Electronics / Circuits
Voltage Divider Calculator (Vout, Vin, R1, R2)
Solve a two-resistor voltage divider for whichever value you need, the output voltage, the input voltage, R1, or R2, with the current through it and a circuit diagram.
Voltage Divider Calculator (Vout, Vin, R1, R2): Two resistors in series share the input voltage in proportion to their resistance, so the output across the lower one is Vout = Vin × R2 ÷ (R1 + R2). Rearranged, the same formula gives Vin, R1, or R2 from the other three. The current through both resistors is Vin ÷ (R1 + R2). Runs 100% locally in your browser with zero server file uploads.
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Vout = Vin × R2 ÷ (R1 + R2), with nothing connected to the output. A load across R2 draws current and pulls the output down, so keep the load's resistance at least ten times R2, or use a regulator to power anything that needs current.
A worked example
To read a 5 V signal on a 3.3 V microcontroller: with R2 = 20 kΩ, R1 = 20 kΩ × (5 − 3.3) ÷ 3.3 ≈ 10.3 kΩ, so the standard 10 kΩ gives 5 × 20 ÷ 30 = 3.33 V.
For current, voltage, and resistance in a single resistor, use the Ohm's law calculator.
Using it with sensors
A light-dependent resistor or thermistor as one half of a divider turns its changing resistance into a changing voltage that an analogue input can read. Pick the fixed resistor close to the sensor's resistance in the middle of its range for the widest swing.
The resistor colour code tool reads the bands on the fixed resistor.
How to use it
- Choose what to work out: Vout, Vin, R1, or R2.
- Enter the other three values.
- Read the answer, the current through the divider, and the ratio, and check the diagram.
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Frequently asked questions
Why does my measured output read lower?
Whatever is connected to the output draws current and acts as a resistor in parallel with R2. Keep its resistance at least ten times R2, or buffer the output.
Can I power a device from a divider?
Not reliably: the output sags as soon as current is drawn. Use a voltage regulator for power, and dividers for signals and reference voltages.
Which resistor values should I choose?
The ratio sets the voltage; the total sets the current. Around 10 kΩ total suits most signal uses; lower wastes power, much higher is easily disturbed by the load.
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