What to check before your first payroll run in a new system

The first payroll run in a new system is not really a payroll run. It is a test of whether the data you imported means what you thought it meant, and the only way to find out is to compare it against a period you already closed somewhere else.

Start by running the new system against last month, in parallel, without paying anyone from it. You know what the answer should be. Any line that differs is either a genuine import error or a rule you never wrote down, and both are worth finding before money moves.

Pay particular attention to the components that only appear sometimes: an allowance that applies to one grade, a deduction that stops after a certain tenure, an advance that is half repaid. Fixed salary almost always imports correctly. It is the conditional parts that get missed, because nobody remembers to test the case that did not occur last month.

Check how the system treats someone who joined or left mid-period. Pro-rata arithmetic is where systems quietly disagree, and it is the calculation your new joiners will check most carefully, because it is their first payslip.

Finally, approve the run and then look at what it stored. A payroll run that keeps a snapshot of the components as they were at approval can be explained six months later. One that recalculates from current values cannot, and you will not discover the difference until someone disputes an old payslip.