How to Determine Calibration Intervals: Methods for UK Quality Teams

"How often should we calibrate this?" is one of the most common questions a quality manager fields, and one of the most commonly answered wrong — with a blanket "annually" applied to everything from a precision reference standard to a workshop tape measure. The honest answer is that the right interval depends on the instrument, and that there is a recognised method for setting it and, just as importantly, for reviewing it. This guide covers how to determine calibration intervals properly.

It pairs with the question of how often test equipment should be calibrated, which looks at typical intervals by equipment type; this post is about the method for choosing and adjusting any interval.

There Is No Legal Interval — and That Is the Point

It surprises people that the standards do not print a calibration frequency. ISO 9001:2015 Clause 7.1.5 requires measuring equipment to be calibrated or verified "at specified intervals, or prior to use" against traceable standards — but it leaves the interval to you to set and justify. The same is true for accredited laboratories under ISO/IEC 17025:2017: the laboratory determines its own intervals as part of controlling its equipment.

This is deliberate. A single fixed interval cannot be right for every instrument. The standard pushes the responsibility onto you to choose an interval with a rational basis — which is more work than copying a default, but defensible at audit in a way an arbitrary annual cycle is not.

The Recognised Method: ILAC-G24

The internationally recognised guidance on this is ILAC-G24:2022, Guidelines for the determination of recalibration intervals of measuring equipment, published by the International Laboratory Accreditation Cooperation. It is the document UKAS-accredited laboratories and quality teams reference when justifying their intervals. It sets out two things: how to choose an initial interval, and how to review it over time.

Choosing the Initial Interval

For a new instrument, with no calibration history of your own, the initial interval is an informed estimate drawn from:

• The manufacturer's recommendation — a sensible starting point, though it is a generic figure for typical use, not your use.
• How the instrument is used — frequency of use, the environment (a clean lab versus a dusty shop floor), and how it is handled all affect drift.
• The consequence of it drifting out of tolerance — an instrument whose failure would invalidate critical product measurements warrants a shorter interval than one whose failure is low-impact. This is a risk judgement.
• The instrument's inherent stability — some instrument types hold calibration far longer than others.

The initial interval is your best estimate. The next step is what makes it rigorous.

Reviewing and Adjusting the Interval

ILAC-G24:2022 describes several methods for reviewing intervals using the instrument's actual calibration results. The principle common to them is straightforward: let the evidence move the interval.

• If an instrument is repeatedly found well within tolerance at each calibration — comfortably inside its permissible error — the interval can be extended, because the evidence says it is stable.
• If an instrument is found near or outside its tolerance limits, the interval should be shortened, because it is drifting faster than assumed.

The named approaches in the guidance include automatic adjustment of the interval at each calibration based on the result, control-chart methods that track an instrument's drift over time, and in-use or in-service checking between full calibrations. The guidance is explicit that it is for each laboratory to choose which method — or none — suits its needs and risk, and to evaluate whether the chosen method works. There is no mandated method; there is a mandated discipline of having one.

Why This Matters at Audit

The interval is a frequent audit topic precisely because it is so often set badly. Two failure modes recur. The first is the universal "annual" — every instrument on a one-year cycle regardless of use or risk, with no basis recorded. The second is the interval set once at purchase and never reviewed, even after an instrument has been found out of tolerance. An assessor looking at Clause 7.1.5 wants to see that your intervals have a rational basis and that you review them against history. A static, unjustified interval is a soft target.

The practical consequence of getting intervals wrong runs both ways. Too long, and you risk an out-of-tolerance instrument being used on real measurements — the out-of-tolerance event, and the out-of-tolerance investigation that follows, is the expensive outcome. Too short, and you over-calibrate stable instruments, paying for calibrations you do not need. Interval review, done from history, narrows both.

Doing It Without a Spreadsheet Nightmare

Reviewing intervals from calibration history is only practical if the history is to hand. In a spreadsheet, pulling each instrument's run of past results to judge whether to extend or shorten its interval is a manual job most teams never get around to — so intervals never get reviewed, which is how the static-interval audit finding happens. Software that holds each instrument's full calibration history, and surfaces whether recent results sit comfortably within tolerance, makes the review a task you can actually do. This is part of why teams move from tracking calibration in Excel to dedicated tools.

How CalProof Fits

CalProof holds each instrument's interval, due date, and full calibration history in one record, so when it is time to review an interval the evidence is in front of you rather than scattered across certificate PDFs. You set the interval per instrument, the recall reminders fire against it, and the history is there to justify extending or shortening it at your next review. The compliance dashboard shows status across the fleet at a glance.

GBP pricing from £29 a month. No per-user fees on Pro and above. UK data hosting. No long-term contract.

To see the shape of the output, the sample audit pack is ungated and downloads as a single PDF.

Sources

• ISO 9001:2015 — Quality management systems — Requirements
• ISO/IEC 17025:2017 — General requirements for the competence of testing and calibration laboratories
• ILAC-G24:2022 — Guidelines for the determination of recalibration intervals of measuring equipment (International Laboratory Accreditation Cooperation)

This guide applies to UK manufacturers and laboratories under ISO 9001 or ISO/IEC 17025. It is general guidance based on the published standards and ILAC guidance; the intervals that apply to your equipment depend on your use, risk, and your auditor's interpretation. Verify against your certification body or UKAS assessor. This is not legal or compliance advice.