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Voltage Divider Calculator

The output of a two-resistor divider from your input voltage and both resistors, plus the current it draws and the heat in each.

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A voltage divider is two resistors in a line. Put a voltage across the pair and the point between them sits at a fraction of it, set by the two resistor values. It is how you drop a voltage to read it, set a reference, or scale a signal into range. Enter your input and the two resistors and this gives the output, the current and the heat.

The formula

The output is the input times R2 divided by the sum of R1 and R2. Two equal resistors give exactly half the input. Make R2 larger than R1 and the output rises toward the input; make it smaller and the output falls toward ground. The ratio is all that sets the voltage, so a 1k and 1k pair divides the same as 100k and 100k.

Current and heat

The same current runs through both resistors, and it is the input divided by their total. Bigger resistors draw less current and waste less power, which is why a divider meant only to be read uses high values. The power in each resistor is small here, but it is worth checking when the voltage is high or the resistors are low, so nothing cooks.

Loading changes everything

The neat formula assumes nothing is connected to the output. The moment a circuit draws current from the tap, it sits in parallel with R2 and pulls the output down. If the load's resistance is near R2 the output sags hard; if it is far higher, barely at all. For anything a circuit will draw from, keep both resistors well below the load, or feed the tap through a buffer.

Picking the values

There is a balance. Low resistor values hold the output steady under load but burn current all the time. High values sip almost nothing but drift with noise and loading. For a simple reference into a high-impedance input, tens of kilohms is a common middle. For a divider feeding a real current, drop the values and watch the power.

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