4 to 20 mA Cheat Sheet
Every formula and number you need for 4 to 20 mA loops on one printable page: scaling, square root, NAMUR fault levels, loop power and PLC raw counts.
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A one page A4 sheet to print, pin up in the workshop or keep on your phone. Everything on it is also shown below.
Download PDF A4 · 89 KBScaling formulas
| Signal from PV | I = 4 + 16 × (PV − LRV) ÷ (URV − LRV) |
| PV from signal | PV = LRV + (I − 4) ÷ 16 × (URV − LRV) |
| Percent of span | % = (I − 4) ÷ 16 × 100 |
| Signal from percent | I = 4 + 0.16 × % |
| Across 250 Ω | 4 to 20 mA gives 1 to 5 V |
Percent to mA
| % | mA | V (250 Ω) |
|---|---|---|
| 0 | 4.00 | 1.00 |
| 10 | 5.60 | 1.40 |
| 25 | 8.00 | 2.00 |
| 50 | 12.00 | 3.00 |
| 75 | 16.00 | 4.00 |
| 90 | 18.40 | 4.60 |
| 100 | 20.00 | 5.00 |
Square root (DP flow)
| Flow from DP | Q% = 10 × √(DP%) |
| Signal | I = 4 + 16 × √(DP ÷ DPmax) |
| DP from flow | DP% = (Q%)² ÷ 100 |
| Example | 25 % DP = 50 % flow = 12 mA |
Apply square root once only: in the transmitter or in the DCS, never both. Use a low flow cut off (typically 5 to 10 % flow).
NAMUR NE 43 signal levels
| Range | Meaning |
|---|---|
| 3.8 to 20.5 mA | Valid measurement (saturation limits) |
| ≤ 3.6 mA | Failure, downscale |
| ≥ 21.0 mA | Failure, upscale |
| 3.6 to 3.8 or 20.5 to 21.0 mA | Undefined; treat as suspect |
Loop power
| Supply needed | Vs ≥ Vtx,min + Imax × (Rload + Rcable + Rbarrier) |
| Maximum loop resistance | Rmax = (Vs − Vtx,min) ÷ Imax |
| Use Imax | about 22 to 23 mA (alarm current, from the data sheet) |
HART essentials
- Digital signal: FSK, 1200 Hz = 1, 2200 Hz = 0, about 1 mA peak to peak on top of the analog signal
- Loop needs at least 230 Ω (250 Ω typical) for HART communication
- Multidrop: transmitters fixed at 4 mA, polling addresses 1 to 63
- Up to 4 variables: PV, SV, TV, QV
PLC raw count scaling
| General | PV = LRV + (raw − rawmin) ÷ (rawmax − rawmin) × span |
| Siemens S7 (4 to 20 mA) | 0 to 27648 = 0 to 100 % |
| 16 bit unsigned | 0 to 65535 |
Quick fault finding
| Reading | Likely cause |
|---|---|
| 0 mA | Open loop, no supply, blown fuse |
| 3.6 mA or less | Transmitter fault (downscale alarm) |
| Stuck at 4 mA | Wrong range, isolated or blocked impulse line |
| Over 20.5 mA | Over range or upscale alarm |
| Noisy reading | Shield earthed at both ends, AC nearby, poor terminal |
2 wire, 3 wire, 4 wire
- 2 wire: loop powered, signal and power share two wires
- 3 wire: common negative for supply and signal
- 4 wire: separate mains or 24 V supply, active output
- Live zero (4 mA) detects broken wires; 0 to 20 mA cannot
Related Calculators and Tools
Frequently Asked Questions
What is 50 % in 4 to 20 mA?
12 mA. The signal is 4 mA plus 16 mA times the fraction of span.
What does 3.6 mA mean on a transmitter?
Under NAMUR NE 43 a current of 3.6 mA or less signals a transmitter failure (downscale alarm).
Why does a 4 to 20 mA loop start at 4 mA?
The live zero powers 2 wire transmitters and lets the receiver tell a real zero from a broken wire (0 mA).
Learn more on the blog
Why 4 to 20 mA is used instead of a voltage signal
A practical guide on instrumentationblog.in that explains the theory behind this cheat sheet.
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