Voltage Divider Calculator
Calculate the output voltage of a two-resistor voltage divider circuit.
What is a voltage divider?
A voltage divider is a simple circuit with two resistors in series that produces an output voltage that is a fraction of the input voltage. The formula is Vout = Vin * R2 / (R1 + R2), where R1 is the top resistor (connected to the input) and R2 is the bottom resistor (connected to ground). The output is taken at the junction between the two resistors.
Voltage dividers are one of the most fundamental circuits in electronics. They are used to scale voltages for measurement (reading a 12V battery with a 3.3V microcontroller ADC), set bias points in amplifier circuits, create reference voltages, and provide level shifting between different voltage domains.
The voltage divider formula
Vout = Vin * R2 / (R1 + R2). The output is always less than the input. To get exactly half the input voltage, use equal resistors (R1 = R2). For one-third of the input, use R2 = R1/2 (for example, R1 = 10K and R2 = 5K). The actual resistor values do not matter โ only the ratio determines the output voltage.
How to use this tool
Enter the input voltage and either the desired output voltage (to find resistor values) or the two resistor values (to find the output voltage). The calculator shows the output voltage, the division ratio, and the current drawn from the source. It suggests standard resistor values that closely match your requirements.
Loading effects
A voltage divider only works correctly when the load connected to its output draws negligible current compared to the current flowing through the divider itself. If the load resistance is too low, it effectively parallels R2, reducing the output voltage. As a rule of thumb, the load resistance should be at least 10 times larger than R2 for less than 10% error.
Frequently asked questions
Can I use a voltage divider as a power supply?
No. A voltage divider is not a regulated power supply. The output voltage changes with load current, and the divider wastes power continuously through both resistors. For powering circuits, use a voltage regulator (like a 7805 or LM317) or a DC-DC converter. Voltage dividers are only suitable for signal-level applications where the current draw is minimal.
What values should I use for R1 and R2?
Only the ratio matters for the voltage, but the absolute values affect current consumption and noise. Higher resistance values (100K range) waste less power but are more susceptible to noise. Lower values (1K range) are more noise-immune but draw more current. A good starting point is 10Kโ100K for most signal-level applications.