4–20 mA Scaling Calculator
By Santaji GadepatilinUpdated October 27, 2026
Process value vs Loop current (mA) — other inputs held at your current values.
How to use this calculator
- Enter the transmitter’s range — the value at 4 mA (low) and at 20 mA (high), in any engineering unit such as bar, °C, m or m³/h.
- Enter the loop current to get the process value and percent of span.
To check a loop the other way round, press × on Loop current and type the process value — the calculator shows the current the transmitter should send. Reverse-acting ranges (for example 100 at 4 mA and 0 at 20 mA) work too.
The formula
% span = (I − 4 mA) ÷ 16 mA
value = low + % span × (high − low)
Worked examples
Pressure transmitter, 0–10 bar, reading 12 mA
- % span = (12 − 4) ÷ 16 = 50%
- Value = 0 + 0.5 × 10 = 5 bar
Temperature transmitter, 0–250 °C, reading 16.8 mA
- % span = (16.8 − 4) ÷ 16 = 80%
- Value = 0.8 × 250 = 200 °C
Quick reference
| Current | % of span |
|---|---|
| 4 mA | 0% |
| 8 mA | 25% |
| 12 mA | 50% |
| 16 mA | 75% |
| 20 mA | 100% |
NAMUR NE 43 fault levels
| Current | Meaning |
|---|---|
| ≤ 3.6 mA | Fault (downscale failure) |
| 3.8 – 4.0 mA | Under-range, still valid |
| 20.0 – 20.5 mA | Over-range, still valid |
| ≥ 21 mA | Fault (upscale failure) |
Check your transmitter’s manual — some manufacturers use slightly different limits.
Converting mA to volts
Many PLC inputs read the loop as a voltage across a 250 Ω resistor, giving 1–5 V. Use the Ohm’s law calculator to check other resistor values.
Frequently asked questions
How do I convert 4–20 mA to a process value?
Value = low + (mA − 4) ÷ 16 × (high − low). For 12 mA on a 0–10 bar transmitter, (12 − 4) ÷ 16 × 10 = 5 bar.
Why does the signal start at 4 mA instead of 0?
The 4 mA "live zero" powers two-wire transmitters and lets the system tell a real zero reading (4 mA) from a broken wire (0 mA).
What do currents below 4 mA or above 20 mA mean?
Many devices follow NAMUR NE 43. Readings below about 3.6 mA or above about 21 mA signal a fault, such as a sensor failure or open circuit.
Can I calculate the mA output for a given value?
Yes. Press × on Loop current, then type the process value. The calculator gives the expected current, useful for loop checks.