ISO 17025 calibration: accreditation for a calibration laboratory
A calibration laboratory checks whether a measuring instrument gives the correct value, compared with a reference that is traceable to the international unit of measurement. The Dutch Accreditation Council (RvA) describes calibration laboratories in this way and accredits them based on ISO/IEC 17025. The difference from a testing laboratory lies in the purpose: you test a property of a product, you calibrate a measuring instrument. To define the recognised scope, use the scope of accreditation, and for the related uncertainty, see the page on measurement uncertainty.
If you are looking for the answer to “what is ISO 17025” rather than calibration accreditation, first read the explanation of ISO 17025 and the application route. This article covers the additional points that matter for a calibration laboratory.
What is the difference from a testing laboratory?
The RvA treats testing laboratories and calibration laboratories as separate disciplines within the same standard. For each, the RvA has its own accreditation specification (ASR, a document setting out the rules for each discipline). For testing laboratories this is ASR105 , and for calibration laboratories ASR106 .
| Feature | Testing laboratory | Calibration laboratory |
|---|---|---|
| Main question | Which property does this material or product have? | How far does this measuring instrument deviate from the reference? |
| Result | A test report with the measured property | A calibration certificate with results and uncertainty |
| Most important evidence | Method, validation and proficiency testing | Uncertainty calculation, reference standards, traceability and proficiency testing |
| Typical client | A manufacturing company that needs inspections | A company with measuring instruments that fall under a quality system |
What does the scope of a calibration laboratory look like?
ASR106 (version 1.0, in force since 15 September 2025) sets out what the scope looks like. A calibration scope is a table. Each line states the measurand, the measuring range, the expanded measurement uncertainty, a note with the calibration method, and the location. In addition, each line carries an HCS code (Harmonised Classification Scheme), a classification code for the type of calibration.
The RvA calls the expanded measurement uncertainty in the scope the CMC (Calibration and Measurement Capability). It is the lowest expanded measurement uncertainty the laboratory can offer under normal conditions, with a coverage probability of 95 per cent, calculated according to EA-4/02 M. EA is the European cooperation for Accreditation, and EA-4/02 M is the guideline on uncertainty in calibration.
ASR106 lists fields of work such as mass and weight, pressure and vacuum, temperature, humidity, force, geometric quantities, time and frequency, volume of gases and liquids, and chemical analyses. A calibration laboratory that only checks weighing scales therefore has a different scope from one that also calibrates thermometers and electrical measuring instruments.
ASR106 also covers two practical points. Calibrations carried out at the client’s site (on site) and in a mobile laboratory are listed separately in the scope, with their own uncertainty, because it can differ from the uncertainty in the laboratory. And a flexible scope is not currently possible for calibration laboratories with the RvA. Your scope therefore remains a fixed list.
What the RvA assesses in calibration
The RvA assesses a calibration laboratory against the same ISO/IEC 17025 requirements as a testing laboratory: staff competence, suitable environment, measuring equipment, traceability, methods and reporting. For calibration, three elements carry extra weight.
Reference standards and traceability
Every reference you compare against must itself be calibrated, by a laboratory that is in turn demonstrably traceable. This is the chain view from the page on metrological traceability. The auditor asks for the calibration chain for each reference, with dates and certificates.
Calibration capabilities and uncertainty
For each measurand and range, you state the uncertainty you can achieve. According to ASR106, the justification under EA-4/02 M plays a prominent role in the assessment. A figure from a catalogue is therefore not enough. See the explanation of measurement uncertainty for the calculation logic.
Environment and equipment
Temperature, humidity and vibration directly affect calibration. The environment must be monitored and recorded, and the measuring equipment must be checked at a set interval.
During an initial assessment, the RvA reviews all fields of work in the application. If the laboratory carries out calibrations outside its premises, the technical assessor attends at least one such calibration at the client’s site or in the mobile laboratory. Proficiency testing also receives attention: the RvA uses the HCS codes as the basis for participation. For that, read the page on proficiency testing.
What is on a calibration certificate?
The RvA checks whether the certificate meets ISO/IEC 17025 and ILAC P14, the policy of the international accreditation body ILAC on measurement uncertainty in calibration. ASR106 includes a sample certificate. Other layouts are permitted, but these are the fields you will find in it:
- a unique certificate number and the client’s details;
- identification of the instrument: manufacturer, type, serial number and the date of receipt;
- the calibration method, the location, the environmental conditions and the date or period of calibration;
- the results with units and uncertainties, where results outside the accreditation or from external suppliers are clearly marked;
- an explanation of the uncertainty: the product of the standard uncertainty and the coverage factor k, for approximately 95 per cent coverage;
- the traceability of the standards used;
- the date of issue and the name and function of the person who releases the certificate.
If the laboratory also issues a declaration of conformity, the certificate must state which results it applies to, which specifications are or are not met, and which decision rule was used. See the page on declaration of conformity. The certificate must also make clear that the results apply only to the calibrated object. This may be omitted if the certificate describes the object unambiguously (as the RvA explains in INF002, clause 7.8.2.1l).
Fictional example: calibrating a thermometer
A fictional calibration laboratory receives two requests: a digital thermometer for a food company and a pressure gauge for a chemical factory. The first falls under temperature, the second under pressure. They have separate references, separate uncertainty calculations and separate scope lines.
| Step | Thermometer (fictional) | Pressure gauge (fictional) |
|---|---|---|
| Reference | Reference thermometer with a calibration certificate | Weight set or reference manometer with a certificate |
| Measurement points | Three points within the client’s range | Five points across the range |
| Uncertainty | Per measurement point, with the k-factor used | Per measurement point, with the k-factor used |
| Outcome | Deviation per measurement point, without a statement on suitability | Deviation per measurement point, plus a declaration of conformity if the client asks for one |
The thermometer’s certificate shows the measured deviations and the uncertainty. It does not state whether the instrument is suitable for the client. That decision belongs to the client, based on their own tolerance. If they ask the laboratory to make that statement, an agreed decision rule must apply.
A calibration certificate is not proof that your whole measurement process is correct. It only proves the calibration of the measuring instrument at the time of calibration, under the conditions stated.
How does a client choose a calibration laboratory?
As a buyer, check three things. First the accreditation certificate, then the scope of that certificate, and finally whether your measurand and range are included.
- Ask for the accreditation certificate and the scope, and compare them with your own measuring instrument.
- Check whether your measurand, range and required uncertainty fall within the scope. The CMC in the scope is the lower limit: the laboratory cannot deliver a better uncertainty than the CMC.
- Ask whether the laboratory takes part in comparison studies and how it uses the results. See the page on proficiency testing.
Also pay attention to the accreditation mark. It may only be used for calibrations that fall within the scope. In ASR106, the RvA gives the use of the mark for an activity that is not accredited as an example of an A-level nonconformity, the most serious category. A certificate without the mark is not an accredited calibration. It can still be useful, for example if you can demonstrate traceability yourself. In that case, ask the laboratory about the chain behind the certificate.
Common calibration mistakes
- Reading the scope too broadly. A laboratory accredited for a measurand is not automatically accredited for every range and uncertainty within that measurand.
- A reference without a certificate. A working standard that has not been calibrated for years makes the whole chain uncertain.
- Uncertainty from memory. An uncertainty statement without a calculation cannot be followed by the auditor.
- An interval without justification. The calibration interval must be based on use, stability and previous results, not on a fixed year someone once chose.
- The accreditation mark on everything. A certificate with the mark, where some results fall outside the scope, must clearly mark those results.
What do you need for an application?
ASR106 gives a list of documents that the laboratory provides to the RvA. For an initial application, these include:
- documentation of the quality system and a cross-reference table between the requirements of the standard and your own system;
- the report of a recent internal audit and of the most recent management review;
- the technical procedures for all requested calibration quantities;
- validation reports and uncertainty calculations according to EA-4/02 for all quantities;
- the results of the proficiency tests in which you took part, plus the plan for participation;
- an example of a calibration report and calibration certificate.
Use this overview as a checklist before you start the accreditation application. The RvA also expects you to have confirmed, through internal audits, that you meet all requirements for the entire requested scope. Costs and lead time depend heavily on the number of measurands and ranges. See costs and lead time.
Frequently asked questions
Must a calibration laboratory also be able to test? No. The discipline applies per activity. A laboratory that both tests and calibrates needs a separate scope and evidence for each activity.
May a company calibrate its own measuring instruments in-house? Yes, the RvA allows this for laboratories that calibrate their own equipment. The in-house calibration must then meet the requirements for a calibration laboratory: an adequate procedure, competent staff, an uncertainty calculation for each step according to EA-4/02, an unbroken chain of traceability and good records. This is set out in INF002 under clause 6.4.6. If a laboratory opts for this, the RvA periodically adds a separate calibration assessor to the assessment team. Equipment only needs to be calibrated if it has a significant effect on the result.
Is every calibration certificate from an accredited laboratory the same? No. The accreditation applies to the listed measurands and ranges. For each certificate, check which measurement points were calibrated and which uncertainty applies.
Where do I start if I want to become a calibration laboratory? Start with the scope and the reference standards. Only once those are in order does an application make sense. The RvA quick scan first gives an initial impression of the conditions.