Instrumentation · 4-20 mA · Signal Conversion
4-20 mA Signal Conversion: Percentage to Current and Current to Percentage
A complete practical guide with an interactive calculator, formula derivation, worked examples in both directions, engineering unit conversion and a full reference table covering all key 4 to 20 mA signal points.
4 to 20 mA Signal Conversion
Every instrumentation technician and process engineer uses the 4-20 mA signal conversion formula every single working day. When a pressure transmitter reads 10.4 mA, what pressure does that represent? When a control system needs to send 75% to a valve positioner, what current should it output? When a calibration procedure asks for 25% of range, what mA value do you inject with the loop calibrator?
These are the conversions you need at your fingertips: instantly, reliably and without reaching for a calculator app that may not give the right answer if you do not know the formula. This article gives you the interactive calculator, the complete derivation of the formula, worked examples in both directions, the formula extended to real engineering units (bar, °C, m³/h), and a complete reference table you can print and keep in your instrument kit.
This page covers signal conversion only. For a full explanation of how the 4-20 mA current loop works, why it uses current instead of voltage and how to wire 2-wire vs 4-wire transmitters, see our complete guide on 4-20 mA current loop explained.
4 to 20 mA Signal Converter: Interactive Calculator
Use the calculator below for instant conversion in both directions. Enter a percentage to find the mA, or enter a mA value to find the percentage. Optionally add your transmitter range (LRV and URV) to also see the engineering unit value.
The Formula: Where It Comes From
The 4-20 mA signal uses a linear relationship between the percentage of measurement range and the output current. The signal range spans 16 mA (from 4 mA to 20 mA). This 16 mA span represents 100% of the measurement range. From this simple relationship, both conversion formulas follow directly.
Figure 1: The 4-20 mA signal is perfectly linear. Each 1% change in measurement produces exactly 0.16 mA change in output. The total signal span is 16 mA (20 minus 4). This linearity is the basis of both conversion formulas.
Deriving the formulas from first principles
The 4-20 mA signal is a linear function. The relationship between percentage (P) and current (I) can be written as a straight-line equation:
The signal spans from 4 mA (at 0%) to 20 mA (at 100%). The total span is 20 minus 4 = 16 mA.
Each 1% of range = 16 mA / 100 = 0.16 mA
Formula 1: Convert percentage to mA
mA = 4 + (Percentage / 100 × 16)
Formula 2: Convert mA to percentage
Percentage = ((mA - 4) / 16) × 100
Formula 3: Convert mA to engineering unit value
Value = LRV + ((mA - 4) / 16 × Span)
Where: LRV = Lower Range Value (reading at 4 mA) | URV = Upper Range Value (reading at 20 mA) | Span = URV minus LRV
Worked Examples: Percentage to mA
Using the formula: mA = 4 + (Percentage / 100 × 16)
| Percentage | Step 1: % / 100 | Step 2: × 16 | Step 3: + 4 | Result (mA) |
|---|---|---|---|---|
| 0% | 0 / 100 = 0.00 | 0.00 × 16 = 0.000 | 0.000 + 4 = 4.000 | 4.000 mA |
| 25% | 25 / 100 = 0.25 | 0.25 × 16 = 4.000 | 4.000 + 4 = 8.000 | 8.000 mA |
| 50% | 50 / 100 = 0.50 | 0.50 × 16 = 8.000 | 8.000 + 4 = 12.000 | 12.000 mA |
| 75% | 75 / 100 = 0.75 | 0.75 × 16 = 12.000 | 12.000 + 4 = 16.000 | 16.000 mA |
| 100% | 100 / 100 = 1.00 | 1.00 × 16 = 16.000 | 16.000 + 4 = 20.000 | 20.000 mA |
| 33.33% | 33.33 / 100 = 0.3333 | 0.3333 × 16 = 5.333 | 5.333 + 4 = 9.333 | 9.333 mA |
| 62.5% | 62.5 / 100 = 0.625 | 0.625 × 16 = 10.000 | 10.000 + 4 = 14.000 | 14.000 mA |
Worked Examples: mA to Percentage
Using the formula: Percentage = ((mA - 4) / 16) × 100
| mA value | Step 1: mA - 4 | Step 2: / 16 | Step 3: × 100 | Result (%) |
|---|---|---|---|---|
| 4.000 mA | 4.000 - 4 = 0.000 | 0.000 / 16 = 0.000 | 0.000 × 100 = 0.00 | 0.00% |
| 6.400 mA | 6.400 - 4 = 2.400 | 2.400 / 16 = 0.150 | 0.150 × 100 = 15.00 | 15.00% |
| 8.000 mA | 8.000 - 4 = 4.000 | 4.000 / 16 = 0.250 | 0.250 × 100 = 25.00 | 25.00% |
| 10.400 mA | 10.400 - 4 = 6.400 | 6.400 / 16 = 0.400 | 0.400 × 100 = 40.00 | 40.00% |
| 12.000 mA | 12.000 - 4 = 8.000 | 8.000 / 16 = 0.500 | 0.500 × 100 = 50.00 | 50.00% |
| 15.200 mA | 15.200 - 4 = 11.200 | 11.200 / 16 = 0.700 | 0.700 × 100 = 70.00 | 70.00% |
| 20.000 mA | 20.000 - 4 = 16.000 | 16.000 / 16 = 1.000 | 1.000 × 100 = 100.00 | 100.00% |
Converting mA to Engineering Units: Worked Examples
Using the formula: Value = LRV + ((mA - 4) / 16 × Span)
The three examples below show the same formula applied to three common transmitter types: a pressure transmitter, a temperature transmitter and a level transmitter.
| Transmitter | LRV | URV | Span | mA reading | % calculation | Engineering value |
|---|---|---|---|---|---|---|
| Pressure transmitter | 0 bar | 10 bar | 10 bar | 10.4 mA | ((10.4-4)/16)×100 = 40% | 0 + (40% × 10) = 4.0 bar |
| Temperature transmitter | 0°C | 200°C | 200°C | 15.2 mA | ((15.2-4)/16)×100 = 70% | 0 + (70% × 200) = 140°C |
| Level transmitter | 0 m | 5 m | 5 m | 12.0 mA | ((12-4)/16)×100 = 50% | 0 + (50% × 5) = 2.5 m |
| Flow transmitter (suppressed zero) | 100 m³/h | 500 m³/h | 400 m³/h | 8.0 mA | ((8-4)/16)×100 = 25% | 100 + (25% × 400) = 200 m³/h |
| Temperature transmitter (negative range) | -50°C | 50°C | 100°C | 6.4 mA | ((6.4-4)/16)×100 = 15% | -50 + (15% × 100) = -35°C |
Out-of-Range Signal Values: NAMUR NE43 Fault Zones
The formulas above work correctly for mA values outside the 4-20 mA live range. This is intentional. During calibration you may inject signals below 4 mA or above 20 mA to test the DCS response to fault conditions. The NAMUR NE43 standard defines specific signal zones below and above the measurement range for fault signalling:
| mA value | % equivalent | NAMUR NE43 zone | Meaning |
|---|---|---|---|
| 0.000 mA | -25.00% | Hard fault | Broken wire or complete loss of loop power |
| 3.600 mA | -2.50% | Fault boundary | NAMUR NE43 lower fault limit. Below this = hardware fault alarm. |
| 3.800 mA | -1.25% | Low alarm saturation | Measurement below LRV or sensor burnout driving downscale |
| 4.000 mA | 0.00% | LRV (live zero) | Measurement at lower range value. Normal minimum. |
| 12.000 mA | 50.00% | Midpoint | Measurement at exactly 50% of range |
| 20.000 mA | 100.00% | URV | Measurement at upper range value. Normal maximum. |
| 20.500 mA | 103.13% | High saturation | Measurement above URV or sensor burnout driving upscale |
| 21.000 mA | 106.25% | Fault boundary | NAMUR NE43 upper fault limit. Above this = hardware fault alarm. |
Complete 4-20 mA Reference Table: Every 5%
This is the table to print and keep in your instrument kit. It covers every 5% increment from 0% to 100% with the exact mA value to three decimal places.
| Percentage (%) | mA output | mA above zero (mA - 4) | Fraction of span |
|---|---|---|---|
| 0% | 4.000 mA | 0.000 mA | 0.0000 |
| 5% | 4.800 mA | 0.800 mA | 0.0500 |
| 10% | 5.600 mA | 1.600 mA | 0.1000 |
| 15% | 6.400 mA | 2.400 mA | 0.1500 |
| 20% | 7.200 mA | 3.200 mA | 0.2000 |
| 25% | 8.000 mA | 4.000 mA | 0.2500 |
| 30% | 8.800 mA | 4.800 mA | 0.3000 |
| 35% | 9.600 mA | 5.600 mA | 0.3500 |
| 40% | 10.400 mA | 6.400 mA | 0.4000 |
| 45% | 11.200 mA | 7.200 mA | 0.4500 |
| 50% | 12.000 mA | 8.000 mA | 0.5000 |
| 55% | 12.800 mA | 8.800 mA | 0.5500 |
| 60% | 13.600 mA | 9.600 mA | 0.6000 |
| 65% | 14.400 mA | 10.400 mA | 0.6500 |
| 70% | 15.200 mA | 11.200 mA | 0.7000 |
| 75% | 16.000 mA | 12.000 mA | 0.7500 |
| 80% | 16.800 mA | 12.800 mA | 0.8000 |
| 85% | 17.600 mA | 13.600 mA | 0.8500 |
| 90% | 18.400 mA | 14.400 mA | 0.9000 |
| 95% | 19.200 mA | 15.200 mA | 0.9500 |
| 100% | 20.000 mA | 16.000 mA | 1.0000 |
0% = 4.000 mA | 25% = 8.000 mA | 50% = 12.000 mA | 75% = 16.000 mA | 100% = 20.000 mA
These five round numbers are the only ones you need to memorise. Every other value can be quickly interpolated between them or calculated using the 0.16 mA per 1% rule.
Common Conversion Mistakes to Avoid
| Mistake | What goes wrong | Correct approach |
|---|---|---|
| Forgetting to subtract 4 when converting mA to percentage | Using 12 mA / 20 mA × 100 = 60% instead of the correct (12-4) / 16 × 100 = 50%. The 4 mA live zero must always be subtracted first. | Always subtract 4 from the mA reading before dividing. The usable span is 16 mA, not 20 mA. |
| Dividing by 20 instead of 16 | The total signal goes to 20 mA but the usable span is only 16 mA (from 4 to 20). Dividing by 20 gives a wrong result at every point except zero. | Always divide by 16. Span = 20 - 4 = 16 mA. |
| Using LRV = 0 when the transmitter has a suppressed zero | A flow transmitter ranged 100 to 500 m³/h has LRV = 100, not 0. Using LRV = 0 in the engineering unit formula gives a reading 100 m³/h too low at all points. | Always read the transmitter's LRV and URV from the HART communicator or instrument datasheet before calculating engineering unit values. |
| Expecting the DCS to display percentage when it is configured for engineering units | If the DCS AI channel is configured for 0-10 bar, the displayed value will be in bar, not percentage. The conversion to engineering units happens inside the DCS configuration, not in the 4-20 mA signal. | Check the DCS channel configuration for the LRV and URV values to understand what units the display is using. The 4-20 mA signal itself always carries 0-100% proportionally. |
Further Reading and External Resources
- Inst Tools: Convert Percentage to Current Tool. The original reference calculator that inspired this article, from the leading instrumentation knowledge base.
- Precision Digital: The Fundamentals of 4-20 mA Current Loops. Essential background on why the 4-20 mA standard works the way it does, including the water pipe analogy for Ohm's law.
- Omega Engineering: 4-20 mA Current Loop Reference. Practical technical reference on wiring, loop power and signal conversion from a leading instrumentation manufacturer.
Frequently Asked Questions: 4-20 mA Conversion
- 4-20 mA Current Loop Explained: How It Works, Wiring and Troubleshooting
- Analog vs Digital Signals in Instrumentation: A Complete Guide
- NAMUR NE43 Standard: Signal Range and Fault Detection Explained
- How to Calibrate a Temperature Transmitter: Step-by-Step Procedure
- Signals in Instrumentation: AI, AO, DI and DO Explained
- Instrument Loop Checking: A Complete Step-by-Step Procedure
What we learn today
- To convert percentage to mA: mA = 4 + (% / 100 × 16). To convert mA to percentage: % = ((mA - 4) / 16) × 100. The span is always 16 mA (20 minus 4), never 20 mA.
- The five key values to memorise: 0% = 4 mA, 25% = 8 mA, 50% = 12 mA, 75% = 16 mA, 100% = 20 mA. Every 1% change is 0.16 mA.
- To convert mA to engineering units: Value = LRV + ((mA - 4) / 16 × Span). The LRV is the engineering value at 4 mA. The Span is URV minus LRV. This formula handles any range including suppressed zero and negative ranges.
- The most common calculation mistake is dividing by 20 instead of 16. Always subtract 4 first to remove the live zero, leaving the 16 mA usable span as the denominator.
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