How a Voltage Divider Works

Two resistors in series

Put two resistors, R1 and R2, in series across a voltage. The voltage at the point between them, measured to ground across R2, is a fixed fraction of the input:

V_out = V_in × R2 ÷ (R1 + R2)

With 12 V in, R1 = 10 kΩ and R2 = 4.7 kΩ: 12 × 4.7 ÷ 14.7 = 3.84 V.

Choosing a resistor for a target voltage

Rearranged: R2 = R1 × V_out ÷ (V_in − V_out). To bring a 5 V signal down to 3.3 V with R1 = 10 kΩ: R2 = 10 × 3.3 ÷ 1.7 = 19.4 kΩ. The nearest standard value is 20 kΩ, which gives 3.33 V. The Voltage Divider Calculator does both calculations.

Pitfalls

• The load changes the result. Whatever you connect to the output is in parallel with R2 and pulls the voltage down. A divider works well feeding a high-impedance input, such as a microcontroller's analog pin, but not powering a motor, LED strip or circuit board. Use a voltage regulator for that.
• Current flows all the time. The divider draws V_in ÷ (R1 + R2) continuously: 0.82 mA in the 12 V example. Larger resistors waste less, but make the output more sensitive to the load.
• Resistor tolerance. With 5% resistors the output can be off by a few percent; use 1% resistors when it matters.

For the current and power in each resistor, use the Ohm's Law Calculator.