How do you calculate FAT duration?

Divide test points by the execution rate for base hours, then add the allowance. Here 120 points at 12 per hour is 10 base hours, and a 10% allowance gives 11 hours adjusted.

What is a reasonable retest allowance for a FAT?

For a mature skid design, 10-15% covers routine punch items and minor retests. First-of-a-kind or complex skids often warrant 20-30% to avoid overrunning the witness window.

Why add an allowance instead of just base time?

Base time assumes every test passes first try, which almost never happens. The allowance budgets for retests and documentation; here it turns 10 clean hours into a realistic 11.

What execution rate should I use?

Use your team's demonstrated rate from prior FATs on similar skids. Twelve points per hour is typical for straightforward functional checks; alarm and interlock testing runs slower.

How do I shorten FAT duration?

Raise the execution rate with pre-FAT dry runs and complete redlines, and cut the allowance by closing punch items before the client arrives rather than during the witnessed test.

Does this include setup and mobilization time?

No, it covers test-point execution only. Add rig-up, utility hookup, and teardown separately so your scheduled window reflects the full day, not just active testing.

Process Skids, Modular Equipment & Packaged Plants calculator

FAT Duration Calculator

FAT Duration estimates how many hours a Factory Acceptance Test on a process skid will actually take, from the number of test points and how fast your team works through them, plus a realistic allowance for retests. Commissioning managers and skid project leads use it to schedule client witness visits and hold the FAT window that anchors the ship date. Because a witnessed FAT ties up the client, third-party inspectors, and the skid itself, an accurate duration protects everyone's calendar. The allowance term keeps the estimate honest by budgeting for the punch items that always surface.

What this calculator does

FAT Duration estimates how many hours a Factory Acceptance Test on a process skid will actually take, from the number of test points and how fast your team works through them, plus a realistic allowance for retests.
Use it when fat duration in process skids, modular equipment and packaged plants is changing rate or allowance and you want to see the impact.
It divides test points by the execution rate for base hours, then applies a retest allowance to give adjusted FAT duration.

Formula used

Base FAT duration time = required work ÷ processing rate
Adjusted time = base time × allowance factor

Inputs explained

FAT test points to execute:
Test point execution rate:
Retest and contingency allowance:

How to use the result

Use it when scheduling a witnessed FAT, booking client and inspector travel, or sequencing FAT against the skid ship date.
It assumes a steady execution rate across all test points; a single major failure that forces a full re-run can blow past even a generous allowance.

Current U.S. benchmarks

Steel mill PPI stands at 348.53 (BLS, May 2026), up 6.7% from a year earlier. New factory orders are up 2.3% year over year (Census).

Common questions

How do you calculate FAT duration? Divide test points by the execution rate for base hours, then add the allowance. Here 120 points at 12 per hour is 10 base hours, and a 10% allowance gives 11 hours adjusted.
What is a reasonable retest allowance for a FAT? For a mature skid design, 10-15% covers routine punch items and minor retests. First-of-a-kind or complex skids often warrant 20-30% to avoid overrunning the witness window.
Why add an allowance instead of just base time? Base time assumes every test passes first try, which almost never happens. The allowance budgets for retests and documentation; here it turns 10 clean hours into a realistic 11.
What execution rate should I use? Use your team's demonstrated rate from prior FATs on similar skids. Twelve points per hour is typical for straightforward functional checks; alarm and interlock testing runs slower.
How do I shorten FAT duration? Raise the execution rate with pre-FAT dry runs and complete redlines, and cut the allowance by closing punch items before the client arrives rather than during the witnessed test.

Last reviewed 2026-05-12.