A biweekly plan takes half of the monthly principal-and-interest payment and collects it 26 times a year. That is one extra monthly payment each year, because 26 half-payments equal 13 full payments. Interest in this tool accrues once per biweekly period, using the annual rate divided by 26. The page compares that path with paying the ordinary monthly amount.
Half of a $2,204.89 monthly payment is about $1,102.45, paid 26 times. The tool then walks the balance forward until it reaches zero or the horizon ends, and it reports the interest difference against the monthly schedule.
Taxes, insurance, and a lender’s biweekly enrollment fee are not included. Some lenders simply draft half the payment every two weeks and still compound monthly. This page does not know which draft method a lender uses.