How a voltage divider works
Two resistors in series between a voltage and ground share that voltage in proportion to their values. Taking the output from the point between them gives a fraction of the input:
Vout = Vin × R2 ÷ (R1 + R2)
R1 is the “top” resistor, connected to the input, and R2 the “bottom” resistor, connected to ground.
Worked example
With 12 V in, R1 = 10 kΩ and R2 = 4.7 kΩ:
| Result | Value |
|---|---|
| Output voltage | 3.837 V |
| Division ratio | 0.3197 |
| Current through the divider | 0.816 mA |
| Power in R1 | 6.66 mW |
| R2 needed for exactly 3.3 V | 3.793 kΩ |
The loading effect
Anything connected to the output draws current and sits in parallel with R2, lowering the output. With a 10 kΩ load on the example above, R2 effectively becomes 3.2 kΩ and the output drops from 3.84 V to about 2.91 V.
A practical rule: keep the load resistance at least 10 times R2. The calculator flags loads below that. If the load is heavier or varies, use a voltage regulator or an op-amp buffer.
Common uses
- Reading a higher voltage with a microcontroller: dividing 12 V down to below 3.3 V or 5 V so an analog input can measure a battery.
- Level shifting slow logic signals from 5 V to 3.3 V.
- Sensors: a thermistor, light-dependent resistor or potentiometer as one half of a divider turns a changing resistance into a changing voltage.
- Setting reference or feedback voltages in regulators and amplifiers.
Choosing resistor values
- Lower values (around 1 kΩ) make the output stiffer against loads but waste more current.
- Higher values (100 kΩ and up) save power, which matters on batteries, but are more sensitive to loads and noise.
- Many designs land between 1 kΩ and 100 kΩ.
To find R2 for a target voltage: R2 = R1 × Vout ÷ (Vin − Vout), then pick the nearest standard value and check the result.
Tips
- A divider can only reduce voltage, never raise it.
- Resistor tolerance (often 1% or 5%) carries into the output; use 1% resistors when accuracy matters.
- Do not use a divider to power a motor, LED strip or other load that draws real current.