Voltage Divider Calculator

A voltage divider is a simple two-resistor circuit that produces an output voltage that is a fraction of the input voltage. It appears in sensor interfaces, ADC input conditioning, bias networks, and level shifters.

The core formula:

V_out = V_in × (R2 / (R1 + R2))

To find R1 for a desired output:

R1 = R2 × ((V_in / V_out) − 1)

Current through the divider:

I = V_in / (R1 + R2)

What each variable means:

• V_in: the input supply voltage applied across both resistors in series
• V_out: the output voltage measured across R2 (the lower resistor)
• R1: the upper resistor (between V_in and V_out)
• R2: the lower resistor (between V_out and ground)
• I: current flowing through both resistors (power consumed = V_in × I)

Worked example: You need to drop a 12V supply to 3.3V to feed a microcontroller ADC input. Choose R2 = 10 kΩ.

R1 = 10,000 × ((12 / 3.3) − 1) = 10,000 × (3.636 − 1) = 10,000 × 2.636 = 26.36 kΩ → use 27 kΩ Actual V_out with 27 kΩ: V_out = 12 × (10,000 / (27,000 + 10,000)) = 12 × 0.270 = 3.24V (close enough) Current = 12 / 37,000 = 0.32 mA, which is a comfortably low quiescent draw

Important limitations: A voltage divider cannot supply significant current. The load resistance has to be at least 10× R2 before the loading error drops to something you can live with, and even then it is several percent. Use high resistor values (10–100 kΩ) to minimize quiescent current draw in battery-powered circuits. Use low resistor values (100 Ω–1 kΩ) when driving low-impedance loads. Never use a voltage divider as a power supply for circuits drawing more than a few milliamps.