How often should I calibrate?

12 de julio, 2025.

Determination of the calibration interval

One of the most common questions in metrology and quality assurance is: How often should I calibrate my measuring instruments? While this may seem like a simple matter of frequency, the answer is deeply related to the reliability of the process, the nature of the instrument, and the risks associated with using equipment outside of tolerance.

In this blog, we explore how to determine the appropriate calibration interval by following the guidelines of the ISO/IEC 17025:2017, ISO 9001:2015, ILAC-G24, OIML D 10 standards, and the best practices in metrological management.

Regulatory framework

ISO/IEC 17025:2018, in clause 6.4.7, states:
The laboratory shall establish a calibration program, which shall be reviewed and adjusted as necessary to maintain confidence in the calibration status (International Organization for Standardization & International Electrotechnical Commission, 2017).

On the other hand, ISO 9001:2015, in section 7.1.5.2, indicates:
When measurement traceability is a requirement, the measuring equipment shall be calibrated or verified at specified intervals, or before use [...] (International Organization for Standardization, 2015).

No standard imposes a specific interval (annual, semi-annual, etc.), but instead requires it to be technically justified.

ILAC-G24 OIML D 10, on the other hand, sets the guidelines for determining the calibration intervals of measuring instruments, which form the basis of this blog (International Laboratory Accreditation Cooperation, 2022).

Terms and Definitions

ISO/IEC 17025:2018, in clause 6.4.7, states:
● Calibration (VIM3, 2.39): operation that, under specified conditions, first establishes a relationship between values and their associated measurement uncertainties obtained from measurement standards, and the corresponding indications with their associated uncertainties; and, in a second stage, uses this information to establish a relationship that allows obtaining a measurement result from an indication (International Vocabulary of Metrology, 2012).
● Instrument drift (VIM3, 4.21): continuous or incremental variation of an indication over time, due to changes in the metrological characteristics of a measuring instrument (International Vocabulary of Metrology, 2012).
● Maximum permissible error (VIM3, 4.26): maximum tolerable error, error limit, extreme value of the measurement error, with respect to a known reference value, allowed by specifications or regulations for a given measurement, measuring instrument, or measurement system (International Vocabulary of Metrology, 2012).

What factors influence the calibration interval?

The determination of the optimal calibration interval should take the following factors into account:
1.  Frequency of instrument use.
2.  Environmental operating conditions.
3.  Type and stability of the equipment (drift).
4.  History of previous calibrations (error trends).
5.  Impact of error on results or product quality.
6.  Manufacturer’s recommendations.
7.  Risk associated with decisions based on erroneous measurements.
8.  Changes in the process or personnel.

It is important to consider and identify which factors influence each type of instrument you use and for which you will determine the calibration interval, since different calculation methods apply depending on the instrument.

1. Method 1: Historical data with drift evaluation

This methodology is based on an observed drift approach (historical method) with risk adjustment, and is considered one of the most reliable methods for establishing the calibration interval:

Step 1: Collect historical data
Gather at least 3 previous calibration records, including:
  ●  Calibration date.
  ●  Results (observed errors).
  ●  Equipment condition.
  ●  Adjustments made (if any).

Step 2: Calculate average drift
Calculate the average drift (D) between calibrations.
Example:

Step 3: Estimate the time to reach the Maximum Permissible Error (MPE)
Define the Maximum Permissible Error (MPE) for the instrument, according to the applicable process or standard. Estimate how much time (T) can be tolerated before the drift reaches the MPE:

This will give you the estimated maximum time before the next calibration.

Step 4: Apply a safety factor
Apply a safety factor (SF) to account for uncertainty, environmental changes, misuse, or other risks. A value between 0.6 and 0.8 is generally recommended:

Step 5: Validate and adjust
Validate the proposed interval by considering:
  ●  Critical equipment within the process.
  ●  Manufacturer’s recommendations.
  ●  Legal or contractual requirements.

Practical example
  ●  Average drift: 0.02 mm/month
  ●  MPE: 0.10 mm
  ●  T = 0.10 / 0.02 = 5 months
  ●  SF = 0.8 → Suggested interval = 4 months

What if I don't have historical data?
If you don’t have previous records, it is recommended to:
  ●  Apply the interval suggested by the manufacturer.
  ●  Set a provisional interval (usually annual).
  ●  Monitor performance and record errors.
  ●  Adjust the interval as data is collected.

2. Method 2: Control chart (Calendar – Time)

This method aims to visually display the metrological behavior of the equipment over time (years, months), using results from past calibrations. It is useful for detecting trends, drift, and out-of-control behavior, and based on this, adjusting the calibration interval. Based on ILAC-G24:2022, section 6.3.

Prerequisites
  ●  At least 4 previous calibrations.
  ●  Numerical results of the instrument error relative to the reference value.
  ●  Date of each calibration.
  ●  Maximum Permissible Error (MPE), tolerance, or acceptance criteria.

Step 1: Gather historical data
Example table:

Step 2: Plot the error vs. time
  ●  X-axis: Time (dates or months).
  ●  Y-axis: Measured error.
  ●  Horizontal lines: Upper MPE (+0.10 mm) and lower MPE (−0.10 mm).
  ●  Plot the measured points for each date.

What are we looking for?
  ●  Linear trends (increasing or decreasing drift).
  ●  Sudden jumps (indicative of failures).
  ●  Stability (if the error fluctuates without exceeding the MPE).

Example chart:

Step 3: Calculate the drift
The slope of the curve is calculated (rate of change of the error over time), for example:

This will give you the estimated maximum time before the next calibration.

Step 4: Estimate the remaining time before exceeding the MPE
If the current error is +0.10 mm (limit) and has already reached the MPE, the interval should be reduced, for example, to 6 months or less.
If the next calibration is expected to exceed the MPE, the previous interval was too long.

Step 5: Adjust the calibration interval
    ●  If the instrument remains stable and within limits, the interval can be extended.
  ●  If the trend shows it is approaching the limit quickly, the interval should be reduced.
  ●  If there are jumps or erratic behavior, an investigation can be applied, and monitoring should be increased.

Decision example:
  ●  With a drift of 0.0044 mm/month, and a remaining margin of only 0.00 mm until the MPE → the equipment has already reached the limit.
  ●  Therefore, the 6-month interval should be reduced to, for example, 3 or 4 months, to prevent the next measurement from exceeding the limits.

Advantages of the method
  ●  Visual and easy to understand for non-metrologists.
  ●  Allows decisions to be made based on actual behavior.
  ●  Detects dangerous trends before failures occur.
  ●  Useful as a defense tool in audits (traceability of the analysis).

Conclusion

The calibration interval should not be arbitrary or fixed, but rather the result of a technical analysis that considers risks, equipment performance, and process requirements. Effective metrological management enables resource optimization, minimizes rejections, and ensures the reliability of measurements.

Remember: timely calibration is an investment in quality.

Final Recommendation

Implement a documented internal procedure to determine, review, and justify calibration intervals. This not only aligns you with ISO 17025 and ISO 9001, but also strengthens your management system and helps avoid findings during audits.

References

International Laboratory Accreditation Cooperation. (2022). ILAC-G24:2022 Guidelines for the determination of calibration intervals of measuring instruments. ILAC.

International Organization for Standardization & International Electrotechnical Commission. (2017). ISO/IEC 17025:2017 General requirements for the competence of testing and calibration laboratories. ISO.

International Organization for Standardization. (2015). ISO 9001:2015 Quality management systems - Requirements . ISO.

International Vocabulary of Metrology. (2012). VIM - Basic and General Concepts and Associated Terms, 3rd edition. VIM.