Electronics Simplified

Voltage Divider Calculator

Turn a big voltage into a smaller one with two resistors.

Output voltage (Vout)
4.5 V
Share of the input
50%
Current through the divider
450 µA
Power in R1 / R2
0.00202 W / 0.00202 W

The formula

Vout = Vin × R2 ÷ (R1 + R2)

R1 is the top resistor (to Vin), R2 the bottom one (to ground); Vout is taken from the point between them. It only stays true if whatever you connect to Vout draws much less current than the divider itself.

Worked example: Reading a 9 V battery with an Arduino (max 5 V input)

  1. Use R1 = 10 kΩ and R2 = 10 kΩ: Vout = 9 × 10 ÷ 20 = 4.5 V. Safe for the Arduino pin.
  2. Current through the divider = 9 V ÷ 20 kΩ = 0.45 mA, so the battery hardly notices.
  3. In the code, multiply the reading by 2 to get the battery voltage back.

Common questions

Can I use a voltage divider to power a motor or a module?

No. As soon as the load draws current, the output voltage sags. For power use a voltage regulator. Dividers are for signals: sensors, reading a battery, level shifting.

Where are voltage dividers used?

Every potentiometer is one, and so is a light sensor (LDR) with a fixed resistor. They also scale battery voltages for microcontroller inputs and shift 5 V signals down to 3.3 V.

Why use big resistors like 10 kΩ instead of 10 Ω?

Big resistors waste less current and power. Too big (megaohms) and the microcontroller's input starts to disturb the reading, so 1 kΩ–100 kΩ is the usual range.

Everything on this subject: Ohm's Law and Circuit Basics, Simplified