Convert a 4-20 mA loop current into the process value it represents, or work out which current a transmitter should send for a given reading or percentage. You also get the voltage across a shunt resistor, a NAMUR NE 43 signal check and a full scaling table.
Last updated:
Convert
Signal
mA
%
Scaling
Value at 4 mA (or 0 mA)
Value at 20 mA
Ω
250 Ω gives 1–5 V
Enter your values to see the result.
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How to use the 4-20mA Calculator
Choose the conversion: mA to process value, process value to mA, or percent to mA.
Enter the transmitter range: the value at 4 mA (LRV), the value at 20 mA (URV) and the unit.
Type the signal you measured or the value you want to simulate.
Add the input resistor to see the voltage your PLC or meter will read.
Tick square-root extraction for a differential-pressure flow transmitter, then read the results and table.
4-20 mA conversion formulas
Find
Formula (4-20 mA)
Percent of span
% = (I − 4) ÷ 16 × 100
Process value
PV = LRV + (I − 4) ÷ 16 × (URV − LRV)
Current from a value
I = 4 + 16 × (PV − LRV) ÷ (URV − LRV)
Shunt voltage
V = I × R (I in amperes)
Square-root flow
Flow % = √((I − 4) ÷ 16) × 100
For a 0-20 mA signal replace the 4 with 0 and the 16 with 20.
Quick reference
Percent
4-20 mA
0-20 mA
Volts at 250 Ω
Square-root flow % (4-20 mA)
0%
4.0
0.0
1.0
0%
25%
8.0
5.0
2.0
50%
50%
12.0
10.0
3.0
70.7%
75%
16.0
15.0
4.0
86.6%
100%
20.0
20.0
5.0
100%
In the last column the percent is the signal percent, and the flow figure is what a square-root transmitter or receiver reports for that signal.
Troubleshooting tips
Reading stuck at 0 mA: check for an open circuit, a blown fuse or a missing loop supply.
Reading around 3.6 mA or 21+ mA: many smart transmitters drive the loop to these levels on purpose to report an internal fault.
Loop supply budget: the supply voltage must cover the transmitter's minimum voltage plus 20 mA times the total loop resistance (resistor, wiring and any indicators).
Reversed ranges such as 100 to 0 are fine: enter the value at 4 mA as the low end even if it is larger.
Frequently asked questions
What is the 4-20 mA scaling formula?
Process value = LRV + (mA − 4) ÷ 16 × (URV − LRV). For a 0 to 150 psi transmitter at 12 mA: 0 + (12 − 4) ÷ 16 × 150 = 75 psi. To go the other way, mA = 4 + 16 × (PV − LRV) ÷ (URV − LRV).
Why does the signal start at 4 mA instead of 0?
A live zero lets the receiver tell a genuine zero reading (4 mA) from a broken wire or failed transmitter (0 mA). The 4 mA also gives loop-powered transmitters enough current to run their electronics.
How do I convert 4-20 mA to 1-5 V?
Pass the loop current through a 250 Ω resistor. By Ohm's law V = I × R, so 4 mA gives 1 V and 20 mA gives 5 V. A 500 Ω resistor gives 2 to 10 V.
What do readings below 4 mA or above 20 mA mean?
Under the NAMUR NE 43 recommendation, 3.8 to 20.5 mA is the measurement range, so slightly under or over range is still a valid reading. A current of 3.6 mA or less, or 21 mA or more, signals a transmitter fault or wiring problem.
What is square-root extraction?
Differential-pressure flow meters produce a pressure that rises with the square of flow. With square-root extraction, flow % = √(signal %). For example, 8 mA is 25% of the signal span, which means 50% of full flow.
What is the percent of span at 12 mA?
12 mA is exactly the midpoint of a 4-20 mA signal: (12 − 4) ÷ 16 = 50%. The quarter points are 8 mA (25%) and 16 mA (75%).
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.