Your first partial payment would not go straight to principal. The only way to pay down your principal is to pay more than the amount due. Many mortgages will hang on to the partial payment until the remainder of the minimum payment is paid before processing the payment. In order to pay down principal early with the method your planning is to pay a half payment every two weeks, not twice a month. So if you pay half on 3/15, you need to do another half on 3/29, then 4/12, 4/26, etc. That way by the end of the year you’ve paid one full extra payment.