There is no specified format when it comes to payroll numbers. They usually consist of a unique series of numbers or letters, or a combination of both and can vary in length. This is often according to the number of employees that are in an organization with longer payroll numbers indicating a large number of employees.  In the UK it is generally recommended that payroll numbers be at least six digits long, to ensure they are unique for each employee. In some cases, employers will use the National Insurance number of their employees as their payroll number.