If you are paid bi-weekly, multiply this bi-weekly amount by 26 to find your annual salary. In the above example, $549.60 times 52 gives you $28,579.20 per year. Divide your annual salary by 12 to calculate your salary per month.For example, divide $28,579.20 by 12 to calculate a salary of $2,381.60 per month.ivide your annual salary by 12 to calculate your salary per month. For example, divide $28,579.20 by 12 to calculate a salary of $2,381.60 per month.