Twenty-six half-payments, thirteen months
A biweekly mortgage plan collects half of the scheduled monthly principal and interest every two weeks. Twenty-six half-payments total thirteen monthly payments in a 52-week year. This calculator estimates the effect by adding 1/12 of the scheduled payment to principal each month (13/12 × the scheduled payment). It does not claim to reproduce a servicer’s day-by-day posting schedule.
Displayed half-payment = monthly payment ÷ 2
Modeled monthly extra principal = monthly payment ÷ 12
Estimated interest saved = original interest − interest on the accelerated schedule
Example
A $300,000 loan at 6.5% for 30 years has a monthly principal-and-interest payment of about $1,896.20, so half is about $948.10. Spreading one extra monthly payment across the year pays the loan off in about 24.2 years in this estimate. A 0% rate still finishes earlier, but interest saved is $0.
Posting and fee limits
Taxes, insurance, and HOA stay outside this principal-and-interest comparison. A servicer may hold partial payments until enough accumulates for a full payment, and a formal biweekly plan may charge fees. Check how and when extra principal is credited. You may be able to pursue the same one-extra-payment goal by adding 1/12 to each monthly payment. Recurring extra amounts after a chosen month belong on the extra-payment calculator.