Voltage Divider Calculator

How to use the voltage divider calculator

1. Fill in the fields — they start with a worked example.
2. Read the answer, and the arithmetic shown beneath it.
3. Copy the results, or adjust the inputs to compare scenarios.

About this tool

Two resistors in series across a supply give an output at their junction of Vin × R2 / (R1 + R2). It is the simplest way to scale a voltage down — reading a 12 V battery on a 3.3 V analogue input, for instance.

The trap is loading. That formula assumes nothing draws current from the output. Connect anything with a comparable resistance and it sits in parallel with R2, lowering the effective resistance and pulling the output below what you calculated. A divider is fine for a high-impedance input such as an ADC pin, and useless as a power supply — the moment real current is drawn, the output collapses.

The rule of thumb is to make the divider's own current at least ten times whatever the load draws. That costs power continuously, which is the trade-off: lower resistances give a stiffer output and waste more current, and on a battery that matters.

The current shown here is what the divider wastes permanently, whether or not anything is connected.

Common uses

• Scaling a battery voltage down for a microcontroller input.
• Setting a reference voltage.
• Checking how much current a divider wastes.

Frequently asked questions

Why is my divider's output lower than calculated?

Something is loading it. Any load draws current through R1, which drops more voltage across it. Use resistances at least ten times lower than the load's impedance.

Can I power a circuit from a voltage divider?

No, in almost every case. As soon as real current is drawn the output collapses. Use a regulator for anything that consumes power.

Should I use large or small resistors?

Small resistors give a stiffer, more accurate output and waste more current. Large ones save power and load more easily. Ten times the load impedance is the usual compromise.