4-20mA Signal Converter — Free Online Calculator
by KOEED
Convert 4-20mA analog signals to physical engineering units in real time. Built for field technicians — type in any field and see instant results. No PLC knowledge needed.
Why 4-20mA? The Industry Standard for Analog Signals
The 4-20mA current loop has been the backbone of industrial process control for decades. Unlike voltage signals (0-10V), current loops offer distinct advantages that make them the first choice in harsh plant environments:
- Built-in fault detection: A reading of 0mA (or near-zero) means the wire is broken, the transmitter has failed, or power is lost. With a 0-20mA signal, you can't tell the difference between a genuine 0% reading and a dead sensor. The "live zero" at 4mA is a safety feature.
- Noise immunity: Current signals are far less susceptible to electromagnetic interference (EMI) from motors, VFDs, and welding equipment than voltage signals. This is critical in industrial settings where cable runs can span hundreds of meters.
- No voltage drop error: In a current loop, the current is the same at every point in the circuit. Voltage signals lose accuracy over long cable runs due to wire resistance, but a 4-20mA signal arrives at the receiver exactly as it left the transmitter.
- Universal compatibility: Virtually every PLC, DCS, RTU, panel meter, and loop-powered indicator on the market accepts 4-20mA inputs — it is the lingua franca of industrial instrumentation.
The Conversion Formula
This tool uses the standard linear interpolation formula to convert between milliamp signals and physical engineering units:
Physical_Value = ((mA - 4) / 16) × (Eng_High - Eng_Low) + Eng_Low
The logic is straightforward:
- Subtract 4mA (the live zero) from your signal
- Divide by 16 (the span: 20 - 4 = 16mA)
- This gives you a fraction between 0 and 1 (0–100% of range)
- Multiply by the engineering range span and add the low limit
Example: A pressure transmitter with range 0–150 PSI reads 12mA. Calculation: ((12 - 4) / 16) × (150 - 0) + 0 = 75 PSI. The 12mA sits exactly halfway between 4 and 20mA, so the pressure is exactly halfway between 0 and 150 — 75 PSI.
Common Applications
| Application | Sensor Type | Typical Range | Common Use |
|---|---|---|---|
| Pressure Transmitters | Strain gauge / Piezoresistive | 0–100 Bar, 0–1000 PSI | Hydraulic systems, pipelines, compressed air |
| Temperature Sensors | RTD (Pt100) / Thermocouple via transmitter | 0–200 °C, -50–150 °C | Ovens, chillers, reactors, HVAC |
| Level Sensors | Ultrasonic / Radar / Hydrostatic | 0–5 m, 0–10 m | Tanks, silos, water treatment |
| Flow Meters | Magnetic / Vortex / Coriolis | 0–100 L/min, 0–500 m³/h | Process dosing, cooling loops, fuel metering |
| Position Sensors | LVDT / Linear potentiometer | 0–100 mm, 0–500 mm | Valve position, actuator feedback, CNC |
Frequently Asked Questions
Why not 0-20mA? What is the "live zero"?
The 4mA offset (the "live zero") is a safety feature. If a 0-20mA transmitter fails or its cable is severed, the loop current drops to 0mA — but a valid 0% reading also produces 0mA. You cannot distinguish between "process is at zero" and "the sensor is dead." With 4-20mA, any reading below 3.8mA is unmistakably a fault. This is critical in safety applications like burner management, chemical dosing, and tank overfill protection.
How do I test a 4-20mA loop with a multimeter?
Set your multimeter to DC current (mA) mode. You must break the loop and insert the meter in series — never measure across the terminals in parallel. The correct procedure: (1) Identify the loop wiring at the transmitter, PLC, or junction box. (2) Disconnect one wire. (3) Connect the red probe to the disconnected wire and the black probe to the terminal you removed it from. (4) The meter now sits in series and reads the loop current. Safety note: Many loop-powered transmitters derive their operating power from the 4-20mA loop itself — disconnecting the loop removes power and the transmitter will restart when reconnected.
What is loop-powered vs self-powered?
Loop-powered (2-wire): The transmitter gets all its operating power from the 4-20mA loop. Only two wires are needed — both carry the signal AND power the device. This is the most common configuration for pressure, temperature, and level transmitters. It simplifies wiring but limits the minimum current to ~3.5mA (the transmitter always draws some power, establishing the 4mA live zero). Self-powered (4-wire): The transmitter has a separate power supply (typically 24VDC or 110/230VAC). Four wires — two for power, two for the signal loop. This allows higher-power sensors like magnetic flow meters or gas analyzers to operate, and the signal can genuinely drop to 0mA on fault.
Can I use this tool for 0-20mA signals too?
Yes. Although 4-20mA is far more common, you can use this converter for 0-20mA signals by simply adjusting the formula mentally: the 0-20mA span is still 20mA wide and maps linearly to your engineering range. Set your low-end reference to 0mA in your PLC, and the same linear scaling applies. However, note that this tool's fault detection (wire break at <3.8mA) is designed for the 4-20mA standard — a valid 0-20mA reading below 3.8mA would trigger a false alarm here.
What accuracy can I expect from 4-20mA?
Typical industrial transmitters achieve 0.1% to 0.5% of full-scale accuracy. The limiting factor is rarely the 4-20mA signal itself — it is the sensor element, the transmitter electronics, and the ADC resolution of the receiving device. A quality pressure transmitter (e.g., Wika, Endress+Hauser) combined with a 16-bit PLC analog input can resolve changes as fine as 0.003% of span. For most industrial processes, 12-bit resolution (1 part in 4096, or ~0.025%) is more than adequate.