Test Uncertainty Ratio (TUR) Calculator for Gages and Calibration
Test uncertainty ratio compares the tolerance you need to verify with the uncertainty of the measurement that verifies it. A classic 4:1 requirement means the calibration is four times better than the tolerance. This tool computes TUR, the margin, the largest uncertainty you can accept and a guard-banded limit.
Your numbers
Half the total tolerance, in the same unit as the uncertainty.
Take it from the calibration certificate or your uncertainty budget.
Your results
Defines TUR as tolerance divided by expanded uncertainty. Some labs use the full tolerance span over twice the uncertainty, which gives the same number. Follow your calibration procedure and customer requirements for the decision rule.
TUR as calibration uncertainty grows
Sweeps Expanded uncertainty of the calibration (k=2) from 0.00005 to 0.0005, holding your other inputs fixed.
Calibration due-date reporting
Optional: we can review how calibration records and gage status are reported from your ERP.
Optional. The tool and every export work without this. No spam.
How to use it
Enter the plus or minus tolerance of the gage or product feature and the expanded uncertainty from the calibration certificate, both in the same unit. The result shows TUR against your minimum, the maximum uncertainty that would still meet it, and an acceptance limit pulled in by the uncertainty. Use the chart to see where the ratio crosses your target line as uncertainty rises.
- Keep units identical, for example inches or millimetres.
- Use the expanded uncertainty at k=2, not the standard uncertainty.
- Apply the same method to product features and to calibration standards.
TAR, TUR and guard banding
Test accuracy ratio uses the accuracy specification of the standard and ignores other uncertainty contributors. TUR uses the full expanded uncertainty and is more defensible. When TUR falls below the target, either use a better standard or apply a guard band so an item near the limit is not accepted on a result that could be wrong. ILAC G8 and ISO/IEC 17025 clause on decision rules describe the options.
Common mistakes
Mixing units, using the manufacturer accuracy as uncertainty, ignoring resolution and repeatability, and applying 4:1 blindly to a tolerance so tight it is unreachable. If you cannot reach the ratio, document the decision rule and the risk you accept rather than hiding the gap.
Frequently Asked Questions
What is a good TUR?
Four to one is the traditional target, and ten to one is common for critical measurements. Many labs accept lower ratios when they use a documented decision rule with guard banding.
How is TUR different from TAR?
TAR compares tolerance to the standard's accuracy specification. TUR compares tolerance to the full expanded uncertainty of the calibration process, so it includes repeatability, resolution and other contributors.
What is a guard band?
A guard band moves the acceptance limit inside the tolerance by an amount tied to the uncertainty, reducing the chance of accepting a nonconforming item. This calculator uses a guard of one uncertainty.
Which uncertainty do I enter?
Enter the expanded uncertainty reported on the calibration certificate, normally at about 95% coverage with k=2. Do not use the standard deviation or the manufacturer accuracy statement unless it is all you have.
If calibration records and gage due dates sit in your ERP or QMS, we can help build alerts and reports so uncertainty and due dates stay visible.
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