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This calculator converts between a 4-20 mA signal and the process value it represents, in both directions, for pressure, level, temperature, and flow ranges. Enter the values your transmitter is ranged for at 4 mA and at 20 mA, then type either a process value or a mA reading. The flow tab includes a square-root mode for differential pressure elements that transmit raw DP.
The math is the standard linear scaling every transmitter and PLC uses. If you want the electrical side, how the loop is powered, what the fault currents mean, and why the range starts at 4 mA, read the guide on how a 4-20 mA current loop works.
Calculator
4-20 mA signal calculator
Readings at or below 3.6 mA or at or above 21 mA are NAMUR NE43 fault signals, not measurements. Results are scaling math, not a calibration.
The 4-20 mA formula
A transmitter maps its calibrated range linearly onto the 16 mA of usable span. Current from process value:
I = 4 + 16 × (PV − LRV) / (URV − LRV) mA
Process value from current:
PV = LRV + (URV − LRV) × (I − 4) / 16
LRV is the value at 4 mA and URV is the value at 20 mA. For the older 0-20 mA signal the same math applies with a 0 mA zero and a 20 mA span; the calculator has a signal-type switch for it, but note that 0-20 mA gives up the live zero, so a broken wire reads the same as the bottom of the range. The same span expressed as percent and as the 1-5 V signal a 250 Ω receiver resistor produces:
| Percent of span | Loop current | Voltage across 250 Ω |
|---|---|---|
| 0% | 4.0 mA | 1.0 V |
| 10% | 5.6 mA | 1.4 V |
| 25% | 8.0 mA | 2.0 V |
| 50% | 12.0 mA | 3.0 V |
| 75% | 16.0 mA | 4.0 V |
| 90% | 18.4 mA | 4.6 V |
| 100% | 20.0 mA | 5.0 V |
Worked examples
Pressure: a transmitter ranged 0-10 bar reading 6.3 bar gives I = 4 + 16 × (6.3/10) = 14.08 mA. Temperature: ranged 0-250 °C, a 150 °C process gives 4 + 16 × 0.6 = 13.6 mA. Level, in reverse: a 0-2 m loop reading 12.0 mA means 2 × (12 − 4)/16 = 1.0 m, half the tank. Ranges do not have to start at zero: a compound transmitter ranged −1 to 1 bar reads 12 mA at exactly 0 bar, which is why offset ranges are a common source of manual-conversion mistakes.
Flow is the one that trips people. If a differential pressure element transmits raw DP without square root extraction, flow follows the square root of the signal: 12 mA is 70.7% of full flow, not 50%. Switch the flow tab to square-root mode for that case; for meters that output current linear with flow, magnetic, vortex, turbine, Coriolis, keep it linear. The physics is in our guide to the flow rate and pressure relationship, and the companion flow rate from pressure calculator sizes the flow itself.
How to measure a 4-20 mA signal
Two ways. Break the loop and put a multimeter in series on the mA range; this reads the true loop current but interrupts the signal while you connect. Or measure voltage across the receiver’s input resistor without breaking anything: across 250 Ω, a healthy signal reads 1 to 5 V, and dividing volts by 250 gives amps. Many loop-powered indicators and clamp meters for mA avoid the break entirely. If the reading sits at 0 mA the circuit is open; at or below 3.6 mA or at or above 21 mA the transmitter is reporting a fault, per the NAMUR NE43 bands explained in the loop guide.
Application example
Automation integrator, flow totalizing (South Asia). The panel needed a flow totalizer taking one 4-20 mA input from a vortex or DP flow meter, with a 4-20 mA retransmission, two alarms, and RS485 to the SCADA. The scaling on both ends had to match: the meter’s URV and the totalizer’s input range set to the same flow value, or every downstream total is silently wrong. Both instruments were quoted as a matched pair with one shared range sheet, so the loop scaling was fixed on paper before any wiring started.
Related guides and products
- 4-20 mA current loop: how it works, scaling and faults
- Flow rate and pressure relationship: formulas and worked examples
- Pressure transmitters with 4-20 mA output
- Temperature transmitter, RTD and thermocouple input
- Product selector: find an instrument by your parameters
FAQ
How to calculate 4 to 20 mA formula?
Use I = 4 + 16 × (PV − LRV) / (URV − LRV), where LRV is the value at 4 mA and URV the value at 20 mA. To go the other way, PV = LRV + (URV − LRV) × (I − 4) / 16. Both are linear; only square-root DP flow signals differ.
What is 50% of a 4-20 mA signal?
12 mA. The usable span is 16 mA wide, so 50% is 4 + 8 = 12 mA, which is 3.0 V across a 250 Ω receiver resistor. For a DP flow element transmitting raw DP, 12 mA corresponds to 70.7% of full flow, because flow follows the square root of the signal.
How many psi is 4 to 20 mA?
There is no fixed answer; it depends on the transmitter’s calibrated range. On a 0-100 psi range, 4 mA is 0 psi, 12 mA is 50 psi, and 20 mA is 100 psi. On a 0-30 psi range the same 12 mA means 15 psi. Enter your range in the pressure tab above to convert exactly.
Request a quote
If the numbers lead to an instrument, send the measurement, range, supply voltage, and what the loop feeds into. We configure the transmitter so 4 and 20 mA land exactly where your control system expects them, and check the loop budget before shipping.