Calibration Plan: What to Put in One

A calibration plan is the document that says how your organisation keeps its measuring equipment fit to measure: what is in scope, who is responsible, how intervals are set, where calibration comes from, and what happens when an instrument fails. It is not the same thing as the spreadsheet of due dates — that is the schedule, and it is one part of the plan. Most organisations have the schedule. The plan is what an assessor asks for when the schedule looks like it was set by guesswork.

This guide covers what goes in one, what the published guidance actually requires, and the difference between a plan that exists and a plan that holds up.

The Requirement, and Where It Is Actually Written

It is worth being precise about this, because a lot of what is written about calibration plans asserts a requirement without pointing at one.

There is no UK statute that demands a calibration plan by name. The obligation comes through the quality standard you work to, and the clearest published statement of it sits in UKAS policy rather than in the standards themselves. TPS 41, Edition 6, the UKAS policy on metrological traceability, puts it in one sentence at section 3.1:

"In order to maintain traceability on a continuous basis, reference standards and measuring equipment shall be subject to further calibrations on an ongoing basis, hence the establishment of a calibration programme is necessary."

That is the whole logic. Traceability is not a one-off property you acquire at purchase — it decays, so it has to be renewed on a cycle, and a cycle has to be organised. The programme is the organising. If you want the underlying idea in more depth, metrological traceability covers why the chain has to be unbroken in the first place.

The word UKAS uses is "programme". "Plan", "policy", "procedure" and "schedule" all get used interchangeably in practice, and no assessor will fail you on the noun. What they will look at is whether the thing exists, whether it covers the ground below, and whether anyone follows it.

What Goes In

Seven sections cover it. This is not a prescribed structure — there isn't one — but a plan missing any of these tends to get questions.

1. Scope: what is in, and what is deliberately out. The list of equipment covered by the plan, and — the part usually missing — a statement of what is excluded and why. Not every instrument in a building needs calibrating, and a plan that silently omits things looks like an oversight rather than a decision. Writing the exclusions down converts it into a judgement you can defend.

2. Responsibilities. Who owns the plan, who books or performs calibrations, who reviews the returned results, and who has authority to accept an instrument back into service or quarantine it. On a small team this may be two names. Name them anyway; "the quality department" is not a person.

3. How intervals are set. Not the intervals themselves — those live in the schedule — but the basis on which they are chosen and changed. This is the section that most often does not exist, and it is the one that turns an arbitrary annual cycle into a defensible one. The six factors below are the ones UKAS names, and it is explicit that they are not the only ones; how to determine calibration intervals covers the review methods in full.

4. Sources of calibration. Where calibration is obtained and why that source is acceptable — an accredited laboratory, a national measurement institute, or your own in-house calibration against traceable references. TPS 41 section 2 sets out which sources establish valid traceability and which need additional evidence. If you calibrate anything yourself, in-house versus outsourced calibration covers what UKAS expects of that arrangement.

5. What happens when something fails. The out-of-tolerance route: quarantine, impact assessment on measurements already taken, corrective action. This is the section that gets used under pressure, so it should be short enough to follow on a bad day. The out-of-tolerance procedure covers the sequence.

6. Records. What is recorded for each calibration, where certificates are held, and how long everything is kept. How long to keep calibration records covers the retention question honestly — neither standard prints a number.

7. Review of the plan itself. When the document is revisited, by whom, and what triggers an unscheduled review.

The Six Factors UKAS Names

The interval section is where plans are weakest, because "annually" is easy to write and hard to justify. TPS 41 section 3.1 says the intervals "will depend on various factors, including but not limited to" the six below — so it is a floor rather than a ceiling, but a usefully concrete one to write your reasoning against:

• The measurement uncertainty required
• The past history of the equipment, including the results of calibrations and the frequency of any necessary maintenance
• The frequency of use of the equipment
• The frequency of cross-checking against other equipment, or of intermediate checks
• The recommendations of the manufacturer
• The environmental conditions to which the equipment is exposed, including any effects due to transportation

Note what the list is doing. Only one of the six is the manufacturer's recommendation — the default most plans lean on entirely. The other five are about your equipment in your use. Two instruments of the same model, one on a temperature-controlled bench and one carried between sites in a van, do not have the same interval, and the transportation clause in the last factor says so explicitly.

You do not need a paragraph per factor per instrument. A plan that groups equipment into a few classes and gives the reasoning per class is proportionate and holds up fine. What does not hold up is a schedule of identical intervals with no reasoning anywhere.

What Separates a Plan That Holds Up

Three things, and none of them is length.

It matches what actually happens. The most common finding is not a missing plan but a plan that describes a process nobody follows — a 30-day quarantine rule when instruments in practice go back into use the same afternoon. A shorter plan that is true beats a thorough one that is fiction.

The reasoning is visible. An assessor is not checking that you picked the right interval. They are checking that there is a basis for it and that you can say what it was. Recording "12 months — manufacturer recommendation, light bench use, stable history since 2024" against a class of instruments takes a line and answers the question before it is asked.

It changes. A plan dated three years ago with no revision history says the intervals have never been reviewed, which is the finding, not the plan. TPS 41 section 3.2 is direct about the trigger:

"When a fresh calibration has been obtained the data provided should be reviewed in order to confirm that the declared performance is still met. It may be necessary to reconsider the calibration interval or the suitability of the equipment in accordance with the outcome of such a review."

So the plan is not really an annual document. It is a document that should move whenever a calibration result tells you something you did not already know — an instrument coming back near its limits, or one that has sat comfortably inside tolerance for four cycles and is being calibrated more often than it needs. For what an assessor looks at more broadly, the calibration audit checklist covers the wider ground.

Where Plans Go Stale

The mechanism is the same everywhere. The plan is written once, properly, usually for an audit. The schedule then lives in a spreadsheet that drifts — instruments get added without a class or a basis, one person's tab diverges from another's, and the reasoning that justified the intervals exists only in the head of whoever wrote it. Two years later the document still says intervals are reviewed against calibration history, and nobody has opened a certificate PDF since.

The review step is what fails first, because it is the one that requires pulling an instrument's past results together. If pulling one instrument's history together is a slow, manual job, the review does not happen. Teams making this switch usually come from tracking calibration in a spreadsheet, where the history exists but is not usable at the moment the review is due.

How CalProof Fits

CalProof holds the interval, the due date and the full calibration history against each instrument, so the review the plan promises is something you can actually do — the past results are on the record when you are deciding whether to lengthen or shorten an interval. Recall reminders fire against the interval you set, and the dashboard shows calibration status across the fleet without assembling it.

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If you are building the schedule side of the plan first, the calibration schedule generator produces one from your equipment list, and the calibration schedule template is a spreadsheet with the due-date logic already in it. To see the shape of the output, the sample audit pack is ungated and downloads as a single PDF.

Sources

• UKAS TPS 41, Edition 6 (December 2022) — UKAS policy on metrological traceability (sections 2, 3.1, 3.2)
• United Kingdom Accreditation Service

This guide applies to UK manufacturers and laboratories working to ISO 9001 or ISO/IEC 17025. It is general guidance based on published UKAS policy; what your own plan needs depends on your scope, your equipment and your assessor's interpretation. Verify against your certification body or UKAS assessor. This is not legal or compliance advice.