Electrical Engineering Tool

Voltage Divider Calculator

Calculate the output voltage of a two-resistor voltage divider from the input voltage, R1 and R2. This tool also calculates divider current, resistor power dissipation and the loaded output voltage when a load resistor is connected.

Voltage Divider Calculator

Enter Vin, R1 and R2. All values should be greater than zero.

Voltage applied across R1 + R2.
Upper resistor between Vin and Vout.
Lower resistor between Vout and ground.
Load connected across the output.
Output Voltage
—
V
Divider Current
—
A
R1 Power
—
W
R2 Power
—
W
R1 R2 Vin Vout GND

Voltage Divider Formula

A basic voltage divider uses two resistors in series to produce an output voltage that is a fraction of the input voltage. The output is normally measured across R2.

Vout = Vin × R2 / (R1 + R2)
Divider current: I = Vin / (R1 + R2)
R1 power: PR1 = I² × R1
R2 power: PR2 = I² × R2

How a Voltage Divider Works

The two resistors form a series circuit. Because the same current flows through both resistors, each resistor develops a voltage proportional to its resistance.

1. Apply Input Voltage

Connect the input voltage across the series combination of R1 and R2. The total resistance is R1 + R2.

2. Current Flows

The divider current is determined by the input voltage divided by the total resistance of the two resistors.

3. Measure Vout

The output voltage is measured across R2 and is proportional to R2 relative to the total resistance.

Voltage Divider Example

Suppose a circuit has a 12 V input, R1 is 10 kΩ and R2 is 5 kΩ.

Output Voltage

Vout = 12 × 5,000 / (10,000 + 5,000) = 4 V.

Divider Current

I = 12 / 15,000 = 0.0008 A, or approximately 0.8 mA.

Voltage Divider Design Considerations

A resistor divider is simple, but its output can change when a circuit connected to Vout draws significant current.

Load Resistance

A load connected across R2 effectively places another resistance in parallel with R2. This can lower the actual output voltage.

Resistor Power

Check the power dissipated by both resistors. Select resistors with an appropriate power rating and suitable operating margin.

Resistor Values

Very large resistor values reduce divider current but can make the output more sensitive to leakage and loading.

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Voltage Divider FAQ

A voltage divider is a simple circuit that uses two resistors in series to produce an output voltage that is a fraction of the input voltage.

For a two-resistor divider, the output voltage is Vout = Vin × R2 / (R1 + R2), where R1 is the upper resistor and R2 is the lower resistor connected to ground.

Yes. When a load resistor is connected across the output, it is effectively placed in parallel with R2. This changes the effective lower resistance and normally reduces the output voltage.

For an unloaded divider, calculate current by dividing Vin by the total resistance: I = Vin / (R1 + R2).

A voltage divider can provide a reference or signal voltage, but it is generally not suitable as a regulated power supply. A changing load can change the output voltage significantly. For powering a circuit, a suitable regulator or power converter may be more appropriate.