Voltage Divider Calculator
Calculate a two-resistor voltage divider's output voltage and current — Vin × R2 ÷ (R1 + R2). Add an optional load resistance to see how much a real downstream circuit (an ADC input, a sensor) shifts the output from the ideal, unloaded value.
Input
In volts (V).
In ohms (Ω) — between Vin and Vout.
In ohms (Ω) — between Vout and ground.
The resistance of whatever's actually connected to Vout (an ADC input, a sensor). Leave blank to see only the ideal, unloaded result.
Output
| Quantity | Value |
|---|---|
| No data yet | |
Guides
A voltage divider is two resistors in series across a supply, with the output taken from the junction between them: Vout = Vin × R2 ÷ (R1 + R2). It's the standard way to scale a voltage down for an ADC input, create a reference or bias voltage, or read a resistive sensor. This calculator gives the ideal output voltage and divider current — and, unlike most divider calculators, also shows what happens once something is actually connected to the output.
How to use it
- Enter the input voltage (Vin), the top resistor (R1, between Vin and Vout) and the bottom resistor (R2, between Vout and ground).
- The Results table shows the ideal, unloaded Vout, the divider current, and the power dissipated in each resistor.
- Optionally enter a Load Resistance — the resistance of whatever is actually connected to Vout (an ADC input's impedance, a sensor, a downstream circuit). Three more rows appear: the effective R2 (R2 in parallel with the load), the loaded Vout, and the loading error as a percentage.
Why the load resistance matters
Every voltage-divider calculator computes the ideal case — no current drawn from the output. In practice, whatever you connect to Vout draws some current, which effectively puts that load's resistance in parallel with R2. That pulls Vout below the ideal value; how far depends on how the load resistance compares to R1 and R2. A load resistance ten times R2 or more (a general rule of thumb) keeps the error small; a load close to R2 can shift the output by tens of percent. Leaving the load field blank shows only the ideal calculation — useful when you're driving something with genuinely negligible input current (most CMOS logic inputs, a true high-impedance op-amp buffer).
Formulas used
- Vout (ideal) = Vin × R2 ÷ (R1 + R2)
- Divider current = Vin ÷ (R1 + R2) — the current flowing through R1 and R2 with nothing connected to Vout
- Power = I² × R, for each resistor
- R2 ∥ Rload = (R2 × Rload) ÷ (R2 + Rload) — R2's effective resistance once the load is connected
- Vout (loaded) = Vin × (R2 ∥ Rload) ÷ (R1 + R2 ∥ Rload)
- Loading error % = (Voutideal − Voutloaded) ÷ Voutideal × 100
Privacy
Everything runs client-side — no values are sent anywhere.