How to Use the 4-20mA Signal Scaling Calculator
The 4-20mA loop is the default analog signal in process instrumentation. Starting at 4mA instead of zero means a dead loop reads differently from a genuine zero measurement, and because the signal is a current rather than a voltage, long wire runs barely affect accuracy.
Scaling is purely linear. Going from current to measurement, value = (mA − 4) ÷ 16 × span + low, and the reverse direction is mA = (value − low) ÷ span × 16 + 4. On a 0 to 100 range, 12mA lands exactly at 50 percent of span, or 50 units. Where the analog input card accepts voltage instead, a 250Ω precision resistor turns the loop into a 1 to 5V signal, and the converted voltage in the results is what a meter should read at the terminals.
When the calculated value disagrees with the display, the cause is usually a transmitter range that no longer matches the drawing, or a drifted zero and span. Negative values cannot be typed into the input boxes, so for a range such as −50 to 50 °F, add 50 to both ends, run the calculation on 0 to 100, and subtract 50 from the result. Confirm the final range against the transmitter data sheet and its calibration certificate.
Frequently Asked Questions
With a 4mA floor, current still flows when the measurement sits at its minimum, so a reading near 0mA clearly indicates a broken wire or a dead supply. This live zero also powers the electronics of a two-wire transmitter.
Instruments following NAMUR NE43 treat 3.8 to 20.5mA as the valid measuring window and reserve values beyond it for fault signaling. If your result falls outside that window, check whether the configured range still matches the real process span.
It converts the loop current into a voltage for input cards that only accept volts: 4mA across 250Ω is 1V and 20mA is 5V, giving the standard 1 to 5V signal. Any error in the resistor becomes measurement error, so a 0.1 percent precision part is used.