# How to run a parallel payroll before you switch systems

Published 3 October 2026 by The PayLoom team. https://payloom.in/blog/parallel-payroll-run-before-switching

> A parallel payroll run calculates the same period in both your current and your new system, compares them line by line, and only lets you go live once every difference is explained.

Switching payroll systems is one of the few software changes where a mistake reaches every employee's bank account. A parallel run is how you find those mistakes before they cost anyone money. You keep paying people from your existing system, and for at least one period you also calculate the same payroll in the new one. Then you compare the two.

## What a parallel run is, and what it is not

A parallel run is a full calculation of a real pay period, using real inputs, in the system you are moving to. It is not a demo on sample data, and it is not a spot check of a few payslips. Nothing is paid from the new system during the run. Its only output is a comparison.

The point is to prove that the new system, configured with your pay elements, policies and employee data, produces the same net pay as the old one. Where it does not, you need to know why before anyone is paid from it.

## What you need before you start

- A complete staff list with salaries, bank details, tax profiles and start dates, loaded into the new system.
- Every pay element you use set up: allowances, deductions, bonuses, recoveries and loan instalments, each with its tax treatment.
- The inputs for the period you will test: approved overtime, unpaid leave, expense reimbursements and any one-off adjustments.
- The final payroll register from your current system for the same period, at employee and line level.

Note: Pick a period that has some movement in it. A month with joiners, leavers, overtime or a bonus tests far more of your configuration than a quiet one.

## How to compare the two runs

1. Compare totals first: gross, total deductions, net and employer cost. If these agree, you are close.
2. Compare each employee's net pay. List everyone whose net differs, even by a small amount.
3. For each difference, compare line by line: which earning or deduction is different, and by how much.
4. Classify every difference: a configuration error in the new system, a data error in either system, or a known and intended change.
5. Fix the cause, recalculate, and compare again until every remaining difference is explained and signed off.

## The differences you will usually find

Most mismatches come from a short list of causes: rounding rules applied at a different step, a deduction capped or prorated differently, a pay element with the wrong tax treatment, mid-period joiners and leavers prorated on a different day count, and data that was correct in the old system but mapped to the wrong field during import.

Occasionally the new system is right and the old one was wrong. Record those cases explicitly, because they change what people are paid and someone has to approve that change.

## When to go live

Go live when the totals agree and every employee-level difference is either fixed or explained and approved. One clean period is the minimum. If the first run needed significant fixes, run a second period to confirm the fixes hold, before you stop calculating payroll in the old system.

## How PayLoom handles the parallel run

In PayLoom the parallel run happens inside the 30 day trial, on your own data. Your implementation lead loads one month of payroll and your staff list, calculates the same period, and sits on the run with your finance team to work through the line by line difference. You decide whether to switch with the comparison already done.