4-20 mA Signal Calculator

How 4-20 mA scaling works

The 4-20 mA current loop is the most common analog signal in process automation. A transmitter converts its measured value into a current between 4 mA and 20 mA. The lower range value (LRV) corresponds to 4 mA and the upper range value (URV) to 20 mA, giving a 16 mA span.

Process value = LRV + (mA − 4) ÷ 16 × (URV − LRV)
Current (mA)  = 4 + 16 × (value − LRV) ÷ (URV − LRV)

Worked example

A pressure transmitter is ranged 0 to 10 bar, and the PLC reads 12 mA. The fraction of span is (12 − 4) ÷ 16 = 0.5, so the pressure is 0 + 0.5 × 10 = 5 bar. In the other direction, 7.5 bar is 75% of span, so the transmitter should output 4 + 0.75 × 16 = 16 mA.

Why the signal starts at 4 mA, not 0 mA

A "live zero" at 4 mA lets the control system tell the difference between a genuine zero reading and a broken wire. A reading of 0 mA can only mean a fault. The 4 mA minimum also provides enough current to power two-wire (loop-powered) transmitters.

NAMUR NE 43 fault levels

Many modern transmitters follow the NAMUR NE 43 recommendation, which the calculator uses to flag suspicious currents:

Current	Meaning
3.8 to 20.5 mA	Valid measurement (including slight under- and over-range)
3.6 mA or lower	Transmitter failure signal (downscale)
21.0 mA or higher	Transmitter failure signal (upscale)

Square-root extraction for DP flow

Flow through an orifice plate, venturi or other differential pressure element is proportional to the square root of the differential pressure. If the DP transmitter's output is linear with DP, the flow must be calculated as:

Flow % = √((mA − 4) ÷ 16) × 100

So 50% flow corresponds to only 25% DP, or 8 mA. Many transmitters can perform the square root internally; make sure it is applied exactly once, either in the transmitter or in the PLC/DCS, never in both. Near zero flow, square-root extraction amplifies noise, so most systems apply a low-flow cutoff.

Practical tips

• Use a loop calibrator to check at least 0%, 50% and 100% (or 0, 25, 50, 75, 100%) during commissioning.
• Confirm the range configured in the transmitter matches the scaling in the PLC or DCS.
• Reverse-acting ranges (for example LRV = 100, URV = 0) are valid; the calculator handles them.
• A steady reading slightly below 4 mA often points to a power supply or loop resistance problem.