Calibration is the process of comparing an unknown value to a known value. To calibrate a device is to compare a characteristic of that device with the characteristic of a similar device called a “standard.” Using load cells as an example, the load cell being calibrated and the standard load cell are mounted together in series and a range of loads is applied. Both load cells experience the same load and therefore the output values should ideally be identical. A calibration result most often indicates a difference between the two values.

A common misconception is that calibration means adjusting the output of a device to bring its performance “within limits.” Any adjustment made to the output of the device is separate from the calibration and verification process, and must be followed by a further calibration and verification to prove the adjustment was successful.

NOTE: It is important not to confuse CALIBRATION with the term "calibrate" that is sometimes used on instruments to refer to the actions of setting up a transducer such as a loadcell or extensometer to work with the system.

Below are the definitions as defined by the International Vocabulary of Metrology.

2.39 (6.11) calibration
Operation that, under specified conditions, in a first step, establishes a relation between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties and, in a second step, uses this information to establish a relation for obtaining a measurement result from an indication

NOTE 1: A calibration may be expressed by a statement, calibration function, calibration diagram, calibration curve, or calibration table. In some cases, it may consist of an additive or multiplicative correction of the indication with associated measurement uncertainty.

NOTE 2: Calibration should not be confused with adjustment of a measuring system, often mistakenly called "self-calibration," nor with verification of calibration.