NABL calibration certificate format: what it must contain

What must a NABL calibration certificate contain?

Every calibration certificate must contain the items in ISO/IEC 17025 clause 7.8.2.1, unless the lab has a valid reason to leave one out. They are the same 16 items as on a test report. The table shows each one and where it usually sits on a calibration certificate.

The note to clause 7.8.2.1 suggests a statement that the certificate must not be reproduced except in full without the lab's approval. Clause 7.8.2.2 makes the lab responsible for everything on the certificate except information supplied by the customer, which must be clearly identified.

For subcontracted work (item p), NABL 133 clause 10 allows subcontracting only to a NABL-accredited lab. You must take full responsibility for the result, have the subcontractor's approval to report it, and identify it clearly.

What does clause 7.8.4.1 add for calibration certificates?

Clause 7.8.4.1 adds six items for calibration certificates. The last three apply only when available, relevant or appropriate.

Clause 7.8.4.3 then sets a rule about due dates, which is covered further down.

Can a certificate say the whole instrument is calibrated?

Only if your scope says so. NABL 133 clause 6.14 says an accredited calibration lab must not issue a certificate for an equipment, assembly, set-up or device unless it is explicitly mentioned in its scope.

Where your accreditation covers only the parameter, the certificate must clearly separate the "Parameter calibrated" from the equipment. It must not suggest that the whole equipment is calibrated. For example, a lab accredited for temperature calibration that checks a customer's oven reports the temperature points it measured. It does not write "Oven calibrated" unless ovens are named in its scope.

Does NABL 129 add anything for your discipline?

It can. NABL 129 (Issue 01, Amendment 01, 1 November 2021) sets specific criteria for mechanical, fluid flow, radiological, electro-technical and thermal calibration, and some of its chapters add certificate content.

Its chapter on weights, for example, says the certificate shall state, as a minimum, the conventional mass of each weight, whether the weight was adjusted before calibration, its expanded uncertainty and the coverage factor. The same chapter says weight calibration is for scientific and industrial purposes, that use in commercial trading needs approval as required by the Legal Metrology department, and that this should be stated on the certificate. Check the chapter for your discipline before you settle your layout.

How should the NABL symbol and ILAC MRA mark be used on a calibration certificate?

The NABL symbol is mandatory for calibrations within your accredited scope, and not allowed for anything outside it. NABL 133 (Issue 09, Amendment 03, 3 September 2024) clause 5.1 also says a narrative reference to accreditation is not acceptable in place of the symbol on calibration certificates.

The main rules in NABL 133:

• a calibration lab's symbol carries its accreditation certificate number, CC-XXXX (clause 4.2), or the new-format number, NABLCMMYYSCXXXXX, once NABL has issued it (NABL announcement of 12 June 2026);
• non-accredited parameters go on a separate certificate, and an asterisk or any other mark to flag them is not permitted (5.1, 6.2);
• no other accreditation body's symbol may be used with the NABL symbol (6.7);
• the symbol may be used only while accreditation is valid, under the accredited name and address, and for the accredited location (5.1, 6.6, 6.8);
• reporting a value outside your accredited range, a method not in your scope, or an uncertainty smaller than your CMC counts as a claim of accreditation outside scope (15.2);
• the certificate must be authorised by a person declared to NABL as responsible for review, reporting and authorising results (17);
• misuse of the symbol leads to adverse action under NABL 216 (16.1).

NABL 142 (Issue 07, 11 January 2021) explains why this matters to your customers. A lab shows that its calibration service is covered by the ILAC Arrangement by using the NABL symbol or the combined ILAC MRA mark on the certificate. That is what a testing lab relying on your certificate for traceability looks for.

When can you use the combined ILAC MRA mark?

The NABL Accredited CAB Combined ILAC MRA Mark is voluntary. You need NABL's written permission first, by signing the agreement in Appendix A of NABL 133 (5.3, 8.2). Once you have it:

• use the mark only on certificates that contain accredited scope alone (8.3);
• place the ILAC MRA mark to the left of the NABL symbol, close to it (9.4);
• print it in black and white only (9.5);
• never put it on the calibrated instrument, or on anything other than a report or certificate (8.12, 8.13).

Can calibration labels carry the NABL symbol?

Yes, on equipment you calibrated within your accredited scope. NABL 133 clause 11 says such a label must not give the impression that NABL approved or calibrated the equipment, and must show at least:

1. the name of the accredited calibration lab;
2. the instrument identification;
3. the date of the current calibration;
4. a cross-reference to the calibration certificate.

The list does not include a due date. That fits the interval rule in the due-date section below.

Do calibration certificates need a ULR number?

Yes, for calibrations within your accredited scope. NABL's ULR clarification of 11 June 2026, revised 15 June 2026, is addressed to accredited testing labs, calibration labs and reference material producers. It says a ULR (Unique Laboratory Report) number must be mentioned on all reports, certificates and RM documents issued within the accredited scope. The ULR is in addition to your own certificate number, not a replacement for it.

NABL introduced the ULR so that the Government e-Marketplace (GeM) can match reports with sellers' claims. NABL's earlier circular of 9 November 2021 noted that GeM also accepts calibration certificates from NABL-accredited labs for equipment that is used after calibration.

What is the ULR format for a calibration certificate?

It depends on the format of your accreditation certificate number. The June 2026 clarification gives three formats. Two apply to calibration labs; the third, for five-digit TC-XXXXX numbers, is for testing labs.

The year is the year the certificate is issued. The running number starts at 00000001 and restarts every calendar year. The clarification allows hexadecimal numbering if needed.

Reading the first example: "CC1234" is the accreditation certificate number without its hyphen, "26" is 2026, and "00000312" means the 312th accredited certificate of the year.

What does the new NABLC certificate number mean?

NABL announced a new format for accreditation certificate numbers on 3 June 2026, revised 6 July 2026. For a calibration lab it looks like this, using NABL's own example:

The new accreditation certificate also refers to your old number, in the form "In lieu of CC-XXXX". Its footer shows the accreditation cycle number, and any revision is numbered.

When do you have to switch to the new format?

• Accreditation certificates with CC-XXXX numbers stay valid until NABL issues you a new-format number.
• After that, you may keep using the old number until your software is updated or your printed stationery runs out, for a maximum of six months. NABL's symbol announcement of 12 June 2026 counts the six months from the date NABL issues the accreditation certificate.
• The ULR clarification gives the same six-month transition for moving to the new ULR format.

The NABL symbol, the certificate number and the ULR all carry your accreditation number. It is simplest to change all three on the same day, so that no certificate mixes the old number and the new one.

What changed from the 2021 ULR?

NABL's circular of 9 November 2021 used an 18-character ULR, for calibration labs as well as testing labs. It was CC and four digits, the year, a location digit, an 8-digit running number and a final "F", meaning every parameter on the certificate was accredited.

The June 2026 formats have no location digit and no final letter. If your certificate template still builds an 18-character ULR, check it against the 2026 clarification.

When should a calibration certificate not carry a ULR?

• When the certificate contains any parameter outside your accredited scope. The clarification says the system "is not to be used in reports containing parameters outside the scope of NABL accreditation".
• The exemptions the clarification lists (in-house petroleum labs, air, waste water and similar environmental parameters, soil and rock, cell culture, veterinary, dope and forensic testing) are all about test reports. It lists no exemption for calibration certificates.

An amended certificate may keep the same ULR. The clarification leaves that choice to the lab.

How should measurement uncertainty be stated on a calibration certificate?

Every result on a calibration certificate needs its measurement uncertainty. On a test report, uncertainty is needed only in some cases (clause 7.8.3.1). On a calibration certificate it is always required (clause 7.8.4.1 a). NABL 143 adds that accredited calibration labs shall evaluate and report the uncertainty of all calibrations covered by their scope.

NABL's rules for stating it are in NABL 143 (Issue 04, 26 March 2021), section 6, which is in line with ILAC P14:09/2020. NABL 141 (Issue 04, 10 February 2020) gives guidance on estimating it.

1. Report the result as y ± U, with units. A table is fine, and you may add the relative uncertainty U/|y| (NABL 143, 6.2).
2. State the coverage factor and the coverage probability, with an explanatory note (6.2).
3. Give U to at most two significant digits. Round the result to the last digit of U, after all calculations are done (6.3).
4. Include short-term contributions during calibration and contributions from the customer's instrument (6.4). This is why the uncertainty you report is usually larger than your CMC.
5. Never report an uncertainty smaller than the CMC in your scope, in any form (6.5).
6. Use SI units. If you must report in a non-SI unit, give the SI value alongside (note to 6.6).

What should the coverage factor note say?

NABL 143 suggests this wording: "The reported expanded measurement uncertainty is stated as the standard measurement uncertainty multiplied by the coverage factor k such that the coverage probability corresponds to approximately 95 %."

NABL 141 (clause 3.6.1) gives the usual case with the number filled in: the combined standard uncertainty multiplied by the coverage factor k = 2, which for a normal distribution corresponds to a coverage probability of about 95%. When the effective degrees of freedom are too few for k = 2, NABL 141 Appendix B takes k from the t-distribution, and the note says so (clause 3.6.2).

A worked example

One point from a fictitious digital thermometer certificate:

The uncertainty has two significant digits, and the error is rounded to the same last digit. Written as ±0.1487 °C, it would have too many digits. If your scope gives a CMC of 0.20 °C at this point, ±0.15 °C could not be reported at all: the reported uncertainty would have to be at least 0.20 °C.

Should a calibration certificate show a due date?

Not unless the customer asks. ISO/IEC 17025 clause 7.8.4.3 says a calibration certificate or calibration label shall not contain any recommendation on the calibration interval, except where this has been agreed with the customer. NABL 160A (Issue 01, 2 January 2026) says the same: if the customer wants the interval on the certificate and label, it is given as agreed with them.

In practice, ask at inward whether the customer wants a due date, record the answer on the job, and print the date only on those certificates. Choosing the interval is the customer's decision. If customers ask you how to choose one, our guide to instrument calibration in a testing lab covers how labs set intervals using ILAC-G24.

Your own master standards are different. Clause 6.4.7 requires a calibration programme for your equipment, and clause 6.4.13 requires records of each item's due date or interval. NABL 129 gives recommended calibration intervals for reference equipment in each discipline, based on the present practice of NPL, India. It lets labs vary them using ILAC G24 and a risk analysis based on calibration history.

How do you state conformity on a calibration certificate?

A statement of conformity says whether the instrument is within a tolerance, for example "in tolerance" or "pass". ISO/IEC 17025 clause 7.1.3 says that when a customer asks for one, the specification and the decision rule must be clearly defined. Unless the rule is part of the specification, it must be communicated to and agreed with the customer.

Clause 7.8.6 then requires the lab to document its decision rule, taking account of the risk of false acceptance and false rejection. The certificate must state:

• which results the statement applies to;
• which specification, or part of it, is met or not met;
• the decision rule applied, unless it is part of the specification.

ILAC G8:09/2019 describes the common rules. Under simple acceptance, the acceptance limit is the tolerance limit. When a result sits right at the limit, the chance that the true value is outside it can be as high as 50%. A guard band moves the acceptance limit inside the tolerance, often by the expanded uncertainty (w = U). A rule can be binary (pass or fail), or non-binary, adding "conditional pass" and "conditional fail" for results inside the guard band.

Here is an illustration. A customer's pressure gauge has a tolerance of ±0.10 bar. At 6 bar its error is +0.08 bar, and your expanded uncertainty is 0.03 bar.

• Under simple acceptance, the gauge passes.
• With a guard band of w = U, the acceptance limit is 0.07 bar. A binary rule gives "fail" and a non-binary rule gives "conditional pass".

The same reading gives different statements, which is why the certificate must name the rule.

Opinions and interpretations (clause 7.8.7) are different from conformity statements. Only authorised people may release them, the basis must be documented, and they must be clearly identified on the certificate.

How do you amend an issued calibration certificate?

An amended certificate must identify every change and, where appropriate, give the reason. ISO/IEC 17025 clause 7.8.8 sets three rules.

1. Any change to an issued certificate is clearly identified, with the reason where appropriate (7.8.8.1).
2. An amendment after issue is a further document that includes the statement "Amendment to Report, serial number..." or equivalent wording, such as "Amendment to Calibration Certificate No. ...", and meets every requirement of the standard (7.8.8.2).
3. A complete new certificate is uniquely identified and refers to the original it replaces (7.8.8.3).

Say an engineer typed the wrong serial number for a customer's micrometer. Either issue an amendment that names the certificate, shows the corrected serial number and gives the reason, or issue a complete new certificate with its own unique number that refers to the one it replaces.

NABL's ULR clarification lets the amended certificate keep the same ULR. Keep the original as well: clause 7.8.1.2 requires all issued reports to be kept as technical records, and clause 7.5.2 requires changes to be traceable to the original.

What calibration certificate mistakes do assessors raise?

This is not a published NABL list. Each item below breaks a specific written requirement, which is why it becomes a finding when an assessor spots it.

1. An uncertainty smaller than the CMC in your scope (NABL 143, 6.5; NABL 133, 15.2).
2. A point outside your accredited range, or a method not in your scope, reported under the NABL symbol (NABL 133, 15.2).
3. Non-accredited parameters on the same certificate, flagged with an asterisk (NABL 133, 5.1 and 6.2).
4. "Equipment calibrated" when your scope covers only the parameter (NABL 133, 6.14).
5. A due date printed when the customer did not ask for one (clause 7.8.4.3).
6. No coverage factor or coverage probability, or an uncertainty given to three or four significant digits (NABL 143, 6.2 and 6.3).
7. No traceability statement, or no environmental conditions where they affect the result (clause 7.8.4.1 b and c).
8. Only "as left" readings for an adjusted instrument, when "as found" readings were available (clause 7.8.4.1 d).
9. A ULR that does not match the June 2026 formats, or a ULR on a certificate with non-accredited parameters.
10. A certificate authorised by someone not declared to NABL for that purpose (NABL 133, 17).
11. Calibration labels with the NABL symbol but no cross-reference to the certificate (NABL 133, 11.2).

For findings across the whole management system, see common NABL non-conformities.

How do calibration labs lose time and money without software?

None of these problems is dramatic on its own. They add up quietly, job after job.

Retyping readings. An engineer writes readings on a paper datasheet. Someone types them into an Excel sheet, then copies the results into a Word certificate. Each copy is a chance to turn 0.25 into 0.52, and the checker has to compare three documents to find it. Clause 7.11.6 of ISO/IEC 17025 requires calculations and data transfers to be checked systematically, so every retyping step adds checking work.

Uncertainty sheets that break. A common set-up is one workbook per type of calibration, copied for each job. When a master standard is recalibrated, its new values have to be updated in every copy. A formula overwritten by hand, or a range not extended for an extra point, keeps producing numbers that look right. Nobody notices until an assessor recalculates one.

Missed recall dates. When a customer asks for a due date, it sits in an issued certificate or a spreadsheet. Unless someone looks it up every month, the reminder call never happens, and the repeat job goes to whichever lab calls first.

Master standards used past their due date. A wall chart of due dates does not stop an engineer picking up an overdue master. Every certificate issued with it rests on a master whose calibration was not current. If its next calibration shows it had drifted, clause 6.4.9 requires you to examine the effect on past work under your nonconforming work procedure. That can mean contacting customers and amending certificates.

Certificate number mistakes. Two engineers working from copies of the same template can issue the same number, or skip one. The ULR running number has to restart at 00000001 every calendar year. Labs moving to the new NABLC number have to change the symbol, certificate number and ULR within six months. Done by hand, each of these is a small error that ends up in a customer's file.

For the wider costs of running a lab on spreadsheets, see the hidden cost of Excel in labs.