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4–20 mA Scaling Calculator

Convert a measured 4–20 mA transmitter signal into engineering units. Useful for pressure, temperature, flow, level, and other linear 4–20 mA instrumentation signals.

Enter the transmitter's lower and upper range values and the measured loop current to calculate the corresponding process value.

Calculate Engineering Units

The engineering value represented by 4 mA.
The engineering value represented by 20 mA.
Examples: PSI, °F, °C, GPM, %, feet.
Enter the measured transmitter loop current.

Result

4–20 mA Scaling Formula

A standard 4–20 mA transmitter uses a 16 mA signal span: 20 mA − 4 mA = 16 mA.

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

Where:

  • LRV = Lower Range Value
  • URV = Upper Range Value
  • mA = measured loop current

Worked Example

Suppose a pressure transmitter is ranged from 0 to 100 PSI.

You measure 12 mA in the loop.

0 + ((12 − 4) ÷ 16) × (100 − 0)

The result is:

12 mA = 50 PSI

Twelve milliamps is halfway through the 4–20 mA signal range, so it represents 50% of the transmitter's configured process range.

Field Application

4–20 mA scaling is commonly used when troubleshooting transmitters, PLC analog inputs, DCS inputs, and control systems.

For example, suppose a pressure transmitter is configured for 0–150 PSI and you measure 10.4 mA at the transmitter output.

You can calculate the expected process value independently of the PLC display. This helps determine whether the problem is at the transmitter, wiring, analog input, or PLC scaling.

Useful 4–20 mA Reference Points

  • 4 mA = 0% of span
  • 8 mA = 25% of span
  • 12 mA = 50% of span
  • 16 mA = 75% of span
  • 20 mA = 100% of span

Common 4–20 mA Scaling Mistakes

  1. Using 20 mA as the signal span. The active span is 16 mA because the signal starts at 4 mA.
  2. Assuming the lower range is always zero. A transmitter can have a negative or non-zero LRV.
  3. Using the wrong transmitter range. Verify the actual configured LRV and URV.
  4. Confusing milliamps with PLC raw counts. A PLC analog input may convert the electrical signal into raw counts before the program scales it.
  5. Assuming the PLC display proves the transmitter is correct. Measuring the actual loop current provides an independent reference when troubleshooting.

Using the Calculation for Troubleshooting

One useful troubleshooting method is to measure the actual loop current and calculate what the process value should be.

For example, with a 0–100 PSI transmitter:

16 mA → 75 PSI

If the PLC displays a substantially different value, investigate the analog input configuration, PLC scaling, signal wiring, grounding, and transmitter configuration.

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Engineering Note

This calculator assumes a linear relationship between the configured Lower Range Value and Upper Range Value. Always verify transmitter configuration, wiring, analog input configuration, and manufacturer documentation when troubleshooting an actual control system.