calculator
Voltage divider calculator
Two resistors in a row split a voltage between them. This is how every light sensor and temperature sensor circuit on this site turns a changing resistance into a changing voltage.
The answer
Vout = 2.25 V
- Total resistance: 10 kΩ + 10 kΩ = 20 kΩ
- Current through both (Ohm's law): 4.5 V ÷ 20 kΩ = 0.225 mA
- Voltage across R2 = I × R2: 0.225 mA × 10 kΩ = 2.25 V
- Same thing as a ratio: 4.5 V × 10 kΩ ÷ 20 kΩ = 2.25 V
- Heat: 0.51 mW in R1, 0.51 mW in R2
Inspect mode is on below: the number at each hole is its voltage measured from the battery's − side.
simulating…
What a voltage divider is
Put two resistors in series across a battery and the same current flows through both. Ohm's law (V = I × R) says each resistor takes a share of the voltage in proportion to its resistance. Equal resistors split it in half; make R2 ten times bigger than R1 and it takes ten elevenths of the voltage. The formula is Vout = Vin × R2 ÷ (R1 + R2).
Why it matters
A light sensor (LDR) is a resistor that changes with light. Put it in place of R1 and Vout rises as the room gets dark — that is the whole trick behind the Automatic Night Light. A thermistor does the same for temperature in the Fever Beeper.
Common questions
Can I use a divider to power something? Not really. Anything you connect to the middle draws current and changes the ratio. Dividers are for making a signal a transistor or a chip can read, not a power supply.
Why do the resistors have to be big? With two 100 Ω resistors on 4.5 V, 22 mA flows all the time and the battery drains for nothing. With 10 kΩ each it is 0.2 mA. Bigger is better until the signal gets too weak to read.
What if R1 is zero? Then Vout equals Vin and R2 is just a load on the battery.