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How to read a calibration certificate

A calibration certificate documents that a measuring instrument has been compared with a known reference, and records the results of that comparison. Taking a few minutes to read the certificate before you file it away tells you whether your instrument really is fit for use. This guide explains the sections usually found on certificates issued by laboratories working to ISO/IEC 17025.

In brief

  • Start with the identification details: does the serial number match the instrument in your hands?
  • The results table gives the deviation and the measurement uncertainty (usually with k=2).
  • A statement of conformity is given only when the customer asks for one, and it rests on a decision rule.
  • The accreditation symbol shows that the calibration was carried out within the scope of accreditation.
On this page

Reading the certificate section by section

  1. Identification details

    At the top of the certificate you will find the laboratory's name and address, the unique certificate number, the page numbers (e.g. "Page 1 of 3"), the customer's name and a description of the calibrated instrument (name, manufacturer, model, serial number). The first check: does the serial number match the instrument in your hands? The date of calibration and the date the instrument was received by the laboratory are also in this section.

    Drawing of a sample calibration certificate with numbered sections: identification, instrument, traceability, results, uncertainty and approval : Enlarge

  2. Measurement results and uncertainty

    The results table usually shows the reference (nominal) value, the value indicated by the instrument, the deviation between the two and the measurement uncertainty for each point. The uncertainty is mostly given in the form "U (k=2)", at a level of confidence of approximately 95%. Check whether the measurement points cover the range in which you actually use the instrument. If an adjustment was made, the values before and after adjustment may be reported separately. For more about uncertainty, see the guide What is measurement uncertainty?

    Sample results table with columns for nominal value, reading, deviation and U (k=2) : Enlarge

  3. Statement of conformity and decision rule

    If the customer asked for it, the certificate includes a statement on whether the instrument conforms to a particular specification or tolerance. This statement rests on a decision rule that describes how the uncertainty is taken into account. If the deviation is very close to the tolerance limit, the uncertainty may make it impossible to say that the result is "definitely conforming" or "definitely non-conforming". If there is no statement of conformity, you assess the results yourself against your own acceptance criterion.

    Diagram: measurements inside and at the edge of the tolerance limits, shown with uncertainty bars : Enlarge

  4. Traceability and accreditation

    The traceability statement shows that the measurement is linked to national or international measurement standards through an unbroken chain of calibrations; the reference equipment used and its certificate numbers may also be listed. If the certificate carries the symbol of an accreditation body (the national accreditation body, e.g. UKAS, DAkkS, TÜRKAK), the calibration was carried out within the laboratory's scope of accreditation. You can check the laboratory's schedule of accreditation to see whether the scope covers the quantity and range that were measured.

    Checklist: serial number, measurement points, uncertainty, conformity, accreditation, approval and date : Enlarge

Points to watch

  • Is the uncertainty small compared with the tolerance? If the uncertainty is close to the tolerance you expect of your instrument, the result may not be enough to support a reliable decision.
  • Do the deviations call for a correction? In some measurements the deviation on the certificate is applied to measurement results as a correction.
  • Environmental conditions: Conditions such as temperature and humidity can influence the results; they are stated on the certificate.
  • Next calibration date: Laboratories do not put a recommended calibration interval on the certificate unless this has been agreed with the customer; the interval is set by the owner of the instrument. See How to determine a calibration interval.
  • Approval: The name, function and signature (which may be an electronic signature) of the person authorising the certificate should be present.

Frequently asked questions

If the certificate does not say "pass", can the instrument not be used?

It can. A statement of conformity is given only when the customer asks for one. Without it, you make the decision yourself by comparing the results and the uncertainty with your own acceptance criterion.

Is a certificate without an accreditation symbol valid?

The certificate is still a record of calibration; accreditation, however, shows that the laboratory's competence has been assessed by an independent body. If your customer or the standard you work to requires accredited calibration, check that the symbol is there.

What does k=2 mean?

It shows that the expanded uncertainty was calculated by multiplying the standard uncertainty by 2. This corresponds to a level of confidence of approximately 95%.

BYK Yazılım Support Team
This guide is written and regularly reviewed by the BYK Yazılım support team. Last updated: 4 October 2026.

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