Voltage Divider Calculator and Tutorial: How to Split Any Voltage

_The voltage divider is the circuit you'll encounter more than almost any other. Logic level shifting. Sensor conditioning. Reference voltages. Volume controls. Battery monitors. Setting the gain of an amplifier. Setting the output of an LM317._

_It's two resistors and an output tap. The simplicity is deceptive — understanding it properly opens up a surprising amount of circuit design._

The formula

Vout = Vin × (R2 / (R1 + R2))

R1 is the top resistor (between input and the output tap). R2 is the bottom resistor (between the tap and ground). The output voltage is always between 0V and Vin — you can only divide down, never amplify.

Worked examples

5V to 3.3V logic level converter:

Target ratio: 3.3/5 = 0.66

Choose R2 = 10kΩ: R1 = 10k × (1/0.66 − 1) = 5.15kΩ → use 5.1kΩ standard

Actual output: 5 × (10/(5.1+10)) = 3.31V

12V battery monitor for 3.3V ADC:

Target ratio: 3.3/12 = 0.275

Choose R2 = 10kΩ: R1 = 10k × (1/0.275 − 1) = 26.36kΩ → use 27kΩ

Actual max output: 12 × (10/(27+10)) = 3.24V — safely within ADC range.

The loading problem — why real results differ

The formula assumes nothing else is connected to the output. The moment you add a load, you've added a third resistor in parallel with R2 — changing the ratio and the output voltage.

Rule of thumb: load impedance should be at least 10× the value of R2 for less than 10% error. For 10kΩ R2, your load should be above 100kΩ.

For stable low-impedance output regardless of load: add an op-amp voltage follower after the divider. The op-amp's high input impedance doesn't load the divider; its low output impedance drives whatever load you need.

Resistor value selection

Signal-level applications (logic shifting, sensors): 10kΩ–100kΩ. High enough to avoid significant current draw, low enough that output impedance doesn't pick up noise.

Power applications (setting regulator voltage, biasing transistors): 1kΩ–10kΩ, to ensure divider current is large relative to any bias currents.

Precision applications: always use 1% tolerance resistors. Two 5% resistors at their worst-case extremes can introduce up to 10% ratio error.

Five real applications

1. 5V to 3.3V logic shifting: R1=1.8kΩ, R2=3.3kΩ drops 5V to 3.3V with no active components.
2. Thermistor temperature sensing: thermistor + fixed resistor form a divider whose output varies with temperature. ADC reads the voltage, firmware converts to temperature.
3. Battery voltage monitoring: scale pack voltage into ADC input range.
4. Audio volume control: a potentiometer is a variable divider. The wiper position sets output level.
5. LM317 output voltage setting: the R1 and R2 in an LM317 circuit form a divider from output to the adjustment pin — the ratio directly sets Vout.

Related Tools & Projects

• [Ohm's Law Calculator](/calculators/ohms-law)
• [LM317 Output Voltage article](/blog/lm317-voltage-calculator)
• [Touch Sensor Project](/projects/touch-sensor)