Most general-purpose test and measurement instruments are calibrated once every 12 months — but the annual default is a starting point, not a rule. The correct interval is the one that keeps the instrument within tolerance for your application between calibrations. It is set from the instrument's stability, how heavily it is used, the environment it works in, and the consequence of an out-of-tolerance reading.
The factors that decide the interval
- Manufacturer recommendation — the baseline (often 12 months).
- Stability / drift history — if past calibrations show little drift, the interval can often be extended; if it drifts, shorten it.
- Usage intensity — a multimeter used daily on a production line ages faster than one used monthly.
- Environment — heat, humidity, vibration, dust and mechanical shock accelerate drift.
- Criticality / risk — the higher the cost of a wrong measurement (safety, product release, billing), the shorter the interval.
Typical starting intervals
| Instrument type | Common starting interval |
|---|---|
| Digital multimeters, clamp meters | 12 months |
| Process & multifunction calibrators | 12 months |
| Reference thermometers / dry-wells | 12 months (references often 12–24) |
| Humidity & temperature loggers | 12 months |
| Pressure calibrators / gauges | 12 months |
| New or critical instruments | Shorten initially, then review |
These are typical defaults. Always confirm against the manufacturer's specification and your own quality system.
Let the data set the interval
The most defensible approach is to review the "as-found" results at each calibration. If an instrument repeatedly returns well within tolerance, you have evidence to extend the interval. If it returns near or beyond the limit, shorten it. This is exactly what auditors want to see: an interval justified by history, not guesswork.
What happens if you find an instrument out of tolerance
If a calibration shows the instrument was out of tolerance, you have a problem that reaches backwards: every measurement made since the last good calibration is now suspect. This is why critical instruments get shorter intervals — it limits how much work you would have to re-check. A good calibration certificate gives you the "as-found" values so you can assess the impact.
Build it into a schedule
Track due dates, keep certificates, and recall instruments before they expire — a lapsed calibration on a critical instrument can fail an audit on its own. Unitest Instruments issues recall reminders and can calibrate most common instruments with a fast turnaround at our SAC-SINGLAS accredited lab.