Voltage divider calculator

How a voltage divider works

Two resistors in series between a voltage and ground split that voltage in proportion to their values. The output, taken between them, is Vout = Vin × R2 ÷ (R1 + R2), where R1 is the top resistor and R2 the bottom one.

Worked examples

• 12 V with R1 = 10 kΩ and R2 = 4.7 kΩ gives 12 × 4,700 ÷ 14,700 = 3.84 V. The divider draws 0.82 mA and wastes about 9.8 mW.
• To get 3.3 V from 5 V with R1 = 10 kΩ, you need R2 = 3.3 × 10,000 ÷ 1.7 = 19.4 kΩ; the nearest standard value up is 20 kΩ, giving about 3.33 V.

Practical tips

• Loading: whatever you connect to the output is in parallel with R2 and pulls the voltage down. Keep its input resistance at least ten times R2.
• Current versus noise: larger resistors waste less power but are more sensitive to noise and to the load. 1–100 kΩ is a common range for signals.
• Not a power supply: a divider cannot supply a varying load. Use a voltage regulator to power circuits.

Common uses include reading a battery voltage with a microcontroller's ADC and shifting a 5 V signal down to 3.3 V logic.

In output mode the page multiplies the input voltage by R2 and divides by R1 plus R2. In resistor mode it rearranges the same formula to R2 = Vout × R1 ÷ (Vin − Vout) and suggests the next standard E24 value. The current through the divider is Vin divided by the total resistance, and the power wasted is Vin squared divided by the total resistance. The output is assumed unloaded.