About this calculator
Two resistors in series across a supply produce an intermediate voltage at the point where they join. That is a voltage divider — the simplest way to scale a voltage down, and the basis of sensor biasing, reference generation and level shifting.
This calculator works in both directions. Give it the supply and both resistors and it returns the output voltage; give it the supply, one resistor and a target output, and it returns the resistor you need.
How it works
The same current flows through both resistors because they are in series. That current is Vin / (R1 + R2), and the output voltage is whatever that current develops across the lower resistor, R2.
Combining those gives the familiar ratio form: the output is the input scaled by R2 / (R1 + R2). Only the ratio sets the voltage — 10 kΩ over 10 kΩ and 100 Ω over 100 Ω both halve the input.
What the absolute values do set is the current wasted in the divider and its output impedance. Small resistors burn more current but drive a load more stiffly; large resistors sip current but sag the moment anything draws from the tap.
Worked example
Reading a 12 V battery with a 3.3 V microcontroller ADC. You need to scale 12 V down to about 3 V.
- Pick R1 = 27 kΩ for the upper leg
- R2 = R1 × Vout / (Vin − Vout) = 27000 × 3 / 9 = 9000 Ω
- Nearest standard value: 9.1 kΩ
- Check: 12 × 9100 / (27000 + 9100) = 3.02 V
- Current wasted: 12 / 36100 = 0.33 mA
27 kΩ over 9.1 kΩ gives 3.02 V from a 12 V input while drawing only a third of a milliamp — safe for the ADC and easy on the battery.
Practical notes
- A divider is not a power supply. Draw any real current from the tap and the output collapses, because the load is effectively in parallel with R2 and changes the ratio.
- Keep the divider current at least 10× whatever the load draws if you want the output to stay near the calculated figure.
- Feeding an ADC is the ideal use: ADC inputs draw almost nothing, so the divider stays accurate. Aim for a source impedance under about 10 kΩ so the ADC's sampling capacitor has time to charge.
- Resistor tolerance stacks. Two ±5% resistors can put the output off by roughly 5% in the worst case — use 1% parts where the reading matters.
- For level shifting a fast digital signal, remember the divider and the stray capacitance form a low-pass filter. Above a few hundred kHz, use a proper level shifter.
Frequently asked questions
How do I calculate a voltage divider?
Multiply the input voltage by R2 divided by the sum of both resistors: Vout = Vin × R2 / (R1 + R2). R2 is the resistor between the output tap and ground.
Can I power a circuit from a voltage divider?
Almost never. The output voltage depends on nothing being drawn from it, so as soon as a real load pulls current the voltage drops. Use a regulator instead — a linear regulator for small loads, a buck converter for larger ones.
What resistor values should I choose?
Only the ratio sets the voltage, so choose the absolute values for current. For battery-powered sensing, totals in the 10 kΩ–1 MΩ range keep the drain negligible. For driving something with a bit of load, drop to 1–10 kΩ.
Why is my divider output lower than calculated?
Something is loading it. Whatever you connected sits in parallel with R2 and pulls the ratio down. Either reduce the divider resistances or buffer the output with an op-amp follower.
How do I scale 12 V down for a 3.3 V ADC?
Use a ratio of about 4:1 — for instance 27 kΩ over 9.1 kΩ, which gives 3.02 V from 12 V. Leave a little headroom below the reference so an input spike does not clip or damage the pin.