Voltage Divider Calculator

Voltage divider: scaling Vin with two resistors

Put two resistors in series and you get a voltage divider: the output, measured across R₂, is a fraction of what comes in, following Vout = Vin · R₂ / (R₁ + R₂). Example: with V_in = 12 V, R₁ = 1 kΩ and R₂ = 2 kΩ, you land on V_out = 8 V. Current through the divider is I = V_in / (R₁ + R₂). Critical limitation: a bare divider with no op-amp buffer feels whatever stage comes next, so any current pulled from V_out nudges the ratio off. Rule of thumb: keep the divider current at roughly 10× the load current and V_out stays stiff. If the load has high impedance, drop a unity-gain op-amp right after the divider.

Applications: sensors, level shifting, audio

A divider is how a resistive sensor (LDR, NTC, potentiometer) gets read into an MCU ADC input (0–5 V on an Arduino, 0–3.3 V on an ESP32). The same trick shifts logic levels from 5 V down to 3.3 V, say from an Arduino TX to an ESP32 RX with R₁ = 1 kΩ and R₂ = 2 kΩ. You will also find dividers doing duty as passive audio attenuators and as bias networks inside amplifier stages.

FAQ

Why does V_out drop under load? Your load ends up in parallel with R₂, which lowers the effective bottom resistance and throws off the ratio. Fix it with smaller resistors, which raises divider current, or with an op-amp buffer.

Can I power a circuit with a divider? Only if the load draws microamps. Anything that needs real power should run off a regulator such as an LDO or a buck, not off a divider.

Which resistor values should I pick? It is a balancing act. Go too low and you burn power; go too high and the circuit gets touchy about loading and noise. For reading sensors on an MCU, somewhere in the 1–100 kΩ range usually works.