The voltage divider formula
Two resistors in series between Vin and ground split the voltage in proportion to their values. The output is taken at the junction, across R2.
Worked examples
Measuring a 12 V battery with a 3.3 V microcontroller: R1 = 30 kΩ and R2 = 10 kΩ give 12 × 10 ÷ 40 = 3.00 V, and the divider draws only 0.3 mA. To turn a 5 V signal into about 3.3 V with R1 = 10 kΩ, R2 = 10 × 3.3 ÷ 1.7 = 19.4 kΩ; the nearest E24 value, 20 kΩ, gives 3.33 V.
Using a divider with an ADC
- Keep Vout below the ADC's maximum: 5 V on a 5 V Arduino Uno, 3.3 V on ESP32, RP2040 and other 3.3 V boards (their pins are not 5 V tolerant).
- The ESP32 ADC is most accurate well below 3.3 V: Espressif documents about 2.45 V as the recommended maximum at the highest attenuation.
- The ATmega328P datasheet recommends a source impedance of 10 kΩ or less; the output impedance of a divider is R1 ∥ R2.
- A divider is for signals only. It cannot power a load: the output voltage drops as soon as current is drawn.
Frequently asked questions
How do I convert 5V to 3.3V with resistors?
Use R1 = 1 kΩ and R2 = 2 kΩ (or 10 kΩ and 20 kΩ): 5 × 2 ÷ 3 = 3.33 V. This is fine for slow signals, not for power.
How do I measure 12V with an ESP32?
Divide it down to below about 2.5–3 V, for example 30 kΩ over 10 kΩ (12 V → 3 V) or 47 kΩ over 10 kΩ (12 V → 2.1 V), then scale the reading in code.
Why does my divider voltage drop under load?
The load is in parallel with R2, which lowers the bottom resistance. Use a regulator or buffer to power anything.