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4–20 mA Signal Scaling

Convert between process value, percent of span, and loop current.

1

Enter the range

Calculate from
2

Calculated output

Loop current12.000 mA
Process value150.000 PSI
Percent of span50.000%
Loop current12.000 mA
Span300.000 PSI
Show calculation

mA = 4 + (PV − LRV) ÷ (URV − LRV) × 16

Equation guide

4–20 mA scaling and process-control equations

A standard 4–20 mA loop represents the complete calibrated process range with a 16 mA span. Because 4 mA represents zero percent and 20 mA represents 100 percent, the live zero makes an open circuit distinguishable from a valid zero reading.

The calculator also handles differential-pressure square-root extraction, common pressure and flow conversions, temperature scales, and hydrostatic head. Confirm the transmitter’s configured LRV, URV, characterization, and engineering units before using any result.

Process value to loop current

mA = 4 + [(PV − LRV) ÷ (URV − LRV)] × 16

PV is the process value. LRV and URV are the lower and upper range values.

Loop current to process value

PV = LRV + [(mA − 4) ÷ 16] × (URV − LRV)

Percent of span equals (mA − 4) ÷ 16 × 100.

Square-root flow from differential pressure

Flow % = √(DP % ÷ 100) × 100Flow = Full-scale flow × √(DP fraction)

Use square-root extraction only when differential pressure is proportional to flow squared and confirm where extraction is performed.

Hydrostatic liquid level

Height (in) = Pressure (inH₂O) ÷ Specific gravity

For a water reference, 1 PSI is approximately 27.68 inH₂O at 4 °C. Density and temperature affect real applications.

Temperature conversions

°F = (°C × 9 ÷ 5) + 32K = °C + 273.15

Rankine uses °R = °F + 459.67.

Linear unit conversion

Converted value = Input value × Unit conversion factor

Pressure and volumetric-flow tools convert through a common base unit to prevent chained-rounding errors.

Worked example: 0–300 PSI transmitter at 12 mA

A 12 mA signal is halfway through the 16 mA span.

Percent of span(12 − 4) ÷ 16 = 50%
Engineering span300 − 0 = 300 PSI
Process value0 + 0.50 × 300 = 150 PSI

Process control calculator FAQs

Why does a 4–20 mA signal start at 4 mA?

The live zero powers two-wire transmitters and helps distinguish a valid zero-percent measurement from a broken wire or loss of loop power.

What does 12 mA represent?

Twelve milliamps is 50% of the 4–20 mA signal span. For a linear 0–300 PSI range, it represents 150 PSI.

When should square-root extraction be used?

Use it for differential-pressure primary elements when DP is proportional to flow squared. Do not apply the square root twice; verify whether the transmitter, control system, or display already performs it.

Why does specific gravity affect hydrostatic level?

Pressure depends on both liquid height and density. A denser liquid produces more pressure at the same height, so the calculated height is pressure head divided by specific gravity.

Can calculated values replace a field calibration?

No. These are reference calculations. Field calibration still requires suitable test equipment, an approved procedure, traceability, and verification of the complete measurement loop.

Field caution: Confirm scaling and square-root configuration at the transmitter and control system before applying simulated signals to an operating process.