Series Resistor Voltage Divider Calculator

Voltage divider with series resistors

The voltage divider is about as fundamental as circuits get. Put two resistors in series across a source and the junction between them sits at some intermediate voltage. That output works out to V_out = V_in · R2 / (R1 + R2). It falls straight out of Kirchhoff's voltage law and Ohm's law once you note the current is the same through both resistors in a series loop.

Say you have a 12 V supply with R1 = 1 kΩ and R2 = 2 kΩ. The midpoint reads V_out = 12 · 2000/3000 = 8 V. That ideal figure only holds if whatever comes next draws almost no current. Draw real current and the load resistance ends up in parallel with R2, dragging V_out below the calculated value. That's the loading effect.

Applications

You'll find voltage dividers just about everywhere. They scale a 0–24 V sensor signal down to the 0–3.3 V range a microcontroller ADC can take. They bias the base of a BJT or the gate of a MOSFET in amplifier stages. They set a reference for op-amp comparators, and they read the wiper of a potentiometer. Introductory circuits courses cover them early (Sedra/Smith, Boylestad), and they sit underneath op-amp gain networks defined by IEEE standards.

FAQ

Why does the output sag under load? Any load impedance Z_L sitting in parallel with R2 lowers the effective bottom resistor. Keep the error under about 10% by picking Z_L ≥ 10×R2, or put a voltage-follower op-amp between the divider and the load.

What power do the resistors dissipate? Current through the series chain is I = V_in / (R1 + R2), and each resistor burns P = I²·R. Pick a standard rating (1/4 W is common) and leave yourself at least 2× of headroom.

Can I use very large resistors to save power? You can, but watch out: above ~1 MΩ noise pickup, op-amp input bias current and parasitic capacitance all start to matter. Go past 10 MΩ and you'll need guard rings and a careful PCB layout.