Voltage Divider Calculator
Output voltage from Vin, R1 and R2, or R2 for a target, with load effects.
About this tool
Choose whether to find the output voltage or the R2 for a target output. Enter the input voltage and resistor values using shorthand like 10k or 4k7, and optionally a load resistance. The calculator shows the output voltage, the divider ratio, the current through the divider and the power it wastes; in design mode it gives the exact R2, the nearest standard E24 value and the real output with it.
How a voltage divider works
A voltage divider is two resistors in series across a supply, with the output taken from the point between them. The same current flows through both, so the voltage splits in proportion to the resistances: Vout = Vin × R2 ÷ (R1 + R2), where R2 is the resistor to ground. From 12 V with R1 = 10 kΩ and R2 = 4.7 kΩ, the output is about 3.84 V. To design one, choose R1 and solve R2 = R1 × Vout ÷ (Vin − Vout); for 3.3 V from 5 V with R1 = 10 kΩ that's about 19.4 kΩ, rounded to a 20 kΩ standard value. Dividers are used to scale sensor and battery voltages for microcontroller inputs, set reference voltages and bias transistors. Anything connected to the output is effectively in parallel with R2 and pulls the voltage down, which is why the optional load field shows the loaded result; keep the load at least ten times R2. Lower resistor values make a stiffer divider but waste more current. Honest limits: a divider can't power a load such as a motor or LED strip, so use a regulator for that, and resistor tolerance shifts the output slightly. The Ohm's law calculator, the series and parallel resistor calculator and the resistor colour code tool are related.
Frequently asked questions
What is the voltage divider formula?
Vout = Vin × R2 ÷ (R1 + R2), where R2 is the resistor connected to ground.
How do I get 3.3 V from 5 V with resistors?
Use R1 = 10 kΩ and R2 ≈ 19.4 kΩ (20 kΩ standard), or a regulator if the 3.3 V must supply current.
Why does my divider's voltage drop under load?
The load sits in parallel with R2, lowering its effective resistance. Keep the load much larger than R2.
Is my data saved?
No. The calculation runs in your browser and nothing is stored or sent.