Instrument Accuracy Calculator (% of Span vs % of Reading)
By Santaji GadepatilinUpdated October 28, 2026
Instrument accuracy can be stated as a percentage of span, of full scale or of reading — and the difference matters most at low readings. A meter that is ±1% of full scale can be ±10% of reading at 10% of range. This instrument accuracy calculator shows the real error — see accuracy reading vs full scale and pressure transmitter accuracy specifications.
How to use the instrument accuracy calculator
Enter the instrument’s range (LRV and URV), the current reading and the accuracy as a percentage of span. You get the error band in engineering units, the same error expressed as a percentage of reading, and the range in which the true value could lie.
Accuracy formulas
Error = ±(% of span) × (URV − LRV) · % of reading = Error ÷ Reading
±1% of full scale vs ±1% of reading (0–100 range)
| Reading | Error at ±1% FS | as % of reading | Error at ±1% of reading |
|---|---|---|---|
| 100 | ±1.0 | ±1.0% | ±1.00 |
| 75 | ±1.0 | ±1.3% | ±0.75 |
| 50 | ±1.0 | ±2.0% | ±0.50 |
| 25 | ±1.0 | ±4.0% | ±0.25 |
| 10 | ±1.0 | ±10.0% | ±0.10 |
| 5 | ±1.0 | ±20.0% | ±0.05 |
Typical accuracy specifications
| Instrument | Typical accuracy |
|---|---|
| Smart pressure / DP transmitter | ±0.04–0.1% of span |
| Industrial pressure gauge (class 1.0) | ±1% of full scale |
| Electromagnetic flow meter | ±0.2–0.5% of reading |
| Coriolis flow meter | ±0.1% of reading |
| Vortex flow meter | ±0.75–1% of reading |
| Variable area meter | ±1–2% of full scale |
| Pt100 Class A | ±(0.15 + 0.002 × t) °C |
Total probable error
Several independent errors (transmitter, ambient temperature effect, static pressure effect, card) combine as the square root of the sum of squares — not a simple sum. For calibration checks see calibration error calculation and accuracy vs precision.
Frequently asked questions
What is the difference between % of span and % of reading?
Accuracy in % of span (or full scale) gives a fixed error in engineering units across the whole range. % of reading scales with the measured value, so it stays tight at low readings.
How do I convert % of span to % of reading?
Error (units) = % of span × span; then % of reading = error ÷ reading. ±0.5% of a 0–100 span at a reading of 25 is ±0.5 units, or ±2% of reading.
Is accuracy the same as precision?
No. Accuracy is closeness to the true value; precision is repeatability. See accuracy vs precision.
Does rangeability change accuracy?
Yes. Re-ranging a transmitter to a smaller span usually increases the error as a % of the new span — check the turndown effect in the datasheet.