Voltage divider calculator
Enter the input voltage and two of the three unknowns. The tool shows the output, the current through the divider and the nearest standard resistors.
The divider is just a ratio
The output sits at the fraction of the input that R2 makes up of the total. Slide the tap and watch the output follow. The resistor values only change how much current flows, not the voltage.
Loading: the catch
Anything connected to the output sits in parallel with R2 and pulls Vout down. Rule of thumb: keep the divider current at least 10× the load current, or use it only for high-impedance inputs such as an ADC or op-amp input. Never use a divider to power a motor, LED strip or anything that draws real current.
Example: 12 V to a 3.3 V ADC
R1 = 10 kΩ, R2 = 3.9 kΩ gives 12 × 3.9 ÷ 13.9 = 3.37 V, drawing 0.86 mA. Swap R1 for 27 kΩ and R2 for 10 kΩ and you get 3.24 V at 0.32 mA, a safer margin below 3.3 V and less current.
Questions people ask
What is the voltage divider formula?
Vout = Vin × R2 ÷ (R1 + R2), where R2 is the resistor between the output and ground.
Can I power a device from a voltage divider?
Usually not. Any load in parallel with R2 lowers Vout. Dividers suit high-impedance inputs such as ADC pins and reference voltages. Use a regulator to power something.
How do I pick resistor values?
Choose a divider current that is at least 10× the load current but low enough to keep power small. For microcontroller ADC inputs, values between 10 kΩ and 100 kΩ are typical.