4-20 mA Loop Calculator
Convert between engineering units, loop current, and loop voltage for instrumentation signals.
1) Process Value ➜ Current (mA)
2) Current (mA) ➜ Process Value
3) Current + Resistor ➜ Voltage
What is a 4-20 mA signal?
A 4-20 mA current loop is a standard analog signaling method used in process control, automation, PLC systems, DCS systems, and industrial instrumentation. It maps a measurement range to a current range: 4 mA = 0% and 20 mA = 100% of span.
Because current is less sensitive to electrical noise and voltage drop than voltage-only signals, 4-20 mA loops are reliable over long cable runs. They are widely used for pressure transmitters, temperature transmitters, flow meters, level sensors, and valve positioners.
Why this 4-20 mA calculator is useful
This page helps you perform fast, practical loop calculations during design, commissioning, and troubleshooting:
- Convert process value to expected loop current.
- Convert measured mA back to engineering units.
- Calculate loop voltage across a resistor (for PLC analog input cards, multimeter checks, and shunts).
- See percent-of-span to verify instrument scaling quickly.
Formulas used in the calculator
Process value to current
I(mA) = 4 + 16 × (PV - LRV) / (URV - LRV)
Current to process value
PV = LRV + (I - 4) / 16 × (URV - LRV)
Loop voltage across a resistor
V = I(A) × R, where I(A) = I(mA) / 1000.
Quick examples
Example 1: 0-100 psi transmitter at 12 mA
12 mA is exactly 50% of span, so the transmitter is reading 50 psi.
Example 2: 0-200 °C transmitter at 25% span
25% corresponds to 8 mA. Temperature is 50 °C and loop current should be about 8 mA.
Example 3: Voltage across a 250 Ω resistor at 20 mA
Current is 0.02 A, so voltage is 0.02 × 250 = 5.0 V. This is why 250 Ω resistors are commonly used to convert 4-20 mA into 1-5 V signals.
Commissioning and troubleshooting tips
- Always confirm LRV/URV in both the transmitter and the control system scale block.
- Check whether the signal is linear or square-root extracted (especially for differential pressure flow).
- Measure loop current in series; measure shunt voltage in parallel.
- Watch for loop resistance limits if cable runs are long.
- Out-of-range values (below 4 mA or above 20 mA) can indicate alarms, faults, or overrange behavior.
Frequently asked questions
Can I use reversed ranges?
Yes. If URV is less than LRV, the calculation still works mathematically and represents reverse-acting scaling.
Why is 4 mA used instead of 0 mA?
The 4 mA “live zero” allows detection of wire breaks and provides operating power for many two-wire transmitters.
What is a normal resistor value for conversion?
250 Ω is common because 4-20 mA becomes 1-5 V, which is easy to read with control hardware and meters.
Bottom line
A reliable 4-20 mA calculator saves time and helps prevent scaling mistakes in instrumentation systems. Use this tool whenever you need quick, accurate loop conversions between mA, engineering units, and voltage.