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 (low quiescent current — good)

Important limitations: A voltage divider cannot supply significant current — the load resistance must be at least 10× R2 to avoid loading error. 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.