Calibration Drift and Interval Calculator
Enter the as found error and the months since the previous calibration for each event. The sheet calculates the drift rate, finds the worst case and recommends a calibration interval with a safety factor.
Download
- Calculator sheet with a worked example
- How to Use sheet with steps
- 24 built in formulas, no macros
- Yellow input cells, grey results
- Calculation sign off block
- Excel, Google Sheets and LibreOffice
Worked Example
These are the results of the example already filled in the workbook. Replace the yellow cells with your own data.
Calibration history
| No | Calibration date | Months since previous | Max as found error (% span) | Drift rate (% span per year) | Within tolerance | Adjusted to zero? | Remarks |
|---|---|---|---|---|---|---|---|
| 1 | 10 Oct 2022 | 12.0 | 0.10 | 0.100 | YES | Yes | |
| 2 | 12 Oct 2023 | 12.0 | 0.12 | 0.120 | YES | Yes | |
| 3 | 08 Oct 2024 | 12.0 | 0.09 | 0.090 | YES | Yes | |
| 4 | 14 Oct 2025 | 12.0 | 0.14 | 0.140 | YES | Yes | |
| 5 | 12 Oct 2026 | 12.0 | 0.11 | 0.110 | YES | Yes |
About this Calculator
Fixed calibration intervals waste effort on stable instruments and miss unstable ones. Using as found drift history gives a data based interval, which ISO 9001 and ILAC G24 encourage.
Basis: Assumes the instrument is adjusted close to zero at each calibration, so the as found error is the drift over the interval. Drift rate = |as found error| ÷ months × 12. Recommended interval = tolerance ÷ worst drift rate × 12 × safety factor, limited to 3 to 24 months. Engineering judgement required, in the spirit of ILAC G24.
How to use
- Enter tolerance, current interval and safety factor.
- For each calibration enter the date, months since the previous one and the maximum as found error.
- Read average and worst drift rates.
- Use the recommended interval as an input to your interval review.
Formulas used
- Drift rate = |as found error| ÷ months × 12.
- Recommended = tolerance ÷ worst rate × 12 × safety factor, between 3 and 24 months.
- Any out of tolerance event: recommended = half the current interval (minimum 3).
Related tools
Frequently Asked Questions
How many calibrations are needed?
At least three to judge drift; more gives better confidence.
Why a safety factor?
Drift varies, so the interval keeps a margin before tolerance is reached.
What if the instrument was not adjusted?
Then errors accumulate; use the change in error between calibrations as the drift.
Learn more on the blog
Calibration interval determination: ISO 9001 and industry best practices
A practical guide on instrumentationblog.in that explains the engineering behind this workbook.
Read Full Article