Control Plan Frequency Calculator

Work out control plan frequency instantly with clear inputs, formula shown and shareable results.

About Control Plan Frequency Calculator

Control plan frequency should follow capability and consequence. A highly capable process (Cpk above 1.67) needs infrequent verification, while a marginal one needs frequent checks. The interval is then tightened where an escaped defect costs far more than an inspection, since the units produced between checks are all at risk if the process shifts.

Formulas

Base interval from capability

Cpk >= 1.67: 4 h; 1.33-1.67: 2 h; 1.0-1.33: 1 h; below 1.0: 0.5 h

Cost adjustment

Interval = base x (20 / (defect cost / inspection cost))^0.25, bounded to 0.25-8 hours

Worked example

With the default values below, Control Plan Frequency Calculator returns Inspection Interval of 1.682. Change any input above and every figure here is recalculated in your browser.

Inputs used

Production Rateminimum 0.01 units/hour: 240 units/hour
Process Capability (Cpk)minimum 0.1: 1.33
Cost Of An Escaped Defectminimum 0: 1,800
Cost Per Inspectionminimum 0.01: 45

Results

Inspection Interval: 1.682
Checks Per 8-Hour Shift: 4.76
Units Between Checks: 404
Defect To Inspection Cost Ratio: 40.0

Step by step

1Values used
2Base interval from capability
3Cost adjustment
4Inspection Interval= 1.682
5Checks Per 8-Hour Shift= 4.76
6Units Between Checks= 404
7Defect To Inspection Cost Ratio= 40.0

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax or compounding frequency into account.
Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why not just inspect every unit?

Because 100% inspection is typically only 80-95% effective, costs a great deal, and does nothing to improve the process. Capable processes with periodic verification outperform incapable ones with full inspection.

What about first-off and last-off checks?

They are mandatory regardless of interval. Setup is where most special-cause variation enters, and a last-off check bounds the quarantine if a problem is found.