Voltage Divider Calculator

In short A voltage divider is two resistors in series with the output taken from between them, and the output is the input multiplied by R2 ÷ (R1 + R2). Two equal resistors halve the voltage, and it is only the ratio that matters — 1 kΩ over 1 kΩ and 1 MΩ over 1 MΩ both give exactly half. What decides the resistor scale is everything else: small values waste current continuously, large values pick up noise and are far more sensitive to loading, and tens of kilohms is the usual compromise. Loading is the part that catches people out, and it is the single most common reason a divider "does not work". The formula assumes nothing draws current from the output. Connect something that does and it sits in parallel with R2, lowering the effective bottom resistance and pulling the output down — a load equal to R2 drags a 5 V output to 3.33 V. The reason this surprises people is that a multimeter has a very high input impedance and barely loads the divider at all, so it measures the right answer, and then the circuit it feeds does not. A useful rule is to keep the load at least ten times R2 and the sag stays under about ten per cent. Where the output must be accurate, buffer it with an op-amp follower rather than chasing bigger resistors: a divider sets a voltage for something that draws almost no current, and it is not a power supply.