Assumptions this tool makes. The biweekly schedule pays half the monthly payment every two weeks and accrues interest every two weeks at one twenty-sixth of the annual rate, which is how the schedule is usually administered. The do-it-yourself comparison adds one twelfth of the payment to each monthly payment and accrues interest monthly. The loan is fixed-rate; the payment is principal and interest only. Fees are taken as paid in cash at the start and at the beginning of each year of the window. The interest figures run to payoff, so they describe a loan held to the end; the balance figure describes a loan still outstanding at the end of the window.