4–20 mA to Voltage Calculator

4–20 mA to Voltage Formula

The voltage across a resistor is calculated using Ohm's Law:

Because the input current is entered in milliamps, the current must be converted to amps before calculating voltage:

Where:

• V = voltage in volts
• I = current in amps
• R = resistance in ohms

Worked Example: 12 mA Through a 250 Ω Resistor

A very common instrumentation application uses a 250 Ω resistor to convert a 4–20 mA signal into a nominal 1–5 V signal.

At 12 mA:

Therefore:

The complete 4–20 mA signal becomes:

• 4 mA → 1 V
• 8 mA → 2 V
• 12 mA → 3 V
• 16 mA → 4 V
• 20 mA → 5 V

Why 250 Ω Is Common in Instrumentation

A 250 Ω resistor produces a 1–5 V signal from a 4–20 mA loop:

This makes it possible for equipment designed to accept a voltage signal to interpret a standard 4–20 mA instrument signal.

This arrangement is commonly encountered with analog input modules, signal converters, data acquisition equipment, and other instrumentation.

Using This Calculation for Troubleshooting

If a device receives a 4–20 mA signal through a known resistor, you can calculate the voltage that should be present across the resistor.

For example, if you measure 16 mA through a 250 Ω resistor:

If you measure substantially less than 4 V, possible areas to investigate include the actual loop current, resistor value, wiring connections, or the measurement point.

Always verify the circuit design before making changes to an operating control loop.

Resistor Power Considerations

The resistor also dissipates power. This can be calculated using:

For a 250 Ω resistor at 20 mA:

A resistor with an appropriate power rating should be selected for the application. The loop's voltage budget and the specifications of the connected devices must also be considered.

Common Mistakes

1. Forgetting to convert mA to A. 12 mA is 0.012 A, not 12 A.
2. Assuming every 4–20 mA loop produces 1–5 V. That only occurs when the resistor is 250 Ω.
3. Measuring voltage at the wrong location. Voltage depends on the resistance between the two measurement points.
4. Ignoring resistor power dissipation. Verify the resistor's power rating for the application.
5. Adding resistance without checking loop voltage. Additional resistance increases the voltage required from the loop power supply.

Related Calculators

• 4–20 mA Scaling
• 4–20 mA Loop Resistor
• Ohm's Law
• Voltage Divider
• 4–20 mA Guide

Engineering Note

This calculator uses Ohm's Law and assumes the entered resistance is the resistance across which the voltage is being calculated. In an actual 4–20 mA loop, verify the complete loop voltage budget, device specifications, wiring resistance, and manufacturer's documentation before modifying the circuit.