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Guard Band Acceptance Limit Calculator

When a measured value sits close to a specification limit, uncertainty means the true value could be on the wrong side. A guard band moves the acceptance limit inward so that accepted items have a controlled chance of being truly in tolerance.

Your numbers

mm
mm
mm

From your uncertainty budget or the calibration certificate.

Your results

Upper acceptance limit
25.021
Lower acceptance limit
24.979
Specification width
0.05
Guard band
0.004
Distance each acceptance limit moves inside the specification limit.
Acceptance zone width
0.042
Tolerance given up
16%
Share of the specification width no longer available for acceptance.
Test uncertainty ratio
6.25
Specification half-width divided by expanded uncertainty, often called TUR.

Guard banding rules differ by standard and customer contract. Follow ILAC-G8, ISO/IEC 17025 clause on decision rules or your own documented rule.

Save results:

Tolerance given up as uncertainty grows

Sweeps Expanded uncertainty (k=2) from 0 to 0.02, holding your other inputs fixed.

Put acceptance limits into your inspection plans

Optional: talk through adding guard-banded limits to ERP inspection characteristics.

Optional. The tool and every export work without this. No spam.

How to use it

Enter the upper and lower specification limits and the expanded uncertainty from your budget or certificate. Choose a multiplier: zero is simple acceptance with no guard band, 1.0 times U is a common rule that cuts the chance of a false accept at the limit to roughly 2.5 percent for a normal uncertainty distribution at k=2. The calculator returns the new acceptance limits, the width of the acceptance zone and the share of the tolerance you give up.

  • Use the expanded uncertainty of the measurement process, not just the instrument.
  • Document the multiplier in your decision rule procedure.
  • Apply the same rule consistently across customers unless a contract overrides it.

The method

Acceptance limits are the specification limits moved inward by the guard band, where the guard band is a chosen multiple of expanded uncertainty. The test uncertainty ratio shown is the specification half-width divided by expanded uncertainty. A ratio of 4 or better is the traditional target, and at that ratio a guard band of one U gives up only a small slice of the tolerance. As the ratio approaches 1, the acceptance zone collapses and the chart shows how quickly it disappears.

Worked example

A 25 mm bore has limits 24.975 and 25.025 mm, so the width is 0.050 mm. With expanded uncertainty 0.004 mm and a 1.0 U guard band, the acceptance limits become 24.979 and 25.021 mm, a zone of 0.042 mm. You have given up 16 percent of the tolerance, and the TUR is 6.25. If a worn gage pushed U to 0.012 mm, the acceptance zone would shrink to 0.026 mm and the TUR would fall to about 2.1.

Common mistakes

People often guard band with the instrument resolution or the calibration certificate uncertainty alone. The relevant number is the full uncertainty of the measurement as performed, including repeatability, operator, temperature and fixturing. Another error is applying guard bands on the way in but not on rejection, so the lab's rule and the customer's incoming inspection rule do not agree. Agree on the decision rule in writing before work starts.

Frequently Asked Questions

Is a guard band required by ISO/IEC 17025?

The standard requires labs to document a decision rule when they state conformity and to account for measurement uncertainty. It does not mandate a specific guard band, so the rule must be agreed with the customer.

What does a 1.0 U guard band mean?

Each acceptance limit sits one expanded uncertainty inside the specification limit. For a normal distribution at k=2 that limits the probability of false accept for a value at the acceptance limit to about 2.5 percent.

Does guard banding reject good parts?

Yes. Values between the acceptance limit and the specification limit are rejected or reworked even though many are truly in tolerance. That is the price of limiting false accepts.

What TUR should I target?

Four to one or better is the traditional target. Below that, guard banding or a better instrument is needed to avoid unacceptable false accept risk.

If your inspection plans in the ERP still use raw specification limits, we can help add acceptance limits and uncertainty fields to the plan characteristics.

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