4–20 mA scaling calculator

How do you scale a 4–20 mA signal?

Any linear analog signal maps a signal span onto an engineering span. Work out how far the signal sits through its span (the signal fraction), then move the same fraction through the engineering range. The calculator above, and the My LoopCal app it mirrors, does it in this order:

1. Signal fraction = (signal − signal low) ÷ (signal high − signal low).
2. Direction: for reverse acting, acting fraction = 1 − signal fraction; otherwise it is unchanged.
3. Transfer: linear keeps the fraction; square root takes √fraction.
4. Engineering value = LRV + fraction × (URV − LRV).

The inverse — the signal a transmitter should send for a given process value — runs the same steps backward: engineering fraction, squared for a square-root range, inverted for reverse action, then mapped onto the signal span. Percent always means percent of the signal span, so 12 mA reads 50% whether the range is linear or square root.

The same formula covers 0–20 mA, 0–10 V and 1–5 V: only the endpoints change. A 1–5 V input is what you get when a 4–20 mA loop drives a 250 Ω resistor, so the percentages line up: 3.75 V on a 1–5 V input scaled 0–10 bar is 68.75% and 6.875 bar.

4–20 mA conversion table: linear and square root

Quick reference for a 0–100 range. The square-root column is flow as a percentage of full scale for a differential-pressure transmitter whose signal is linear in DP.

Worked examples with real ranges

Temperature transmitter, −50 to 150 °C, reading 12 mA

An offset range does not change the method; 0 °C sits at 8 mA here, not at 4 mA.

Pressure transmitter 0–100 psi: what current at 75 psi?

DP flow transmitter with square-root extraction

Because flow varies with the square root of differential pressure, small DP signals mean large flow changes near zero. Near zero flow a tiny DP error becomes a large flow error, so check how the transmitter and the control system treat the bottom of the range (many offer a low-flow cutoff setting) before you trust readings there.

Reverse-acting valve position, 0–100 %, signal 6 mA

Where do scaling mistakes usually come from?

• Using 20 instead of 16 as the span. 4–20 mA has a 16 mA span; dividing by 20 makes every reading low.
• Mixing up the configured range. The transmitter, the DCS or PLC input and the indicator must share the same LRV and URV. Read them from the device, not from a drawing.
• Extracting the square root twice. If the transmitter already outputs flow, the control system must scale it linearly. Set square root in one place only.
• Clamping out-of-range signals. 3.6 mA or 21 mA may be a fault or alarm signal defined by the device; seeing the raw extrapolated value helps diagnosis.

From the conversion to a calibration record

My LoopCal: 4-20mA Calibration has this converter on its first tab, free and without limits, with the substituted formula under every result. “Verify this loop” carries the range straight into a guided check: expected output at each point, observed value, error, percent-of-span error and pass/fail against your tolerance, as found and, after adjustment, as left. The result is a PDF worksheet with your company name and logo. A new install includes one three-point as-found check; more checks, five-point and custom procedures, as-left runs, saved instruments and exports need an in-app purchase. The app is a calculation aid and field worksheet, not a calibrator: follow lockout/tagout, operations authorization and the manufacturer’s procedure.