What is a voltage divider?

A voltage divider is a simple circuit that uses two resistors in series to create a smaller output voltage from a larger input voltage.

What is the voltage divider formula?

The unloaded formula is Vout = Vin × R2 ÷ (R1 + R2), where R2 is the resistor connected from the output node to ground.

What is a loaded voltage divider?

A loaded voltage divider includes the resistance of the circuit connected to the output. The load appears in parallel with R2 and usually lowers the output voltage.

Why is my voltage divider output lower than expected?

The most common reason is loading. If the connected circuit has low input resistance, it changes the divider ratio and pulls the output down.

Can I use a voltage divider as a power supply?

Only for very small signals or light loads. A resistor divider wastes current and has poor regulation, so it should not be used as a real power supply for variable loads.

What resistor values should I use?

Choose values low enough that load current does not create large error, but high enough to avoid wasting too much current. For ADC inputs, also check the ADC source impedance recommendation.

What is Thevenin resistance in a divider?

Thevenin resistance is R1 in parallel with R2. It helps estimate how strongly the divider can drive a load or ADC input.

Does resistor tolerance affect output voltage?

Yes. Real resistors are not exact, so 1% resistors usually give better divider accuracy than 5% resistors.

Engineering

Voltage Divider Calculator

Calculate resistor divider output voltage, loaded output voltage, divider current, resistor power, and resistor values for a target output. Includes both unloaded and loaded voltage divider calculations.

Voltage divider formula

For an unloaded two-resistor divider, the output is taken across the bottom resistor R2.

Vout = Vin × R2 ÷ (R1 + R2) Divider current = Vin ÷ (R1 + R2) Loaded divider: replace R2 with R2 || Rload

When a load is connected to the output, it sits in parallel with R2 and lowers the effective bottom resistance. That is why high-resistance dividers can become inaccurate when connected to low-resistance loads.

Worked example

Vin = 12 V R1 = 10 kΩ R2 = 10 kΩ Vout = 12 × 10000 ÷ (10000 + 10000) Vout = 6 V

If a load is added at the output, the result may fall below 6 V because the load changes the divider ratio.

Practical design notes

Use lower resistor values when the output must drive an ADC input, reference pin, or measurable load. Use higher resistor values when saving battery current matters, but remember that leakage current, ADC sample capacitance, and input impedance can cause error.

Trusted references

For deeper non-competitor reading, see Analog Devices – Voltage Divider, Analog Devices – Practical Resistive Dividers, and Analog Devices – source resistance and ADCs.

Common questions

A voltage divider is a simple circuit that uses two resistors in series to create a smaller output voltage from a larger input voltage.
The unloaded formula is Vout = Vin × R2 ÷ (R1 + R2), where R2 is the resistor connected from the output node to ground.
A loaded voltage divider includes the resistance of the circuit connected to the output. The load appears in parallel with R2 and usually lowers the output voltage.
The most common reason is loading. If the connected circuit has low input resistance, it changes the divider ratio and pulls the output down.
Only for very small signals or light loads. A resistor divider wastes current and has poor regulation, so it should not be used as a real power supply for variable loads.
Choose values low enough that load current does not create large error, but high enough to avoid wasting too much current. For ADC inputs, also check the ADC source impedance recommendation.
Thevenin resistance is R1 in parallel with R2. It helps estimate how strongly the divider can drive a load or ADC input.
Yes. Real resistors are not exact, so 1% resistors usually give better divider accuracy than 5% resistors.