Pro rata is Latin for "a proportion of." In terms of salary it refers to the the amount a part-time worker would make if they worked full-time. This is often calculated for part-time positions with "benefits" such as pensions, holidays, maternity pay and parental leave, but you only receive these benefits in proportion to how much you work compared to a 40-hour work week.
Gather your data. You will need the number of hours worked for a full-time employee (A); the number of hours you work per week (B); and the number of weeks you will be working (C). As an example, let's define these variables as A = 40, B = 28, C = 52. A full-time annual salary is $100,000.
Divide the full-time annual salary by 52. The equation is $100,000 / 52 = $1,923.
Divide Step 2 ($1,923) by the full-time number of hours (40). The equation is $1,923 / 40 = $48. This is your hourly rate.
Multiply Step 3 ($48), the hourly rate, by the number of hours you work each week (28). The equation is $48 * 28 = $1,344. This is your weekly salary.
Multiply your weekly salary ($1,344) by 52. The equation is $1,344 * 52 = $69,888. This is your annual salary.
Divide your annual salary ($70,000) by 12 for your pro rata salary. The equation is $70,000.00 / 12 = $5,833.33.