The above calculators are based on 2080 labor hours per years. This is derived from 40 hours per week and 52 weeks per year. Weekly time is 40 hours. Bi-weekly time is for two weeks (80 hours). Monthly is a year (2080 hours) divided by 12. Semi-monthly is a year (2080 hours) divided by 26 to compute pay twice per month (e.g., 1st and 15th).