Calibration of an instrument is the process of configuring it to provide a result for a sample within an acceptable range. It is a comparison between a known measurement (the standard) and the measurement using the instrument. The primary objective of calibration is to maintain instrument accuracy by eliminating or minimizing factors that cause inaccurate measurements. Calibration involves using the instrument to test samples of one or more known values called "calibrators" to establish a relationship between the measurement technique used by the instrument and the known values. The process "teaches" the instrument to produce results that are more accurate than those that would occur otherwise. Calibration also includes repair of the device if it is out of calibration, and a report is provided by the calibration expert, which shows the error in measurements with the measuring device before and after the calibration.

Calibration is important because the accuracy of all measuring devices degrades over time due to normal wear and tear, electric or mechanical shock, or a hazardous manufacturing environment. Calibration checks the accuracy of the instrument and determines the traceability of the measurement. It is required for a new instrument to ensure that it is working properly and according to specified standards, when the instrument has been exposed to adverse conditions, turbulent processes, etc., after an instrument has been repaired or modified, and after the specified usage time has elapsed.

The calibration process begins with the design of the measuring instrument that needs to be calibrated. The design has to be able to compare the output of the instrument to be calibrated with the values of a proven accurate reference instrument. The calibration standard is normally traceable to a national or international standard held by a metrology body. The calibration process involves comparing an unknown to a known and recording the results. Adjustments are then made if the accuracy of the equipment is out of its allowed tolerance. Calibration is often performed in situ, but it can also be performed in a calibration laboratory. A calibration certificate is usually electronically provided, and the equipment is usually labeled as calibrated and dated before being returned to service.