What is 4-20 mA and why does the world use it?
4-20 mA is the dominant analog signal standard in industrial process instrumentation — pressure transmitters, temperature transmitters, level probes, flow meters, valve position feedback, and thousands of other field devices output a current between 4 milliamps and 20 milliamps to represent their measured value. The reason it beat voltage-based signaling: current doesn’t drop across long cable runs, works with two-wire loop-powered devices, and — critically — 4 mA is a “live zero”. A broken wire or dead sensor reads 0 mA, which is distinguishable from a valid “zero measurement” at 4 mA. Voltage signals can’t do this.
How to use this calculator
Four modes cover the four questions engineers actually ask:
- mA → EU — you measured or received a mA value, what does it mean in engineering units? Use this when reading transmitter output at the field terminals or during calibration.
- EU → mA — you know the process value, what mA should the transmitter be outputting? Use this to verify a transmitter’s output during commissioning or troubleshooting.
- PLC raw counts — inside the PLC, the analog input isn’t a mA value — it’s a scaled integer count (0–27648 on Siemens, 0–32767 on Allen-Bradley, and different again on other platforms). This mode converts raw counts to mA and engineering units, or vice versa.
- Scaling table — generates a complete table showing mA, signal %, engineering value, and Siemens/AB raw counts across the full range. Useful for commissioning documentation.
Set your EU range once at the top (EU_min at 4 mA, EU_max at 20 mA, unit label, and scaling mode) and it applies to all four modes.
The linear scaling formula
For 99% of transmitters, the relationship between mA and engineering units is linear:
EU = (mA − 4) / 16 × (EU_max − EU_min) + EU_min
The reverse:
mA = (EU − EU_min) / (EU_max − EU_min) × 16 + 4
Example: a pressure transmitter ranged 0–10 bar reads 14 mA:
- Signal percentage: (14 − 4) / 16 = 0,625 or 62,5%
- EU value: 0,625 × (10 − 0) + 0 = 6,25 bar
Reverse-acting transmitters
Some devices — commonly tank level probes and certain safety-critical instruments — are configured “reverse acting”: high process value = low mA. This lets the fail-safe direction (loss of signal reads as high alarm) match the process safety requirement.
Formula for reverse-acting:
EU = EU_max − (mA − 4) / 16 × (EU_max − EU_min)
The scaling mode selector at the top handles this — the math stays consistent across all modes.
Square-root extraction (DP flow meters)
Differential pressure flow meters (orifice plates, venturis, flow nozzles) don’t have a linear relationship between flow and DP. The physics gives:
DP ∝ flow² → flow ∝ √DP
So a DP transmitter outputs 4-20 mA linearly with DP, but if you want flow directly, you need to square-root the signal:
EU = EU_min + √((mA − 4) / 16) × (EU_max − EU_min)
Example: a DP-based flow meter ranged 0–100 m³/h at 12 mA:
- Signal fraction: (12 − 4) / 16 = 0,5
- Flow fraction: √0,5 = 0,707
- Flow: 0,707 × 100 = 70,7 m³/h
Note: 50% of signal ≠ 50% of flow for square-root scaling. Many transmitters can be configured to do square-root extraction internally, in which case use linear scaling in this calculator. If square-root is done in the PLC, use square-root mode here.
PLC raw counts — the vendor mess
Inside the PLC, the analog input module doesn’t hand your code a floating-point mA value. It gives you an integer count representing where the signal is between the “nominal 4 mA” and “nominal 20 mA” points. Different vendors use different count ranges:
- Siemens S7-1200 / S7-1500 — 0 to 27648 for 4-20 mA. Overrange up to ~32511. Underrange down to negative values.
- Allen-Bradley Logix (CompactLogix / ControlLogix) — 0 to 32767 for 4-20 mA (full 16-bit signed positive range). Some modules use −32768 to +32767 for a bipolar signal.
- Allen-Bradley Micro850 — 0 to 4095 (12-bit resolution). Much coarser than the higher-end AB range.
- Mitsubishi FX series — 0 to 4000 (16-bit signed but scaled).
- Omron NX / NJ series — 0 to 6000 typical for 4-20 mA input.
The Siemens 27648 quirk trips up engineers moving between platforms. Why 27648 and not 32767? Because Siemens preserves headroom on both ends for over/under-range detection: the range 0–27648 is the “normal” measurement range, but the module can output up to about 32511 to represent up to ~22.8 mA before flagging over-range. The full 16-bit signed range provides the diagnostic bandwidth.
NAMUR NE43 signal integrity
NAMUR NE43 is a widely-adopted standard defining what different mA ranges mean for signal integrity — not just the measured value, but whether the value can be trusted at all:
- < 3,6 mA — Fault (downscale burnout). The transmitter is dead, wiring is broken, or sensor has failed. Do not use this reading as a process value.
- 3,6 – 3,8 mA — Under-range diagnostic. The signal is genuinely below the measuring range (process value is below the configured minimum).
- 3,8 – 4,0 mA — Live zero band. Valid measurement at the low end of range.
- 4,0 – 20,0 mA — Normal measuring range. Signal is within the calibrated span.
- 20,0 – 20,5 mA — Over-range diagnostic. Signal is genuinely above the measuring range (process value is above the configured maximum).
- 20,5 – 21,0 mA — Fault (upscale burnout). Similar to downscale burnout but in the other direction — the transmitter is signalling failure by driving the loop high.
- > 21,0 mA — Fault (wiring / circuit fault). Something has gone very wrong at the electrical level.
Modern PLCs and DCSs handle these bands automatically — the analog input module reports “under range” or “sensor fault” as separate status bits, not just extreme values. But if you’re reading a mA value on a multimeter or from raw counts in older logic, this calculator surfaces the NAMUR status directly so you can spot problems.
Common range examples
Some typical field transmitter ranges to try in the calculator:
- Pressure — 0–10 bar (industrial), 0–100 bar (hydraulic), 0–4 bar (utility)
- Temperature — −50 to +150°C (HVAC), 0–400°C (process heating), 0–1200°C (furnace)
- Level — 0–100% (any tank), 0–5 m (specific tank height)
- Flow — 0–100 m³/h (typical mid-range), often with square-root extraction if DP-based
- Position feedback — 0–100% (valve position), 0–360° (rotary encoders)
Troubleshooting with this calculator
Some common commissioning scenarios where this tool saves time:
- Transmitter reads wrong value in the HMI — measure the actual mA at the field terminals, calculate what EU that should represent, and compare to the HMI value. Discrepancy tells you whether the transmitter is calibrated correctly or the PLC scaling is wrong.
- Raw counts in the PLC don’t match expected mA — enter the raw counts and your platform to see what mA they represent, then check against a field measurement. Mismatch usually means the PLC is configured for a different range (e.g. 0-20 mA instead of 4-20 mA) or the analog module is broken.
- Verifying calibration during commissioning — inject known mA values into the loop and check that the HMI displays the expected EU values, using the scaling table for reference.
- Diagnosing a “value pinned at 4 mA” or “pinned at 20 mA” problem — check whether it’s genuinely at a range limit (valid) or signalling a fault (per NAMUR NE43).
What this calculator doesn’t cover
- HART protocol digital data — 4-20 mA can carry a superimposed HART digital signal for configuration and diagnostics. This calculator only handles the analog value, not the HART digital layer.
- Smart transmitter status bits — modern devices provide diagnostic status separate from the analog output. Read those from the device itself or over HART / fieldbus.
- Analog input module error codes — vendor-specific error codes (open wire, over-range, calibration error) are reported by the PLC module hardware, not derivable from the mA value alone.
- Non-standard signal ranges — 0-20 mA (older), 0-10 V (some HVAC), 1-5 V (rare industrial), 24 V binary inputs. This calculator assumes standard 4-20 mA.
- Damping and filter effects — real transmitters and PLC inputs apply damping / low-pass filtering that affect dynamic response but not the steady-state calibration.
For safety-critical loops, always verify signal integrity handling in the PLC logic — don’t rely on the raw mA calculation alone.