Voltage Divider Calculator

Calculate the output voltage of a two-resistor voltage divider. Voltage dividers are commonly used with sensors, analog inputs, reference circuits, test equipment, and control-system electronics.

How a Voltage Divider Works

A voltage divider uses two resistors in series to produce a lower voltage from a higher voltage.

The output voltage is measured between the junction of R1 and R2 and the circuit common.

The important detail is that R2 is the resistor below the output connection. If R1 and R2 are swapped, the calculated output voltage changes.

Understanding the Resistors

Worked Example

Common Voltage Divider Ratios

Voltage Dividers in Controls

Voltage dividers show up in many control and instrumentation applications.

• Scaling a sensor voltage to an analog input range
• Creating a reference voltage
• Reading potentiometers
• Conditioning certain sensor signals
• Reducing a voltage for measurement
• Creating test signals for electronic circuits

Example: Reducing 24 V to Approximately 10 V

Suppose a circuit has a 24 V signal but an input can only accept approximately 0–10 V.

One possible divider ratio is:

Therefore, the resistor ratio should be selected so that:

Actual resistor values should then be selected based on the required accuracy, loading, current consumption, and the specifications of the receiving input.

The Most Important Practical Concept: Loading

The basic voltage-divider equation assumes that the output is not significantly loaded by whatever is connected to it.

In the real world, an analog input has some input impedance. If that input impedance is comparable to the resistor values in the divider, it becomes another path for current and changes the actual output voltage.

Using a Multimeter to Troubleshoot a Divider

1. Measure the input voltage between Vin and common.
2. Measure the voltage at the junction of R1 and R2 relative to common.
3. Compare the measured Vout with the calculated value.
4. If the values disagree, check resistor values and wiring.
5. Check whether the connected input is loading the divider.

If Vout Is Too High

• R1 may be too low.
• R2 may be too high.
• The resistor values may not be what the drawing specifies.
• R1 and R2 may have been installed in the wrong positions.
• The output circuit may not be connected as expected.

If Vout Is Too Low

• R1 may be too high.
• R2 may be too low.
• One resistor may have changed value or failed.
• The output may be heavily loaded.
• There may be a wiring or grounding problem.

Common Mistakes

• Swapping R1 and R2: The resistor connected to common is R2.
• Ignoring loading: The simple formula assumes the load does not significantly alter the circuit.
• Using extremely large resistors: High resistance reduces current consumption but makes the divider more susceptible to loading and electrical noise.
• Using extremely small resistors: Low resistance reduces loading effects but wastes more current and dissipates more power.
• Forgetting resistor tolerance: Real resistors have tolerances, so the actual output may differ from the nominal calculated value.
• Assuming a divider is a power supply: A voltage divider is generally intended for signal/reference applications, not for supplying significant load current.

Resistor Power

Current flows through both resistors because they are in series. Therefore, both resistors dissipate power.

The divider current is:

For a properly designed divider, check the power dissipated by each resistor and make sure the resistor's power rating is appropriate.

Related Calculators

Engineering Note

A voltage divider is simple electrically, but designing one for an actual control system requires more than calculating the ratio. Consider the receiving input impedance, resistor tolerance, temperature effects, noise, available current, and required signal accuracy.

For a production control system, always verify the divider against the specifications of the actual sensor, controller, analog input, or signal-conditioning device being used.