How the 4–20 mA to PV conversion works
A 4–20 mA current loop is the most common way to carry an analog measurement in industrial plants. The transmitter maps its calibrated range — from the Lower Range Value (LRV) to the Upper Range Value (URV) — linearly onto a current between 4 and 20 mA. Because the signal spans 16 mA (20 − 4), every milliamp above 4 represents one-sixteenth of the instrument span.
The reason the scale starts at 4 mA rather than 0 mA is called a live zero. If 0 mA meant 0%, you could never tell a healthy loop reading zero from a cut cable or a dead power supply. With a live zero, 4 mA is a valid measurement and 0 mA is unambiguously a fault — and the ever-present 4 mA also powers two-wire transmitters through the same pair of conductors.
The formulas
Current to process variable:
Process variable to current:
Percentage of span from current:
Here I is the loop current in mA, and URV − LRV
is the instrument span. The formulas are simple
linear interpolation — the same y = mx + b you learned in school,
with the line anchored at (4 mA, LRV) and (20 mA, URV).
Worked example
A pressure transmitter is ranged 0 to 250 kPa (LRV = 0, URV = 250). The loop current measures 13.6 mA. What is the pressure?
- Subtract the live zero: 13.6 − 4 = 9.6 mA
- Divide by the signal span: 9.6 ÷ 16 = 0.60 → the process is at 60%
- Apply to the instrument span: 0.60 × (250 − 0) = 150
- Add the LRV: 0 + 150 = 150 kPa
Try it in the calculator above — set LRV 0, URV 250, and type 13.6 in the current field.
Quick reference table
| Current | % of span | PV (range 0–100) | PV (range −50 to +150) |
|---|---|---|---|
| 4 mA | 0 % | 0 | −50 |
| 8 mA | 25 % | 25 | 0 |
| 12 mA | 50 % | 50 | 50 |
| 16 mA | 75 % | 75 | 100 |
| 20 mA | 100 % | 100 | 150 |
Notice the second range: with an elevated zero (LRV = −50), 8 mA corresponds to a PV of 0. This is why you should never assume "8 mA = 25% = 25 units" without knowing the range.
Common mistakes in the field
- Forgetting the live zero. Dividing the current by 20 instead of using (I − 4)/16 is the classic error — it reads about 10–15% high across most of the scale.
- Confusing span with URV. A transmitter ranged 50–250 has a URV of 250 but a span of 200. The formulas use span (URV − LRV), not URV.
- Applying linear math to a DP flow signal. Flow from a differential-pressure element follows a square-root law. If the transmitter isn't already doing the extraction, 12 mA is 50% DP but about 70.7% flow.
- Ignoring out-of-range currents. Per NAMUR NE 43, readings between about 3.8–4 mA and 20–20.5 mA are valid saturation, while ≤ 3.6 mA or ≥ 21 mA typically signal transmitter failure. The calculator warns you when your entry crosses these limits.
Frequently asked questions
Why does a 4-20 mA loop start at 4 mA and not 0 mA?
The 4 mA "live zero" separates a genuine 0% process reading (4 mA) from a dead loop or broken wire (0 mA), and it leaves enough current to power two-wire transmitters over the same pair of wires.
What is the formula to convert 4-20 mA to a process variable?
PV = LRV + ((I − 4) / 16) × (URV − LRV), where I is the loop current in mA, LRV is the lower range value and URV is the upper range value of the instrument.
What does 12 mA mean on a 4-20 mA signal?
12 mA is exactly 50% of span. For a transmitter ranged 0–100 °C it means 50 °C; for a range of −50 to +150 °C it means +50 °C. The percentage is always the same; the PV depends on the range.
What if the current is below 4 mA or above 20 mA?
Readings slightly outside 4-20 mA (roughly 3.8-20.5 mA per NAMUR NE 43) indicate the process is just beyond the calibrated range. Currents near 3.6 mA or above 21 mA are usually deliberate failure signals from the transmitter.
Provided for reference and education. Verify independently before use in safety-critical work. See our disclaimer.