4 to 20 mA Calculator
Convert a transmitter signal into percent of span and engineering units, or work backwards from a process value to the expected current. Supports 4 to 20 mA, 0 to 20 mA, 1 to 5 V, 0 to 10 V and custom ranges, with NAMUR NE 43 fault checks.
Step 1: Signal and range
Step 2: Enter any one value
Signal = 4 + (PV − LRV) ÷ (URV − LRV) × 16 mA
Signal Scaling Table for Your Range
Use this as a five point calibration check sheet. It updates with your range.
| Percent of span | Signal | Process value |
|---|---|---|
| 0 % | 4 mA | 0 bar |
| 10 % | 5.6 mA | 1 bar |
| 25 % | 8 mA | 2.5 bar |
| 50 % | 12 mA | 5 bar |
| 75 % | 16 mA | 7.5 bar |
| 90 % | 18.4 mA | 9 bar |
| 100 % | 20 mA | 10 bar |
How the 4 to 20 mA Formula Works
Analog transmitters send a process value as a live zero signal, usually 4 to 20 mA. This tool converts between the signal, percent of span and the engineering value for any range, including 0 to 20 mA, 1 to 5 V and 0 to 10 V.
The relationship between signal and process value is a straight line. The span of the signal is 16 mA (20 − 4), and the span of the process is URV minus LRV. Every reading is simply a position on that line.
PV = LRV + Percent ÷ 100 × (URV − LRV)
I = 4 + 16 × (PV − LRV) ÷ (URV − LRV)
Where I is the loop current in mA, LRV is the lower range value (process value at 4 mA) and URV is the upper range value (process value at 20 mA).
NAMUR NE 43 signal levels
| Signal | Meaning |
|---|---|
| ≤ 3.6 mA | Failure signal (low) |
| 3.8 to 4.0 mA | Under range, still a valid measurement |
| 4.0 to 20.0 mA | Normal measuring range |
| 20.0 to 20.5 mA | Over range, still a valid measurement |
| ≥ 21.0 mA | Failure signal (high) |
Frequently Asked Questions
What is a 4 to 20 mA signal?
It is the most common analog signal in process instrumentation. A transmitter sends 4 mA at the lower range value (0 %) and 20 mA at the upper range value (100 %), so the current is directly proportional to the measured process value.
How do I convert mA to a process value?
Use PV = LRV + (I − 4) ÷ 16 × (URV − LRV). For a 0 to 10 bar transmitter reading 12 mA: 0 + (12 − 4) ÷ 16 × 10 = 5 bar.
How do I convert a process value to mA?
Use I = 4 + (PV − LRV) ÷ (URV − LRV) × 16. For 7.5 bar on a 0 to 10 bar range: 4 + 7.5 ÷ 10 × 16 = 16 mA.
Why does the signal start at 4 mA instead of 0 mA?
A live zero lets the control system tell a real 0 % reading (4 mA) apart from a broken wire or dead transmitter (0 mA). It also leaves enough current to power two wire, loop powered transmitters.
What are the NAMUR NE 43 limits?
NAMUR NE 43 defines 3.8 to 20.5 mA as the valid measuring range. A signal at or below 3.6 mA, or at or above 21 mA, indicates a transmitter failure so the control system can raise an alarm.
Can I use this for 0 to 10 V or 1 to 5 V signals?
Yes. Choose the signal type from the list, or select Custom to enter any low and high signal limits. The same linear scaling formula applies to all of them.
Learn the theory behind it
4 to 20 mA current loop explained
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