Pro rata and per annum are different. Pro rata refers to the proportional reduction in something based on the amount used, consumed or worked, e.g. the reduction in your salary when you work 20% fewer hours. Per annum, on the other hand, refers to the time frame of each year, e.g. 25 days holiday per annum refers to the number of holidays you receive per year. Pro rata and per annum are often used together, such as describing the annual change in someone’s salary when working fewer days.