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calibration interval

With the pandemic we are experiencing, affecting every industry in the world, one strategy to avoid an overdue instrument and ensure an updated calibration program is to extend the calibration due date or calibration interval of our instruments.

But doing this needs a planned strategy to still ensure confidence and reliability of our instruments. This strategy is to implement a calibration interval analysis procedure.

Calibration interval or frequency of calibration is one of the most asked questions. Below are the actual questions that I received:

1. Is a recalibration a requirement?
2. How can we do interval analysis and how long can we extend?
3. Is there a standard that controls the interval between each calibration process? 
4. Is there a standard guide that is set to be followed?
5. Is there a set of rules for the frequency of calibration of an instrument?
6. It is difficult to understand the logic behind the 12-month interval, shouldn’t this depend on the number of usages?

I have read a lot of documents online, and honestly, most are complicated and very technical for a beginner to understand.  I will present the simplest and the one that makes more sense to me and I hope you can understand it as well.

In this post, I will present a simple method with an example that you can follow as a guide in order to analyze and provide clear details about why you need to extend or reduce the calibration interval of your instruments.

Furthermore, I will also present the following:

1. What is a calibration Interval?
2. Why Do We Need to Determine Calibration Frequencies of Instruments
3. How to Determine Calibration Interval of Instruments

a. 4 Major Objectives For the Implementation of Calibration Interval Analysis
b. How to Establish the Initial Calibration Interval
c. How to Determine the Fixed Calibration Interval

> Example of Calibration Interval Calculation of Pressure Test Gauge – Method Implementation Procedure

4. References Guides that We can Use to Perform Other Methods of Analysis
5. Some Techniques that I observe others are doing in relation to calibration Interval
6. Conclusion

I hope that in this post, all those questions will be answered. 

Let us start with the basics, Continue to read on….

What is A Calibration Interval?

Calibration interval is also known as calibration frequency, calibration period, or simply calibration due date is the number of days between scheduled calibrations.

It will answer the question,” how often should we calibrate our instruments?” Or, “how long before we calibrate our instruments again?

Having a calibration interval is a must. Saying this, laboratories are not allowed to give a calibration frequency. This is as per ISO 17025 standard, under clause, which states that  “A calibration certificate or calibration label shall not contain any recommendation on the calibration interval, except where this has been agreed with the customer.”

One of the reasons is that once the instrument is out of the lab, the lab has no control over it, therefore the calibration interval is not guaranteed. 

The user should determine what are the Calibration Intervals needed for his instruments. This will now become part of the In-house Calibration program that should be properly documented.

Any instruments that will not fall under the calibration Interval analysis may fall under ‘calibration-not-required’ status. Check my other post in this link >> calibration-not-required implementation

Some Reasons Why Do We Need to Determine Calibration Frequencies of Instruments

We need to determine the Calibration Frequency in order to satisfy below reasons:

1. To save cost. – by calibrating less frequently because of a longer calibration interval but ensuring reliability.
2. To satisfy requirements of customer or regulatory bodies
3. The need to perform recalibration –The reason why we “calibrate” also applies to why we “recalibrate”

a. The need to recalibrate.  Below are some technical reasons why we recalibrate:

> Because of drift
> To detect any calibration problems before it affects quality
> Exposure to harsh environment
> Over usage
> To assure accuracy and reliability
> To ensure traceability

4. Compliance to the Requirements of ISO Standards – under ISO 17025:2017 and ISO 9001:2015 Standards which  controls and requires calibration interval analysis

a. Calibration Frequency requirements  As Per ISO 17025;2017 Standards are for:

i. Establishing calibration program for calibrated Instruments

> As per clause 6.4.7 The laboratory shall establish a calibration program, which shall be reviewed and adjusted as necessary in order to maintain confidence in the status of calibration.

ii. Monitor Validity of results

 > As per clause 7.7.1 The laboratory shall have a procedure for monitoring the validity of results. g) retesting or recalibration of retained items;

iii. For documentation  as part of Technical Records,

> As per clause 6.4.13 Records shall be retained for equipment that can influence laboratory activities. The records shall include the following, where applicable: e) calibration dates, results of calibrations, adjustments, acceptance criteria, and the due date of the next calibration or the calibration interval;

b. Related Calibration Interval Requirements under ISO 9001:2015, as per clause Measurement traceability

i. When measurement traceability is a requirement or is considered by the organization to be an essential part of providing confidence in the validity of measurement results, measuring equipment shall be: a) calibrated or verified, or both, at specified intervals, or prior to use, against measurement standards traceable to international or national measurement standards; 

ii. The organization shall determine if the validity of previous measurement results has been adversely affected when measuring equipment is found to be unfit for its intended purpose, and shall take appropriate action as necessary

If you read these requirements, there are no specific guides or standards that are required to follow for calibration interval analysis or method, therefore, any methods that work for you are ok. But it is better to have published documents or guides as a reference like the ILAC G24 which is free to download.

How to Determine Calibration Interval of Instruments

There are 2 types of calibration interval that we need to complete here, these are the:

  1. Initial Calibration Interval
  2. Final or Fixed Calibration Interval

4 Major Objectives For Implementing Calibration Interval Analysis

  1. To designate Initial Calibration Interval – this is the starting calibration interval based on experience and recommendations.
  2. Use to extend interval of calibration – this means that a given calibration interval will be extended for a specific period until a fixed interval is reached. This is also applicable like for example during this pandemic where calibration is difficult to access and we want to extend a little more temporarily.
  3. Use  to reduce interval of calibration – this is applicable when we encounter out of tolerance where calibration is reduced as per implementation rule. See below example presentation
  4. Use to determine the fixed calibration interval (final interval) – this is the main calibration interval that we need to achieve based on actual use which should be adequately justified

How to Establish the Initial Calibration Interval

Initial Calibration Interval means an interval that we used initially as per the decision of the expert,  this should  not be the final calibration frequency to be used, this is just our starting point. Since we do not have  yet data to justify this interval (in most cases for start up), it is known as ‘Engineering Intuition’.

I will quote this statement from ILAC G24, It states that:

The so-called “engineering intuition” which fixed the initial calibration intervals, and a system which maintains fixed intervals without review, are not considered as being sufficiently reliable and are therefore not recommended. “

The decision on where to base the initial calibration interval depends solely on you as the user. These could be based on below criteria:

1. Manufacturer Requirements – recommended by manufacturer
2. On the frequency of use – the more it is used, the shorter the calibration interval
3. Required by the regulatory bodies (example: required by the government)
4. Past experience of the user with the same type of instrument
6. Based on the criticality of use. – more critical instruments have higher accuracy or very strict tolerance, therefore shorter calibration interval
7. Customer Requirements
8. Conditions of the environment where it is being used.
9. Published Documents

Initial calibration intervals in some cases could become the ‘fixed/final calibration interval’, considering that we  already have evidence to justify why we decide this calibration interval for a specific instrument. I will share 1 method to be used for justification.. Just continue reading.

Remember that the above choices are the basis for an initial interval, our job is not finished here yet. The next move is to determine or to establish the final or ‘fixed Interval of calibration’ using a specific method or procedure. And this is where the method that I will present here will be used.

How to Determine the Fixed Calibration Interval 

My purpose here is to present and help you understand the calibration interval analysis procedure that you can implement if it suits your needs.

 A disclaimer should be noted. These are my implementation based on my understanding and experience, it may be different with your situations so a proper review should be given. It is your responsibility as the user to evaluate the effectiveness of this method if you implement and take responsibility for the consequences of the decisions taken as a result of using this method. 

Now let us start…

I will present here a modified method based on the principle of a control chart and a calendar-time method as per ILAC G24 (OIML D 10:2007). I will call this method the ‘Floating Interval Method’ because it can increase or decrease in a given condition.

To establish a fixed interval, below is the calibration interval analysis procedure:

1. Gather historical records of the instrument using its calibration report for the past 2 years or more.
2. Choose at least 3 readings that are mostly used, for example, the min, middle, and max to determine stability or drift. 
3. Use excel to summarize the readings.
4. Using the specifications and the calibration results, create a chart with control limits  (a control chart)-see below the table using excel (there are many tutorials about control chart in Google that you can follow).
5. Understand and analyze the trend.

> Based on the trend, you can decide whether to increase or decrease the calibration interval based on the Implementation Rule that you created for the Floating Interval Method. (See below presentation)

a. Implementation Rule #1: 100% Increased on the next calibration interval after the 2 years  if results are within 80% – Max 24 months interval

b. Implementation Rule #2:  50% reduced on the next calibration interval every time As-Found results are within warning interval or OUT-OF-TOLERANCE (and adding back the 50% after passing on the next recalibration) –see the flow below second scenario.

6. Determine the final interval.

Example of Calibration Interval Calculation of Pressure Test Gauge – Method Implementation Procedure

To understand better, below is an example. The instrument is a Pressure Test Gauge. We will establish its fixed calibration interval using the method ‘Floating Interval Method’ (I just made this up, you can call it anything you want)

Assuming you have a test gauge that is calibrated every 6 months as an initial interval. Then after 2 years, you have generated a performance history of 6 calibration reports. 

We can now use the results to analyse the stability or drift. We will choose 3 test points, the min, mid and max range. But in this example, I will plot only the mid range using MS Excel (If you have special software to perform this, it is much easier).

Again, for simplicity, I will use the manufacturer specifications as the tolerance limit.

A. For steps 1 to 3 above. The first four 6 months is the data for 2 years. These are the basis of the fixed interval analysis. I only choose mid-range to simplify the presentation.

Calibration Interval Summary List
Tabulated results of the calibration performance history of the Test Gauge with specifications. The first four calibration intervals (6 months) are the initial data. This will be analyzed more in a chart.

B. For steps 4 to 6, see the below presentation. These will show on “HOW TO INCREASE OR DECREASE A CALIBRATION INTERVAL’


Increasing a calibration interval analysis chart
This is the chart from the table above. It shows the stability trend of the Test Gauge with the Implementation rule for Increasing a calibration Interval. The FIRST SCENARIO table shows the flow sequence.

As a basis for increasing the calibration interval, if you observed that the readings are within the control limits you specified, it means that the instrument is stable. Based on your history, the Test Gauge is stable for 2 years, as per implementation rule, we can extend it for 1 year up to a maximum of  24 months (this is as per my implementation rule, you can adjust it but don’t overdo it.)

On the opposite side, if readings failed as per the As-found Data, then you need to decrease it. As per my decision, I will decrease it by 50%. This is I believe the most suitable range. See below presentation.

Decreasing  a Calibration Frequency Implementation
This is a Decreasing Calibration Interval Implementation as per the Implementation Rule. The analysis of the chart is presented in the SECOND SCENARIO flow sequence table.

Please take note that this is not a one size fits all method. You can change or adjust based on your own needs and implementation rule.

This will tell us the importance of recording the history of the performance of our instruments for this purpose. Past records are used to justify why we extend or reduce calibration intervals.

I also want to emphasize that I did not include the Measurement Uncertainty as part of the Decision Rule to make the presentation much simpler.

Reference Guides for Other Methods of Analysis

There are no specific standards  that provide the ‘exact’ interval to be followed as a basis for a calibration interval of any instruments in a calibration lab. All publications and guides are only recommendations and usually optional to follow or implement unless required by a regulatory body.

For those who want to explore more regarding other methods, check on below free guides:

  1. ILAC-G24 Guidelines for the determination of calibration intervals of measuring instruments
  2. GMP 11 Good Measurement Practice for Assignment and Adjustment of Calibration Intervals for Laboratory Standards
  3. 5 Best Calibration Interval Guides 

Some Techniques I Observe Others Are Doing in Relation to Calibration Interval

  1. They include a ‘calibration interval tolerance’. I smiled at first when I heard this but it’s true, the calibration interval has a tolerance limit.   An example is a calibration interval of 1 year +/- 2  months.
  2. An instrument that is being kept for a longer period and is calibrated only every time it is used, or before it will be used for measuring. 
  3. Some instruments have no calibration due date at all, considering that this kind of instrument is calibrated before use in everyday operation.
  4. There are calibration intervals based on the number of times the instrument is used. For example, we will only calibrate an instrument after it used 500 times.

As you can see, there is no one exact solution that fits all, it all depends on the user. The important thing is we have a procedure to follow that is properly planned with justifications or proper documents as evidence why we choose such a method.

You can tweak or add additional details or criteria to suit your needs.  You can gather most methods from reference guides and combine each of them. It is up to you on how you can create the best calibration interval analysis program for you.


Determining the calibration interval is one of the main requirements when it comes to instrument maintenance and quality control. And as per my exposure to different users, this is one of the most asked topics.

In establishing a Calibration interval, there are 2 main steps that we need to follow, first is to establish the Initial Interval, second is to determine the Final or Fixed Calibration Interval as we progress. Either we choose the initial interval as the fixed interval or establish a new fixed interval is ok, the important thing is we need to have data to justify our decisions. There should be a method that we follow in order to implement this. I call the method in this post, the ‘Floating Interval Method’.

In this article, I have presented:

1. What is a calibration Interval?
2. Why Do We Need to Determine Calibration Frequencies of Instruments
3. How to Determine Calibration Interval of Instruments

a. 4 Major Objectives For the Implementation of Calibration Interval Analysis
b. How to Establish the Initial Calibration Interval
c. How to Determine the Fixed Calibration Interval

> Example of Calibration Interval Calculation of Pressure Test Gauge – Method Implementation Procedure

4. References Guides that We can Use to Perform Other Methods of Analysis
5. Some Techniques that I observe others are doing in relation to calibration Interval

If you have any concerns about this method,  please feel free to comment, this is not a perfect procedure but I believe it is a good start to understand the principle behind the analysis.

If you have your own method, please share by commenting below.

If you liked this article, please share on FB, and subscribe to my email list..

Thanks and regards,


12 Responses

    | Reply

    Thank you very much for the article
    A query is missing placing uncertainties

    1.- Instead of those limits of 100% and 80%, you can place the limits of a control chart graph
    2.- For the calculation you can use the intermediate checksA query is missing placing uncertainties

    • edsponce
      | Reply

      Hi Arturo,

      You are welcome.

      Yes, you are correct for the limits. The limits as per the control chart is the second method under ILAC G24. We can also implement it as you have suggested.

      We can also use include the uncertainty results as per the decision rule of ISO 17025 to make our analysis procedure more strict. Good point for the intermediate checks to acquire more data as the basis for analysis.

      I appreciate your comments.


  2. Amiel Eleazar Macadangdang
    | Reply

    HI Sir Edwin,

    Thank you for sharing this. I can now start to plan a good calibration program for our laboratory.


    • edsponce
      | Reply

      Hi Amiel,

      You are welcome. Good luck with the implementation of calibration interval procedure on your calibration program.


  3. abdelkarim mezghiche
    | Reply

    thank you very much, clear and concise

    • edsponce
      | Reply

      Hi Abdelkarim,
      You are welcome. Thank you for reading.

  4. Wajdi adnan
    | Reply

    Thank you Mr. Edwin long live your hands support for these clarifications.

    • edsponce
      | Reply

      Hi Wajdi,
      You are welcome. I am glad you liked it.


  5. Zainal Abidin
    | Reply

    Thank for your knowledge sharing …..

    • edsponce
      | Reply

      You are welcome Zainal. Thank you for reading.

      Best regards,

  6. sharifa
    | Reply

    Thank you Edwin. This is very good information

    • edsponce
      | Reply

      Hi Sharifa,
      You are welcome. Thank you for reading.

      Thanks and regards,

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