The comparison is your mortgage rate versus after-tax investment returns — prepaying a 6.5% mortgage is a guaranteed, risk-free 6.5% return, which historically beats bonds and matches conservative stock expectations with zero volatility. At a 3% pandemic-era rate, investing wins on expectation; at 7%+, prepaying is genuinely hard to beat risk-adjusted. The middle path most planners land on: capture the full 401(k) match first (instant 50–100% return), fund Roth/emergency reserves, THEN attack mortgage principal. Liquidity is the silent cost — money in home equity needs a refinance to come back, so never prepay into an empty emergency fund.