How do I calculate a calibration interval?

Start from the instrument's own as-found history, which is the strongest evidence you have. Work out how fast the error is drifting per year, compare that with how much of the tolerance you are willing to let drift consume, and the interval is the time between those two. The arithmetic is straightforward; the judgement is in the margin you choose and in the consequence of the instrument being wrong, neither of which a calculation can settle for you.

Is there a standard calibration interval?

No. ISO/IEC 17025 requires the interval to be justified and reviewed rather than fixed at a particular figure, and ILAC-G24 / OIML D 10 describes several methods for adjusting intervals on evidence without prescribing the numbers. Annual is the most common starting point in GxP use, but it is a starting point and not a requirement, and an interval nobody can explain is the weakness auditors find.

Why does adjustment change how drift is calculated?

Because adjustment resets the error. If an instrument is adjusted back to zero at every calibration, its as-found results might read 0.05 every year — a flat line with a slope of nothing — while the instrument is in fact consuming a fixed share of its tolerance every single year. Fitting a line through those as-found points would report no drift at all. What actually drifts is the gap from each as-left to the next as-found, so that is what gets measured when the history shows adjustment.

Can I extend a calibration interval?

Yes, where the as-found history supports it and the risk assessment agrees. A run of in-tolerance results with small, steady drift is exactly the evidence that justifies lengthening an interval, and documenting that reasoning is what makes the longer interval defensible. What does not justify it is that nothing has gone wrong yet, or that the instrument is inconvenient to send away.

What happens if an instrument comes back out of tolerance?

The interval is the lesser half of the problem. An out-of-tolerance as-found result means every measurement made with that instrument since its previous calibration is in doubt, so it triggers an impact assessment on that work as well as a shorter interval. This tool flags any such result in the history you enter, because it changes what you have to do next.

What is the difference between as-found and as-left?

As-found is the error the laboratory measured on arrival, before touching anything — it is the evidence about how the instrument behaved in service, and it is the number that matters for setting intervals. As-left is the error after any adjustment, and it is where the next interval starts from. A certificate that reports only as-left has thrown away the information you need to set an interval at all, which is worth checking before you choose a laboratory.

Calibration interval calculator from as-found history

Paste an instrument's as-found history and see how fast it is actually drifting, how much of its tolerance that consumes, and how long the evidence supports trusting it. The strongest input to an interval is the instrument's own record — this reads it.

Calibration historyone per line: date, as-found error, as-left error (optional)

Tolerancepermitted error, as ± in °C± °CUnitwhatever the certificate reports

How much of the tolerance you will let drift consume

Your decision, not a standard’s. No standard sets this figure — ILAC-G24 describes methods for adjusting intervals on evidence and leaves the acceptance figures to you. Pick the margin your quality system can defend, and the projection follows from it.

● as-found○ as-left- - fitted drift- - conservative

Drift rate

+0.1246°C/yr

as-left to next as-found

Tolerance used

standing at 0.000 °C

Interval in use

12 months

mean of 4 intervals

Projected to margin

2.3 years

3.2 years at the fitted rate

At the conservative drift rate this instrument reaches 80% of its tolerance in 2.3 years, which supports a schedule step of 24 months against the 12 months it is on now — but this instrument is not drifting at a steady rate, so that projection is not evidence for a longer interval. See below.

The conservative figure is the one to schedule on. It uses the drift rate plus its 95% expansion for 5 calibrations, so it answers “how fast could this instrument plausibly be drifting?” rather than “how fast does the best-fit line say it is?” With few records those two are far apart, and the gap is the honest cost of thin history.

The drift rate has risen in every interval, from 0.0886 to 0.1592 per year — 1.8 times. This instrument is not drifting at a steady rate, so a projection from the average understates how fast it is now moving. Use the most recent interval's rate as the planning figure, and treat an accelerating instrument as one heading for replacement rather than a longer interval.

Drift in each interval, as-left to the next as-found

Calibration history with the interval and drift rate between each pair
From	To	Interval	Drift	Rate
2021-03-02	2022-03-08	12 months	0.090 °C	+0.0886 °C/yr
2022-03-08	2023-02-27	12 months	0.110 °C	+0.1129 °C/yr
2023-02-27	2024-03-04	12 months	0.140 °C	+0.1378 °C/yr
2024-03-04	2025-03-06	12 months	0.160 °C	+0.1592 °C/yr

This instrument is adjusted at each calibration, so the drift that matters is from what it was left at to what it was next found at. A line through the as-found results of an adjusted instrument mostly measures how well it was adjusted each time, which is why this tool does not fit one.

What this does not decide

There is no standard interval, and this does not invent one. ILAC-G24 / OIML D 10 describes several methods for setting intervals on evidence, and every one of them leaves the acceptance figures to the laboratory. The margin above is a control for that reason: the projection is arithmetic, the margin is your policy, and the interval is your decision.
Drift history is the strongest input, not the only one. Severity of use, transport, the environment the instrument lives in, the manufacturer’s stability specification and — above all — the consequence of an undetected error all belong in the decision. An instrument whose failure would invalidate a year of batch records earns a shorter interval than this projection alone suggests.
A projection assumes the past continues. It cannot see a knock in transit, a change of operator, a new location or the point where an ageing sensor starts to turn. That is what the conservative rate is for, and why a run of results near the margin deserves a shorter interval than the arithmetic alone would give.
Write the rule down. The weakness of most interval policies is not that they are wrong but that they cannot be stated — everything is annual, with exceptions nobody can explain. A rule based on as-found history, with the margin written into the SOP, is defensible in a way that habit is not.

A projection from the history you entered — not a compliance decision, and not a statement that any interval is acceptable.

Runs entirely in your browser · nothing you enter is sent to a server

How the calculation works

01Drift is fitted by least squares through the as-found results when the instrument is never adjusted, so the slope of that line is the drift rate.
02When the history shows adjustment, drift is measured per interval instead — from each as-left to the following as-found — because a line through the as-found results of an adjusted instrument mostly measures how well it was adjusted each time.
03The projection starts from the last as-left for an adjusted instrument and from the fitted value at the last date otherwise, so a single noisy reading does not set the whole schedule.
04A conservative projection uses the drift rate plus its 95% expansion for the number of records available, which answers how fast the instrument could plausibly be drifting rather than what the best-fit line says.
05ILAC-G24 / OIML D 10 describes methods for adjusting intervals on evidence and leaves the acceptance figures to the laboratory, so the margin of tolerance is a control on the page rather than a constant in the code.

Limitations

This projects; it does not decide. No standard sets a calibration interval, and nothing here states that any interval is acceptable — the margin is your policy and the schedule is your decision.
Three calibrations is the fewest it will work from and it is thin evidence. A rate from three points carries a wide confidence interval, which is why the conservative projection is so much shorter than the fitted one.
A projection assumes the past continues. It cannot see a knock in transit, a change of location or the point where an ageing sensor begins to turn, and drift is often not linear over long periods.
As-found history is the strongest input to an interval but not the only one. Severity of use, environment, the manufacturer's stability specification and the consequence of an undetected error all belong in the decision.
Dates must be written as YYYY-MM-DD. A date like 01/02/2021 is January in one country and February in another, so lines it cannot read unambiguously are skipped and counted rather than guessed at.

Frequently asked questions

How do I calculate a calibration interval?: Start from the instrument's own as-found history, which is the strongest evidence you have. Work out how fast the error is drifting per year, compare that with how much of the tolerance you are willing to let drift consume, and the interval is the time between those two. The arithmetic is straightforward; the judgement is in the margin you choose and in the consequence of the instrument being wrong, neither of which a calculation can settle for you.
Is there a standard calibration interval?: No. ISO/IEC 17025 requires the interval to be justified and reviewed rather than fixed at a particular figure, and ILAC-G24 / OIML D 10 describes several methods for adjusting intervals on evidence without prescribing the numbers. Annual is the most common starting point in GxP use, but it is a starting point and not a requirement, and an interval nobody can explain is the weakness auditors find.
Why does adjustment change how drift is calculated?: Because adjustment resets the error. If an instrument is adjusted back to zero at every calibration, its as-found results might read 0.05 every year — a flat line with a slope of nothing — while the instrument is in fact consuming a fixed share of its tolerance every single year. Fitting a line through those as-found points would report no drift at all. What actually drifts is the gap from each as-left to the next as-found, so that is what gets measured when the history shows adjustment.
Can I extend a calibration interval?: Yes, where the as-found history supports it and the risk assessment agrees. A run of in-tolerance results with small, steady drift is exactly the evidence that justifies lengthening an interval, and documenting that reasoning is what makes the longer interval defensible. What does not justify it is that nothing has gone wrong yet, or that the instrument is inconvenient to send away.
What happens if an instrument comes back out of tolerance?: The interval is the lesser half of the problem. An out-of-tolerance as-found result means every measurement made with that instrument since its previous calibration is in doubt, so it triggers an impact assessment on that work as well as a shorter interval. This tool flags any such result in the history you enter, because it changes what you have to do next.
What is the difference between as-found and as-left?: As-found is the error the laboratory measured on arrival, before touching anything — it is the evidence about how the instrument behaved in service, and it is the number that matters for setting intervals. As-left is the error after any adjustment, and it is where the next interval starts from. A certificate that reports only as-left has thrown away the information you need to set an interval at all, which is worth checking before you choose a laboratory.

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