4–20 mA Scaling Calculator (Current Loop to Process Value)
The process value a 4–20 mA signal stands for, its position in the span, the current for a given process value, the voltage across a sense resistor and what the NAMUR NE43 limits say about the reading.
Results
The dashed line marks the value you entered.
What this tool does
A 4–20 mA loop sends the measurement as a current between 4 mA (bottom of the range) and 20 mA (top), a straight line, so mA = 4 + 16 × (value − low) ÷ (high − low). The 4 mA offset is deliberate: a dead zero then means a broken wire, not a low reading. NAMUR NE43 sets 3.8 to 20.5 mA as the valid range and treats 3.6 mA or less, or 21 mA or more, as a fault. A 250 Ω resistor turns 4–20 mA into 1–5 V, which is why that value is so common at the input of a controller or data logger.
Formula
value = low + (mA − 4) ÷ 16 × (high − low) · mA = 4 + 16 × (value − low) ÷ (high − low) · V = I × R (NAMUR NE43: valid 3.8–20.5 mA)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
lo | Process value at 4 mA | — |
hi | Process value at 20 mA | — |
ma | Loop current you measure | mA |
v | Process value you want the current for | — |
rs | Resistor that turns the current into a voltage | Ω |
VA | Process value for that current | — |
PC | Position in the span | % |
CU | Current for that process value | mA |
VS | Voltage across the resistor | V |
ST | State of the signal (NAMUR NE43) | — |
Worked example
- Process value at 4 mA0
- Process value at 20 mA100
- Loop current you measure12 mA
- Process value you want the current for50
- Resistor that turns the current into a voltage250 Ω
- Process value for that current50.000
- Position in the span50.00 %
- Current for that process value12.000 mA
- Voltage across the resistor3.000 V
- State of the signal (NAMUR NE43)Normal measuring range (3.8 to 20.5 mA)
Limitations
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.
- The default values are typical reference figures, not measurements of your situation. Replace them with your own data whenever you have it.