How do I convert 4–20 mA to a temperature or pressure?

Subtract 4, divide by 16, multiply by the span (high minus low) and add the low end. For 0–100 °C, 12 mA is (12 − 4)/16 × 100 = 50 °C.

Why does a 4–20 mA loop start at 4 mA and not 0?

So a broken wire or dead power supply reads 0 mA and is recognised as a fault, instead of being mistaken for a real zero. The 4 mA also powers two-wire transmitters.

What are the NAMUR NE 43 limits?

Signals from 3.8 to 20.5 mA are valid measurements. At or below 3.6 mA, or at or above 21 mA, the device is signalling a fault. This calculator marks which side of those thresholds a current falls.

How do I scale a square-root flow transmitter?

A differential-pressure transmitter's current is linear in DP, but flow goes as the square root, so 12 mA (half the DP span) is 70.7 % of maximum flow, not 50 %. Choose the square-root mode, enter the flow at 20 mA as the high end, and the conversion and the five-point table follow that curve. If the transmitter extracts the square root itself, use the linear mode.

What calibration points should I check on a transmitter?

The usual set is 0, 25, 50, 75 and 100 % of span, which is 4, 8, 12, 16 and 20 mA, in rising and falling order if hysteresis matters. The table below the calculator is that set for your range.

4–20 mA current loop calculator

Scale a 4–20 mA signal to engineering units and back, see where it falls against the NAMUR NE 43 fault thresholds, and get the five-point calibration table for your range.

The transmitter's range

4 mA is (low)°C20 mA is (high)°CUnit

Transmitter output

Runs entirely in your browser · nothing you enter is sent to a server

From a current to a value

Loop current (mA)mASense resistorΩ

50 °C

50 % of span

Within 4–20 mA

3 V across 250 Ω

Pneumatic equivalent 9 psi (0.6 bar) on a 3–15 psi signal

From a value to a current

Process value (°C)°C

12 mA

50 % of span

Within 4–20 mA

Five-point check

Span	Current	Value
0 %	4 mA	0 °C
25 %	8 mA	25 °C
50 %	12 mA	50 °C
75 %	16 mA	75 °C
100 %	20 mA	100 °C

Error as % of span%

= 16 µA of loop current

How the calculation works

01The live zero: 4 mA is 0 % of span and 20 mA is 100 %, so value = low + (mA − 4)/16 × (high − low) and its inverse. A dead loop (0 mA) is then distinguishable from a true zero reading.
02Percent of span is (mA − 4)/16. An error of 0.1 % of span is therefore 16 µA of loop current, which is the figure to compare with a loop calibrator's accuracy.
03NAMUR NE 43 treats 3.8 to 20.5 mA as a valid measurement and a signal at or below 3.6 mA or at or above 21 mA as a transmitter fault; between those it is outside the measuring range but not a failure.
04The voltage across a sense resistor is V = I × R, so a 250 Ω resistor turns 4–20 mA into 1–5 V.
05For a differential-pressure flow transmitter the current is linear in DP but flow goes as its square root, so flow = low + √((mA − 4)/16) × (high − low): 12 mA is 70.7 % of maximum flow. The pneumatic equivalent is 3–15 psi (0.2–1.0 bar) over the same span.

Limitations

The linear mode assumes a linear transmitter. Choose the square-root mode for a head-type flow transmitter, and note that some transmitters extract the square root internally, in which case the output is already linear in flow.
NE 43 failure levels are a convention a transmitter may or may not follow: its configured burn-out direction decides what it actually outputs on a fault.
It checks the arithmetic, not the loop: supply voltage, burden resistance and wiring are outside it.

Frequently asked questions

How do I convert 4–20 mA to a temperature or pressure?: Subtract 4, divide by 16, multiply by the span (high minus low) and add the low end. For 0–100 °C, 12 mA is (12 − 4)/16 × 100 = 50 °C.
Why does a 4–20 mA loop start at 4 mA and not 0?: So a broken wire or dead power supply reads 0 mA and is recognised as a fault, instead of being mistaken for a real zero. The 4 mA also powers two-wire transmitters.
What are the NAMUR NE 43 limits?: Signals from 3.8 to 20.5 mA are valid measurements. At or below 3.6 mA, or at or above 21 mA, the device is signalling a fault. This calculator marks which side of those thresholds a current falls.
How do I scale a square-root flow transmitter?: A differential-pressure transmitter's current is linear in DP, but flow goes as the square root, so 12 mA (half the DP span) is 70.7 % of maximum flow, not 50 %. Choose the square-root mode, enter the flow at 20 mA as the high end, and the conversion and the five-point table follow that curve. If the transmitter extracts the square root itself, use the linear mode.
What calibration points should I check on a transmitter?: The usual set is 0, 25, 50, 75 and 100 % of span, which is 4, 8, 12, 16 and 20 mA, in rising and falling order if hysteresis matters. The table below the calculator is that set for your range.

Need the reasoning, not just the number?

Ask ValiTrac AI and see the standards evidence and the engine calculation behind the answer.

Ask ValiTrac AI

Calibration by Parameter

Pressure gauge calibration: procedure, points and uncertainty

Rising and falling series, hysteresis, and the head correction people forget — about 98 Pa per centimetre of water, which usually beats the stated uncertainty.

2026-09-30 · 5 min read

Calibration