mA to Voltage Calculator

Calculate current, resistance, or voltage with Ohm's law by entering any 2 values and choosing mA, ohm, and mV-to-kV units for the missing value.

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mA to Voltage Formula

The conversion from milliamps to voltage is a direct application of Ohm’s Law:

V = (I_{mA} / 1000) \times R

Where V is the resulting voltage in volts, I is the current in milliamps, and R is the resistance in ohms. The division by 1000 converts milliamps to amps before applying V = IR. Without a known resistance value, milliamps cannot be converted to volts because current and voltage are fundamentally different electrical quantities linked only through impedance.

The formula can also be rearranged to solve for current (I = V/R) or resistance (R = V/I), which is why the calculator above accepts any two of the three values.

Why Convert mA to Voltage?

Most analog-to-digital converters (ADCs), microcontrollers, PLCs, and data acquisition systems accept voltage inputs, not current. However, in industrial process control, sensors and transmitters frequently output a 4-20 mA signal because current is immune to voltage drops over long cable runs. At the receiving end, a precision shunt resistor converts that loop current back into a voltage the controller can read.

Common scenarios that require mA to voltage conversion include reading a 4-20 mA pressure, temperature, or flow transmitter with a voltage-input data logger; interfacing industrial sensors with Arduino or Raspberry Pi platforms that accept 0-3.3 V or 0-5 V analog inputs; and troubleshooting control loops with a multimeter by measuring voltage across a known resistor instead of breaking the loop to measure current directly.

The 4-20 mA Current Loop Standard

The 4-20 mA current loop, defined by ANSI/ISA-50.1, is the dominant analog signaling standard in industrial instrumentation. It maps a measured process variable (pressure, temperature, flow, level, pH) to a current range where 4 mA represents 0% of scale and 20 mA represents 100% of scale.

The “live zero” at 4 mA is a deliberate design choice. A reading of 0 mA indicates a broken wire or failed transmitter, not a zero-value measurement. This built-in fault detection is one reason current loops remain the preferred analog signaling method in process plants, refineries, water treatment facilities, and HVAC systems, even as digital protocols like HART and fieldbus gain adoption.

Current loops are powered by a 24 VDC supply in most installations. The transmitter regulates current flow through the loop, and because current in a series circuit is identical at every point, the signal is unaffected by wire resistance. This allows reliable transmission over distances exceeding 1,000 meters, something voltage signals cannot achieve without amplification or signal conditioning.

Common Shunt Resistor Values and Voltage Outputs

To extract a voltage from a 4-20 mA loop, a precision shunt resistor is placed in series at the receiver. The table below shows the resulting voltage at key points across the signal range for the most commonly used resistor values.

Resistor Precision Requirements

The accuracy of a mA to voltage conversion depends directly on the shunt resistor’s tolerance. A 1% tolerance resistor introduces up to 1% measurement error before any other sources of inaccuracy are considered. For process control applications where instrumentation accuracy matters, the following specifications are recommended: tolerance of 0.1% or better, temperature coefficient of 25 ppm/C or lower to minimize thermal drift, and a power rating of at least 0.25 W for reliability margin. Metal film and wire-wound precision resistors are the preferred types for this application.

At 20 mA through a 250 ohm resistor, actual power dissipation is only P = I^2 x R = (0.02)^2 x 250 = 0.1 W, well within the rating of standard quarter-watt resistors. However, in high-temperature environments or enclosures with limited airflow, derating the power specification by 50% is good engineering practice.

Scaling mA to a Process Variable

In process control, the voltage reading from a shunt resistor ultimately represents a physical measurement. The linear scaling formula to convert any current within the 4-20 mA range back to the measured process variable is:

PV = \frac{I - 4}{16} \times (PV_{max} - PV_{min}) + PV_{min}

For example, a pressure transmitter calibrated for 0 to 100 PSI that outputs 12 mA: PV = ((12 – 4) / 16) x (100 – 0) + 0 = 50 PSI. With a 250 ohm resistor, that 12 mA produces 3.0 V, so 3.0 V corresponds to 50 PSI. The same math works for any calibrated range: a temperature transmitter spanning 0 to 500 degrees F at 8 mA reads ((8 – 4) / 16) x 500 = 125 degrees F, producing 2.0 V across 250 ohms.

Maximum Loop Resistance and Compliance Voltage

Every 4-20 mA transmitter has a maximum loop resistance it can drive, determined by the power supply voltage minus the transmitter’s minimum operating voltage (compliance voltage). The relationship is: R_max = (V_supply – V_transmitter_min) / 0.020. For a typical 24 VDC supply with a transmitter requiring at least 12 V to operate, the maximum total loop resistance is (24 – 12) / 0.020 = 600 ohms. This budget must cover the shunt resistor, wire resistance, and any other series loads. A 250 ohm shunt consumes the largest share, leaving 350 ohms for wiring, which at approximately 5 ohms per 100 meters of 18 AWG copper wire, supports cable runs up to about 3,500 meters one way.

Microcontroller and Arduino Applications

When interfacing a 4-20 mA sensor with a microcontroller, the shunt resistor value must be chosen to match the ADC’s input voltage range. An Arduino Uno with a 10-bit ADC (0-5 V range) pairs well with a 250 ohm resistor, producing 1-5 V from the 4-20 mA signal. A 3.3 V microcontroller (ESP32, STM32, Raspberry Pi Pico) should use a 150 ohm resistor, producing 0.6-3.0 V and staying safely below the 3.3 V reference. For extra protection, a 3.3 V Zener diode clamp across the resistor prevents ADC damage if the loop current exceeds 20 mA during a fault condition.

The ADC reading can then be converted to the process variable in firmware using the same linear scaling formula: PV = ((ADC_value – ADC_at_4mA) / (ADC_at_20mA – ADC_at_4mA)) x (PV_max – PV_min) + PV_min.