Electronics & General Electrical
4–20 mA Loop Scaling Calculator
Convert between a 4–20 mA loop current and the process value it represents, with the percentage of span shown explicitly.
About this calculator
A 4–20 mA current loop carries an analogue measurement as a current rather than a voltage, because current is unchanged by the resistance of a long cable run. The transmitter is calibrated so that 4 mA represents the lower range value (LRV) of the measurement and 20 mA the upper range value (URV); everything in between is linear.
The 4 mA offset is deliberate and is called a live zero. Because a healthy loop never sits below 4 mA, a reading of 0 mA is unambiguously a broken wire or a dead transmitter rather than a genuine measurement of zero — a distinction a 0–20 mA loop cannot make. It is also the source of the most common arithmetic error with these loops: the span is 16 mA, not 20 mA, so the fraction of span is (I − 4) / 16 and never I / 20.
This calculator works in both directions. Give it a process value and it returns the current a correctly calibrated transmitter would produce; give it a current and it returns the process value that current represents. The percentage of span is shown as its own step, because that is the number worth checking when a reading looks wrong.
Values outside 4–20 mA are reported rather than clamped. A current below 4 mA or above 20 mA still maps to a process value under the same straight line, and seeing that value — together with the under-range or over-range flag — is usually how a calibration or wiring problem gets identified.
Design notes & common mistakes
- The span is 16 mA, not 20 mA. Dividing by 20 is the single most common error with these loops and gives an answer that looks plausible at every point except the endpoints.
- A live zero is a diagnostic feature, not an inconvenience: 0 mA means a broken loop, which a 0–20 mA signal cannot distinguish from a genuine zero reading.
- Re-ranging a transmitter changes what a given current means. A loop sheet that disagrees with the device is a calibration record nobody can trust.
Assumptions
- The transmitter's output is exactly linear between its calibrated 4 mA and 20 mA endpoints.
- The transmitter is correctly calibrated — 4 mA corresponds to the LRV entered and 20 mA to the URV.
- The loop is intact and the receiving input's impedance is within what the transmitter can drive.
When to use this calculator
Appropriate for
- Converting between a measured loop current and the process value it represents
- Checking a transmitter's calibration against a known process value
- Working out the current a transmitter should produce at a given process value during a loop check
- Teaching or learning how live-zero analogue signalling works
Not suitable for
- Sizing a loop power supply or checking compliance voltage — that is a load-resistance calculation, not a scaling one
- Interpreting a fault or alarm current from a specific device, which follows that device's documented behaviour rather than this straight line
- Establishing measurement uncertainty, which needs the sensor and transmitter accuracy specifications
- Designing or verifying a safety-instrumented function
Engineering use
Intended use. Measurement analysis and educational calculation for a linear 4–20 mA transmitter loop.
Applicable for
- Converting between loop current and process value in both directions
- Verifying the percentage of span implied by a reading
- Loop checks and calibration verification against a stated range
Does not account for
- Transmitter linearity, hysteresis and repeatability errors
- Sensor accuracy and drift, which usually dominate the total uncertainty
- Loop resistance, compliance voltage and supply limits
- Device-specific fault or alarm currents outside 4–20 mA
- Damping, response time and filtering
Verification required
- Confirm the transmitter's calibrated LRV and URV from the device or the loop sheet, not from memory
- Confirm the measured current with a calibrated meter or a known reference
- Check the device's documentation for how it behaves outside 4–20 mA before interpreting an out-of-range reading
What this calculator does not cover
- Assumes an ideal straight line; a real transmitter has its own linearity, hysteresis and repeatability errors, quoted on its datasheet.
- Does not model the sensor's own accuracy, which is usually larger than the loop's contribution and is a separate specification.
- Currents outside 4–20 mA are extrapolated along the same straight line. Real transmitters saturate a little outside the range and many implement a NAMUR-style fault current instead, so an extreme reading should be interpreted from the device's documentation rather than from this line.
- Does not account for loop resistance, compliance voltage, supply voltage or cable length — a loop can be correctly calibrated and still fail because the transmitter cannot drive the total resistance.
- Damping, response time and filtering in the transmitter are outside this model.
- As with every calculator on this site: results are preliminary and educational, are not verified for any specific installation, and must be reviewed against the applicable code edition and stamped by a licensed Professional Engineer before real-world use.
Frequently asked questions
What process value does 12 mA represent?
Exactly mid-span. The span of a 4–20 mA loop is 16 mA, so 12 mA is (12 − 4) / 16 = 50 % of span. On a 0–100 range that is 50, not 60 — dividing by 20 mA instead of the 16 mA span is the most common mistake with these loops.
Why does the signal start at 4 mA instead of 0?
So that zero current is distinguishable from a zero measurement. This is called a live zero: a healthy loop never sits below 4 mA, so a reading of 0 mA unambiguously indicates a broken conductor or a dead transmitter. A 0–20 mA loop cannot tell those two situations apart.
How do I convert a process value back to a current?
Work out the fraction of span, (PV − LRV) / (URV − LRV), then apply it to the 16 mA span above the 4 mA live zero: I = 4 mA + fraction × 16 mA. Switch this calculator to “Process value → current” and it shows both steps.
What does a reading below 4 mA or above 20 mA mean?
It is outside the calibrated range. This calculator still reports the process value the straight line implies and flags the reading as under-range or over-range, because that value is often what identifies a miscalibration. Interpret an extreme reading from the transmitter's own documentation, since many devices drive a specific fault current rather than continuing linearly.
References
- Standard practice for analogue current-loop transmission in process instrumentation; the 4–20 mA signal range and live-zero convention are defined in IEC 60381-1.
- Bela G. Liptak (ed.), Instrument Engineers' Handbook, Volume 1: Process Measurement and Analysis.
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