Instrumentation guide

Measurement uncertainty, accuracy, error, tolerance and resolution

Measurement terms are often treated as interchangeable even though they answer different questions. Keeping them separate prevents false precision and weak acceptance decisions.

GUMUncertaintyErrorRepeatability

Last reviewed: August 10, 2026 · By Martin E. Calderwick

Error compares indication with a reference

Measurement error is the difference between a measured indication and a reference quantity value under the defined comparison. Error can be positive or negative and may change across the range.

Uncertainty describes doubt about the result

The JCGM/BIPM Guide to the Expression of Uncertainty in Measurement provides the internationally used framework for evaluating and expressing measurement uncertainty. Uncertainty is associated with the measurement result and reflects relevant sources of variability and incomplete knowledge.

Accuracy is qualitative

Accuracy describes closeness of agreement with the quantity value; it is not normally used as a numerical quantity by itself. Instrument data sheets may use “accuracy” informally to label a numerical limit, so always read the manufacturer's definition.

Repeatability and reproducibility

Repeatability concerns variation under tightly similar measurement conditions. Reproducibility involves changed conditions such as operator, laboratory or equipment. A device can repeat very well while still being biased away from the reference.

Resolution

Resolution is the smallest change in the indicated quantity that produces a perceptible change in indication. More displayed digits do not improve the underlying uncertainty if the sensor and measurement chain cannot support them.

Tolerance is an acceptance limit

A tolerance is a permitted limit or range set by a requirement, drawing, process or specification. Measurement uncertainty should be considered when deciding whether a measured item conforms to a tolerance.

This page explains measurement principles. Real instruments, hazardous processes, electrical circuits and safety functions must be handled under applicable procedures, specifications and qualified supervision.