Free tool
4–20 mA scale calculator and analog scaling tool
Convert 4–20 mA, 0–20 mA, 0–10 V, ±10 V or a raw PLC count (e.g. Siemens 0–27648) to engineering units and back. The formula is shown step by step, with warnings for broken wires and out-of-range values.
Signal to value
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Raw PLC count: –
Value to signal
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Raw PLC count: –
Check table
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How analog scaling works
Field transmitters turn what they measure (pressure, temperature, level, flow) into a linear current or voltage. The PLC's analog input module turns that signal into an integer; on Siemens S7-1200/1500 the nominal range of a 4–20 mA input is 0–27648. Getting to engineering units takes two linear steps: signal to raw count, and raw count to engineering value. The calculator shows both.
Most scaling errors come from mismatched ranges: the transmitter is set to one range and the PLC is written for another, or nobody knows which range the raw count uses. That is why every analog signal's range and unit go into the PLC signal list before data collection starts, and are checked at the machine against a known point. The check table is there for that.
Out-of-range and failure signals
For 4–20 mA inputs, warnings follow NAMUR NE 43: 3.8–20.5 mA is measurement, 3.6 mA or less and 21 mA or more are failure signals. Siemens raw counts are classified using the tables in the S7-1200 System Manual (A5E02486680-06, Tables A-134 and A-135): 27649–32511 overshoot (up to 22.81 mA on 4–20 mA), 32512–32767 overflow; on current inputs −1 to −4864 undershoot (down to 1.185 mA), −4865 and below underflow; on ±10 V, −32513 and below underflow. Overflow and underflow values are not measurements, and the calculator does not convert them. How a broken wire reads depends on the module and its diagnostics settings; it can show as underflow. On S7-1500 and other modules, check the ranges in the module manual. Your device's failure current may be configured differently; check its manual.
Our PLC data collection guide covers how analog and digital values are read from PLCs, and the Siemens S7 data collection page covers Siemens specifics. For continuous monitoring of analog signals, see process monitoring. All tools are on the tools page.
Frequently asked questions
Common questions
How do you scale a 4–20 mA signal?
4–20 mA is linear: 4 mA is the bottom of the range and 20 mA the top. Value = low + (mA − 4) / 16 × (high − low). For a 0–10 bar pressure transmitter, 12 mA means 5 bar.
What does 27648 mean on a Siemens PLC?
Siemens S7-1200 and S7-1500 analog modules turn the nominal measuring range into an integer from 0 to 27648 (−27648 to 27648 for bipolar signals). On a 4–20 mA input, 4 mA reads 0 and 20 mA reads 27648. Per the S7-1200 manual, 27649–32511 is the overshoot range and 32512–32767 overflow; on current inputs −1 to −4864 is undershoot and −4865 and below underflow. Overflow and underflow are not measurements.
Why use 4–20 mA instead of 0–20 mA?
With 4–20 mA, zero is sent as 4 mA (a "live zero"). A reading of 0 mA therefore does not mean a zero measurement: the wire is broken or the sensor has lost power. That difference lets you tell a fault from a reading.
What is NAMUR NE 43?
NAMUR NE 43 is a recommendation that separates measurement from failure on 4–20 mA devices: 3.8–20.5 mA is measurement, 3.6 mA or less and 21 mA or more are failure signals. The calculator's warnings use these limits; check your device manual for its own failure current.
What raw range do other PLC brands use?
The raw range depends on the brand, the module and how the module is configured. That is why only Siemens' documented range is built in; for other brands, enter the range from the module manual under "Custom range".
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