Enter the range of the transmitter (the value at 4 mA and at 20 mA) and either a current to get the process value, or a process value to get the current. It also gives the percent of span and the voltage across a loop resistor, such as 1 to 5 V across 250 ohms.
Worked example
These are example values. Change any entry above and press Calculate to run your own numbers.
Current
8 mAmA = 4 + 16 × (value − low) ÷ span
Percent of span
25 %
Voltage across 250 Ω
2 V at 4 mA: 1 V; at 20 mA: 5 V
Signal status
Within the 4 to 20 mA range
The 3.6, 3.8, 20.5 and 21 mA limits are the commonly used NAMUR NE 43 levels for a valid measurement and for fault signalling. A given transmitter can be configured differently, so check its datasheet.
How it works
A 4-20 mA loop carries a measurement as a current, so it does not lose accuracy over a long cable. 4 mA is the bottom of the range and 20 mA is the top, which leaves 0 mA free to mean a broken wire. The relationship is a straight line, so scaling is simple proportion.
The formulas
Value = low + (mA − 4) ÷ 16 × (high − low)
mA = 4 + (value − low) ÷ (high − low) × 16
Percent of span = (mA − 4) ÷ 16 × 100
Voltage across a resistor: V = I × R (250 Ω gives 1 to 5 V)
Reference tables
4-20 mA against percent of span
Current (mA)
Percent of span
Volts across 250 Ω
4
0
1
8
25
2
12
50
3
16
75
4
20
100
5
Frequently asked questions
How do you convert 4-20 mA to a process value?
Subtract 4 from the current, divide by 16 and multiply by the span, then add the value at 4 mA. A transmitter ranged 0 to 100 reading 12 mA is at (12 − 4) ÷ 16 = 50% of span, so 50.
What mA does a value correspond to?
mA = 4 + 16 × (value − low) ÷ (high − low). A level of 25 on a 0 to 100 range is 4 + 16 × 0.25 = 8 mA.
Why does 4-20 mA start at 4 and not 0?
A live zero. A reading of 4 mA is a valid bottom of range, while 0 mA can only mean a broken loop or no power. That lets a controller tell a real zero from a fault. Many transmitters also use a current below 4 mA or above 20 mA to signal a failure; the exact levels are set by the maker, and NAMUR NE43 gives a common recommendation.
How do I turn the current into a voltage for an analog input?
Pass the loop current through a precision resistor and read the voltage across it. 250 ohms gives 1 to 5 V for 4 to 20 mA, and 500 ohms gives 2 to 10 V. Many PLC analog cards have a current input with the resistor built in, so check the card before adding one.
What is the 4-20 mA percent of span?
It is the position within the range as a percentage: 4 mA is 0%, 12 mA is 50% and 20 mA is 100%. Each 1 mA is 6.25% of the span.
