PLC Analog Scaling Calculator (Raw Counts to Engineering Units)

How to use the PLC analog scaling calculator

Enter the raw count range of the analog input card for your signal type, the engineering range of the transmitter and the raw count the PLC is reading. You get the value in engineering units, the percent of range and the resolution of one count. To go the other way, clear the raw count and enter a value in engineering units.

PLC analog scaling formula

EU = EU_min + (Raw − Raw_min) ÷ (Raw_max − Raw_min) × (EU_max − EU_min)

This is what the Siemens NORM_X / SCALE_X pair, the Rockwell SCP instruction and most PLC “scale” blocks calculate.

Example

A 0–10 bar transmitter on a Siemens S7-1200 input (0–27648) reads 20736 counts:

• Percent of range = 20736 ÷ 27648 = 75.0%
• Pressure = 0 + 0.75 × 10 = 7.5 bar
• Resolution = 10 ÷ 27648 = 0.000362 bar per count

Raw ranges and resolution of common analog inputs

Card / converter	Raw range	One count (% of span)	One count on 4–20 mA
Siemens S7-1200 / S7-1500 (nominal range)	0 – 27,648	0.00362%	0.579 µA
12-bit converter	0 – 4,095	0.02442%	3.907 µA
Mitsubishi Q64AD (normal resolution)	0 – 4,000	0.02500%	4.000 µA
Modicon M340 (default)	0 – 10,000	0.01000%	1.600 µA
15-bit unsigned	0 – 32,767	0.00305%	0.488 µA
16-bit unsigned	0 – 65,535	0.00153%	0.244 µA

Raw ranges depend on card model and configuration — confirm with the manufacturer’s manual.

Tips

• Scale in floating point (REAL) to avoid integer truncation.
• Clamp or alarm values outside the range: below about 3.8 mA or above 20.5 mA usually means a sensor fault (NAMUR NE 43).
• If the transmitter already does square-root extraction, scale linearly; otherwise apply the root after scaling.

Frequently asked questions

What is the PLC analog scaling formula?

EU = EU_min + (raw − raw_min) ÷ (raw_max − raw_min) × (EU_max − EU_min). It is the equation of a straight line through the two range end points.

Why does Siemens use 27648 instead of 32767?

Siemens maps the nominal range to 0–27648 and keeps the counts above it (up to 32511) for overrange, so the program can detect a signal above 20 mA before it saturates.

How do I convert engineering units back to raw counts?

Clear the raw count, enter the value in engineering units and calculate — raw = raw_min + (EU − EU_min) ÷ (EU_max − EU_min) × (raw_max − raw_min).

Does the raw range change for 4–20 mA versus 0–20 mA?

On many cards, yes — some map 4–20 mA to the full raw range while others give a non-zero count at 4 mA (for example 3277–16384 on older 16384-count cards). Always check the input card manual.

Related calculators

• 4–20 mA Scaling CalculatorConvert a 4–20 mA loop current to a process value (and back) for any measuring range — with live scaling chart.
• ADC Resolution Calculator (LSB and Step Size)Calculate ADC step size (LSB), number of levels and the digital code for an input voltage from bit depth and reference voltage — including 4–20 mA PLC inputs.
• Signal Converter Calculator (4–20 mA ↔ 0–10 V ↔ 1–5 V)Convert between 4–20 mA, 0–20 mA, 0–10 V and 1–5 V analog signals and percent of range — for transmitters, PLCs and signal isolators.
• 5-Point Calibration Calculator (0–25–50–75–100% Table)Generate a 5-point calibration table for any transmitter — input values and expected 4–20 mA (or other) outputs at 0, 25, 50, 75 and 100% of range.
• Instrument Accuracy Calculator (% of Span vs % of Reading)Convert instrument accuracy between % of span, % of full scale and % of reading — and find the measurement error at any point in the range.
• 4–20 mA Loop Resistance Calculator (Maximum Loop Load)Check whether a 4–20 mA loop has enough voltage — maximum loop resistance from supply and transmitter lift-off voltage, with cable and barrier resistance.

Embed this calculator