What is measurement uncertainty?

Measurement uncertainty quantifies the doubt about a measurement result. It defines a range within which the true value is expected to lie with a stated probability (typically 95%). Required by ISO 17025 for all calibration and testing laboratories.

What is the difference between Type A and Type B uncertainty?

Type A uncertainty is evaluated by statistical analysis of repeated measurements (standard deviation of the mean). Type B is evaluated by other means: manufacturer specifications, calibration certificates, experience, or published data.

What is the coverage factor k?

The coverage factor k multiplies the combined standard uncertainty to give expanded uncertainty. k=2 corresponds to approximately 95% confidence level (assuming normal distribution). k=3 gives about 99.7%.

How do I combine uncertainty sources?

For independent (uncorrelated) sources, the combined standard uncertainty is the root sum of squares (RSS): uc = √(u1² + u2² + u3² + ...). This is the law of propagation of uncertainty from GUM.

GUM (Guide to the Expression of Uncertainty in Measurement) is the international guide for evaluating and expressing measurement uncertainty. Published by JCGM (Joint Committee for Guides in Metrology), it is the basis for ISO 17025 uncertainty requirements.

Free Engineering Tool

Measurement Uncertainty Calculator

Calculate combined and expanded measurement uncertainty per GUM / ISO 17025. Enter up to 6 uncertainty sources with Type A or B classification.

ISO 17025GUMk=2 (95%)

Results

Result Expression

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Combined Standard Uncertainty (uc)

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Expanded Uncertainty U (k=2, 95%)

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Coverage Factor k

Relative Uncertainty

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Combined Standard Uncertainty

Expanded Uncertainty

Where k = coverage factor (k=2 for ~95% confidence, k=3 for ~99.7%).

Divisors by Distribution

Type A (statistical): divisor = 1 (value is already a standard uncertainty from s/√n).
Type B (Normal, from certificate U at k=2): divisor = 2.
Type B (Rectangular, half-width a): divisor = √3 ≈ 1.732.

Measurement Uncertainty Calculator | ISO 17025 / GUM

Published by Nikolai Shelkovenko on February 15, 2026 February 15, 2026

Results

Combined Standard Uncertainty

Expanded Uncertainty

Divisors by Distribution