Before you start: check your year-to-date figures
If you’re switching part-way through the tax year, your results can only match if the year-to-date (YTD) figures are right. Most differences in a parallel run trace back to these.- Use the right period. YTD figures must be as at the last period your old system finalised, the one immediately before the period you parallel run. If the old system has already processed part of the period you’re about to run, the figures will be counted twice.
- Tie out the totals. Add up each employee’s YTD gross pay, taxable pay, tax, employee NI, employer NI, student loan and pension in Flow. The totals should equal your old system’s year-to-date payroll summary or P11 report, and the year-to-date values on the last FPS it sent to HMRC.
Run the period in both systems
1
Process the period in your old system
Run the period as you normally would: submit the FPS and pay your employees from your old system. Keep its payslips and payroll summary to compare against.
2
Open the same period in Flow
Go to Pay runs and select Start new pay run. Check the pay schedule, tax period and payment date match your old system.Running a pay run →
3
Enter exactly the same inputs
Use the same hours, overtime, bonuses, absences, starters, leavers and pay changes as in your old system. If you can, import the same payments file into both: in Flow, click Import CSV in the pay run’s top bar. Using one source means any difference comes from the calculation, not the data entry.Importing payments from CSV →
4
Compare the results
Review each employee’s pay lines in the Payments step and preview their payslips. Payslips viewed before you lock are marked Preview. Compare them against your old system using the checklist below.
5
Lock the parallel pay run if you're happy
Once everything balances, you can lock the parallel pay run in Flow, but you don’t have to. Locking doesn’t send anything to HMRC. If you do lock it, don’t select Submit FPS to HMRC for this period: your old system has already reported it.
What to compare
Check every employee thoroughly. Gross to net and year-to-date figures must balance to the penny, for each employee and in total. Don’t accept a difference because it’s small: a few pence on one payslip can point to a set-up error that grows every period.If something doesn’t match
If any figure doesn’t balance to the penny, find out why before you go live. A penny can occasionally come from rounding, but confirm that’s the cause rather than assuming it. These are the usual causes:
Fix the cause in Flow (an employee’s details, a pay element or the pay schedule) and the pay run recalculates. Then compare again.
Go live: Flow takes over
When gross to net and year-to-date figures balance to the penny for every employee, and the totals agree, you’re ready to switch.1
Refresh your year-to-date figures
Update each employee’s YTD figures in Flow to match the last period your old system submitted, so Flow starts from exactly where your old system finished. Tie out the totals again.
2
Stop using your old system
Your old system’s last FPS is for the period before go-live. From your go-live period onwards, it doesn’t submit anything to HMRC or make any payments.
3
Run and lock your first live pay run in Flow
Process your go-live period in Flow, check it as carefully as the parallel run, then lock it and select Submit FPS to HMRC. From now on, Flow sends every FPS.
Next steps
Switch to Flow Payroll
Closing down your old provider once your first pay run has gone through.
HMRC & RTI
How your FPS is sent to HMRC, and what to do if it fails.