4–20 mA scaling calculator
Turn a loop current into pressure, level, temperature or any other value, and see what a PLC analogue input should read. Useful at commissioning, and when a reading looks wrong.
Process value
5bar
50 per cent of span
Siemens normalised value
13824
0 at 4 mA and 27648 at 20 mA on S7-300, S7-1200 and S7-1500 analogue inputs. Other brands use different ranges.
Within the valid measuring range (3.8 to 20.5 mA).
Reading looks wrong?
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How it works
A 4 to 20 mA signal spans 16 mA. The value is Low + (mA − 4) ÷ 16 × (High − Low). At 4 mA the transmitter reports the bottom of its range and at 20 mA the top. The live zero at 4 mA lets the system tell a real zero from a broken wire.
NAMUR NE 43 treats 3.8 to 20.5 mA as the valid measuring range and signals below 3.6 mA or above 21 mA are treated as faults. A reading of 0 mA usually means a broken loop or no supply.
Siemens analogue inputs on the S7-300, S7-1200 and S7-1500 present a 4 to 20 mA signal as a normalised value from 0 to 27648. Other brands use different raw ranges, so check your input module's manual.
Common questions
My PLC reads zero but the transmitter has power. What should I check?
Measure the loop current with a meter in series. 0 mA points to a broken wire or missing loop supply. 4 mA with a zero reading is genuine. Also check that the input channel is configured for current and not voltage.
Why can't my loop reach 20 mA?
The loop supply may be too low for the total resistance in the circuit, including the input resistor, cable and any indicators. Check the supply voltage against the transmitter minimum and the burden.
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