4–20 mA Loop Scaling

I = 4 + 16 × (val − min) / (max − min)

Quick answer

This calculator solves I = 4 + 16 × (value − min) / (max − min) mA for Loop Current. With Process Value 0.5, Range Minimum 1.1, Range Maximum 2.35, the result is -3.68 mA. It solves in 2 directions, so you can enter the values you have and get the one you need.

Calculator

Loop Current (mA)
—

Formula

I = 4 + 16 × (value − min) / (max − min) mA

Description

The 4-20 mA current loop is the dominant analog signaling standard in industrial process control. The process variable is linearly mapped so that 4 mA represents the minimum value and 20 mA represents the maximum. The 4 mA live zero provides several critical advantages: it powers the transmitter (two-wire loop powered), it distinguishes a valid zero reading from a broken wire (which reads 0 mA), and it provides consistent power to the transmitter regardless of signal level. The 16 mA span (4 to 20) represents 100% of the measurement range.

Variables

  • I — Loop current (mA)
  • val — Current process variable value
  • min — Process variable at 4 mA (range minimum)
  • max — Process variable at 20 mA (range maximum)

Practical Notes

NAMUR NE43 recommends that currents below 3.6 mA or above 21.0 mA indicate a fault condition. Current signals are immune to voltage drops in long cable runs (up to several km) and electrical noise, making them far more reliable than voltage signals in industrial environments. HART protocol overlays digital communication on the 4-20 mA signal without disturbing the analog reading.

Symbols and units

Symbol Meaning Default unit Accepted units
I Loop Current mA pA, nA, µA, mA, A, kA
val Process Value — —
min Range Minimum — —
max Range Maximum — —

Step by step

  1. 1. What you know: Process Value 0.5, Range Minimum 1.1, Range Maximum 2.35
  2. 2. The relation: I = 4 + 16 × (value − min) / (max − min) mA
  3. 3. Substitute: val = 0.5, min = 1.1, max = 2.35
  4. 4. Result: Loop Current = -3.68 mA

Worked examples

Minimum range

Given: Process Value 0.025, Range Minimum 0.055, Range Maximum 0.118

Result: Loop Current = -3.619 mA

Low range

Given: Process Value 0.25, Range Minimum 0.55, Range Maximum 1.18

Result: Loop Current = -3.619 mA

Typical

Given: Process Value 0.5, Range Minimum 1.1, Range Maximum 2.35

Result: Loop Current = -3.68 mA

High range

Given: Process Value 0.8, Range Minimum 1.76, Range Maximum 3.76

Result: Loop Current = -3.68 mA

Maximum range

Given: Process Value 8, Range Minimum 17.6, Range Maximum 37.6

Result: Loop Current = -3.68 mA

Solving for each variable

Solve for Loop Current

Process Value 0.025, Range Minimum 0.055, Range Maximum 0.118 → -3.619 mA

Solve for Process Value

Loop Current 100 µA, Range Minimum 0.055, Range Maximum 0.118 → 0.03964

How Loop Current changes with Process Value

Process Value (input) Loop Current (result)
0.05 -9.44 mA
0.125 -8.48 mA
0.25 -6.88 mA
0.375 -5.28 mA
0.5 -3.68 mA
0.75 -0.48 mA
1 2.72 mA
2 15.52 mA
3.5 34.72 mA
5 53.92 mA

The same result in other units

Unit Value
nA -3680000
µA -3680
mA -3.68
A -0.00368

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Frequently asked questions

How do you calculate 4–20 mA Loop Scaling?

Use I = 4 + 16 × (value − min) / (max − min) mA. With Process Value 0.5, Range Minimum 1.1, Range Maximum 2.35, Loop Current works out to -3.68 mA.

What is the formula for 4–20 mA Loop Scaling?

The formula is I = 4 + 16 × (value − min) / (max − min) mA. The 4-20 mA current loop is the dominant analog signaling standard in industrial process control.

How do you find Process Value when you know the other values?

Rearrange for Process Value: with Loop Current 100 µA, Range Minimum 0.055, Range Maximum 0.118, Process Value is 0.03964.

What units does 4–20 mA Loop Scaling use?

Loop Current is shown in nA, µA, mA, A. Values are held in SI base units internally, so you can mix prefixes without converting by hand.

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