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Engineering Toolbox / 4–20 mA Signal Scaling

Instrumentation & Calibration

4–20 mA Signal Scaling

Convert a measured loop current to percentage of calibrated span and engineering value, or calculate the expected current for a known process value.

Calculator

Linear 4–20 mA scaling. Values are not clamped at 0–100%.

Enter the actual measured current in mA. Out-of-range values are allowed for troubleshooting.

Field use

A linear loop check, not a transmitter configuration tool.

Use it to verify expected indication during calibration, loop checks and troubleshooting where the instrument output is linearly scaled from 4 to 20 mA.

Calculation basis

For a linear 4–20 mA output:

% span = ((mA − 4) / 16) × 100
Process value = LRV + (% span / 100) × (URV − LRV)
mA = 4 + 16 × ((PV − LRV) / (URV − LRV))

LRV: lower range value.

URV: upper range value.

PV: process value in the selected engineering unit.

Applicability: linear 4–20 mA scaling. Square-root extraction, characterized outputs and device-specific alarm currents require separate treatment. ISA50 documentation continues to identify 4–20 mA as a widely used industrial analog signal; the historical ANSI/ISA-50.00.01 document was archived by ISA in 2024.

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