The Parallel Run: How Long You Actually Need to Run Two Systems

Finance teams want to run old and new systems in parallel until they’re confident. Implementation partners want to cut over and close the engagement. The tension between those two interests is where parallel run decisions get made — and where the consequences of getting it wrong become most visible.

The parallel run is the period after a new ERP goes live when the company continues to process transactions in both the old system and the new one simultaneously. The purpose is validation: by running the same period in both systems, you can compare outputs and verify that the new system is producing the same results. When the outputs agree, you have confidence to cut over completely.

In theory, this is sensible risk management. In practice, it’s expensive, operationally demanding, and frequently ends before the validation is as complete as the finance team wanted.

What parallel runs actually cost

Running two systems simultaneously doubles the transaction processing work for anyone who has to enter data in both. For a finance team already stretched by the implementation, adding parallel run workload is genuinely punishing. Someone has to enter each invoice, payment, and journal entry twice — once in the old system, once in the new — and then compare the results. At any meaningful transaction volume, this is exhausting.

The cost isn’t just labor. It’s accuracy. Humans making duplicate entries in two systems under time pressure make mistakes. When the old and new systems disagree, someone has to investigate whether the discrepancy reflects a system error or an entry error. Investigating entry errors in a parallel environment is time-consuming and psychologically demoralizing for a team that’s simultaneously trying to learn a new system.

How long is actually appropriate

The honest answer is: one full accounting period for most companies, two if the first period surfaces significant issues that need resolution, and rarely more than that. The value of parallel running diminishes quickly once the first period’s comparison has been completed. What additional periods add is additional labor and delay, not meaningfully additional confidence.¹

The common mistake is treating the parallel run as a proxy for confidence rather than as a specific validation exercise. A finance team that says “we want to run parallel until we’re comfortable” is expressing a reasonable desire for certainty, but the comfort being sought doesn’t come from continuing to double-enter transactions indefinitely. It comes from completing the specific tests that would confirm the new system is working correctly: the close process, the key reconciliations, the critical reports. Once those have been validated through one live period, the parallel run has done its job.

What to validate during the parallel run

The parallel run should be structured around specific validation points, not general observation. The key items:

The opening balance in the new system matches the closing balance from the old system, confirmed at the account level. The monthly close process completes in both systems and the ending trial balances agree within an acceptable tolerance. Key operational reports — AR aging, AP aging, cash position — produce consistent results in both systems. The specific modules or processes that were the most complex to implement are exercised with real transactions and produce expected results.

When those validations are complete and the results are acceptable, the parallel run has served its purpose. Continuing beyond that point costs more than it protects.

The cutover commitment

The decision to cut over from parallel to single-system operation requires a commitment that some finance teams are reluctant to make, because it means accepting the new system as the authoritative record. This reluctance is understandable — the old system is familiar, and its errors are known errors. The new system’s errors, if any exist, are unknown.

Managing this transition requires being explicit about what level of confidence is sufficient. Perfect confidence is not achievable, because it requires experience with the new system that can only come from using it as the primary system. At some point, the only way to gain confidence is to commit. The parallel run creates the conditions for an informed commitment; it doesn’t eliminate the need for one.

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