Automation & Instrumentation
4-20 mA Scaling Calculator
Convert an instrument’s current signal into engineering units, or find the mA signal for a desired process value. Use a preset pressure, temperature or flow range, then adapt the endpoints and unit to your transmitter.
Your estimate
Within configured span (0–100%). This does not verify sensor accuracy or hardware health.
- Engineering value
- 5 bar
- Signal current (mA)
- 12 mA
- Percentage of span
- 50 %
- Normalized value
- 0.5
4 mA maps to the first endpoint and 20 mA to the second. Negative and reversed engineering ranges are supported. Out-of-range readings are extrapolated; device-specific fault thresholds are not inferred.
Within configured span (0–100%). This does not verify sensor accuracy or hardware health.. Engineering value: 5 bar. Signal current (mA): 12 mA. Percentage of span: 50 %. Normalized value: 0.5
4–20 mA scaling formula
For a linear transmitter, normalized span n = (mA − 4) / 16. Engineering value EU = EUmin + n × (EUmax − EUmin), and percentage of span = 100 × n. The 16 mA denominator is the difference between 20 and 4, not the full 20 mA signal.
For reverse conversion, n = (EU − EUmin) / (EUmax − EUmin), then mA = 4 + 16 × n. The two engineering endpoints must differ. Negative and reversed ranges are supported: the first endpoint always maps to 4 mA and the second to 20 mA.
Live zero and transmitter range
The 4 mA lower endpoint is a live zero: the loop carries current at the bottom of the measurement range. In two-wire systems that current also supports transmitter electronics. A lost loop can therefore be distinguished from a valid lower-range signal, subject to device diagnostics.
Four mA does not necessarily mean zero physical quantity. On a −50 to 150 °C transmitter it means −50 °C; 12 mA is the midpoint, 50 °C. The transmitter range and the PLC engineering endpoints must match.
PLC analog input and engineering units
An analog input module converts measured current to raw counts. Configure the input for the actual signal type and use the module’s documented counts for the 4 mA and 20 mA endpoints. The PLC analog scaling calculator handles that raw-count step.
Unit text labels the numeric result; it does not convert units. Enter both endpoints in the same unit. These equations assume a linear signal. A square-root-extracted flow signal or another nonlinear transmitter characteristic needs the appropriate transfer function.
Under-range and over-range conditions
The calculator extrapolates instead of clamping. A 3 mA signal represents −6.25% of span; 21 mA represents 106.25%. This helps expose configuration errors or readings beyond the calibrated measurement interval.
An out-of-range calculation is not a diagnosis. Devices may use particular currents for faults or saturation, and a value within 4–20 mA does not prove the measurement is healthy. Check transmitter documentation and module diagnostics before using the value in a control or safety decision.
Examples
Pressure midpoint
- Input
- 12 mA, 0–10 bar
- Result
- 5 bar; 50% span; normalized value 0.5.
Pressure reverse conversion
- Input
- 75 bar on a 0–100 bar range
- Result
- 16 mA; 75% span.
Temperature midpoint
- Input
- 12 mA, 0–100 °C
- Result
- 50 °C; 50% span.
Negative temperature range
- Input
- 8 mA, −50 to 150 °C
- Result
- 0 °C; 25% span.
Flow near full scale
- Input
- 16 mA, 0–1000 L/min
- Result
- 750 L/min; 75% span.
Frequently asked questions
What does 12 mA represent?
It is 50% of the 4–20 mA span. For 0–10 bar it means 5 bar; for −50 to 150 °C it means 50 °C.
Why does 4 mA equal zero?
It represents 0% of configured span, not always zero engineering units. Live zero helps distinguish the bottom of the measurement range from loss of loop current.
How do I scale 4–20 mA in a PLC?
Determine which raw counts represent 4 and 20 mA for the configured module. Normalize the raw value between those counts, then map the normalized result to the transmitter’s engineering endpoints.
How do I convert pressure to mA?
Select Engineering units → mA, enter the calibrated pressure endpoints, and enter the desired pressure. For 0–10 bar, 7.5 bar gives 16 mA.
Can the engineering range start below zero?
Yes. A −50 to 150 °C range has a span of 200 °C. The lower endpoint maps to 4 mA regardless of its sign.
Are out-of-range readings clamped?
No. They are displayed with an explicit below-span or above-span message. Hardware fault thresholds must be interpreted from the device documentation.
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