Calibration · Glossary

What is Coverage Factor?

Symbol: kmultiplier for expanded uncertaintyCalibration

Coverage factor k multiplies the combined standard uncertainty to give an expanded uncertainty at a chosen confidence level.

Coverage Factor Explained

For a normal distribution, k = 2 gives about 95 % coverage, the usual level on calibration certificates. With few degrees of freedom, a larger k from the t distribution is needed.

Certificates should state k, so users can recover the standard uncertainty.

Coverage Factor Formula

U = k × u_c

U	expanded uncertainty
u_c	combined standard uncertainty

Coverage Factor: Typical Values

k = 1	about 68 %
k = 2	about 95 %
k = 3	about 99.7 %

Calculators for Coverage Factor

CalculatorCoverage Factor CalculatorFree online calculator with formula and worked example.CalculatorMeasurement Uncertainty CalculatorFree online calculator with formula and worked example.

Free Downloads

Free downloadMeasurement Uncertainty Budget Excel SheetFree download in Excel and PDF.Free downloadCalibration Certificate Format in ExcelFree download in Excel and PDF.

Frequently Asked Questions

What is Coverage Factor?

Coverage factor k multiplies the combined standard uncertainty to give an expanded uncertainty at a chosen confidence level.

Why is 2 not exactly 95 %?

Exactly 95.45 %; 1.96 gives 95.0 %. Certificates commonly use k = 2.

How do I combine a certificate value?

Divide the stated U by its k to get the standard uncertainty first.

Learn more on the blog

Calibration traceability: 6 proven links for trusted results

A practical guide on instrumentationblog.in with more detail on coverage factor and how it is applied on real plants.

Read Full Article

More Calibration Terms

Calibration termTest Uncertainty RatioTest uncertainty ratio compares the tolerance of the instrument under test with the uncertainty of the…Calibration termDecision RuleA decision rule defines how measurement uncertainty is taken into account when stating that an instrument…Calibration termTotal Probable ErrorTotal probable error combines independent error sources, such as reference accuracy, temperature and…Calibration termTraceabilityTraceability links a measurement result to national standards through an unbroken chain of calibrations,…Calibration termReference StandardA reference standard is the highest accuracy standard at a site, used to calibrate working standards…Calibration termDeadweight TesterA deadweight tester generates a precise pressure by placing known masses on a piston of known area.

All calibration terms · Full glossary A to Z