Voltage Divider Calculator

Put two resistors in series across a voltage and the same current flows through both. By Ohm's law each resistor drops a voltage proportional to its resistance, so the node between them sits at a fixed fraction of the input. Tap that node — the voltage across the bottom resistor, R2 — and you have a voltage divider. It is the most common two-component circuit in electronics: scaling a battery voltage into an ADC's range, setting a regulator's output through its feedback pin, biasing a transistor, building a crude 5 V-to-3.3 V level shifter, or reading a thermistor or potentiometer.

The derivation is one line. The series current is I = Vin / (R1 + R2), and Vout is that current through R2, so Vout = Vin × R2 / (R1 + R2). The ratio k = R2 / (R1 + R2) is all that matters for the output voltage; the absolute values only set the current, the power, and how stiff the output is when something loads it.

Vout = Vin × R2 / (R1 + R2)
Vin  = Vout × (R1 + R2) / R2
R1   = R2 × (Vin − Vout) / Vout
R2   = R1 × Vout / (Vin − Vout)
I = Vin / (R1 + R2)     P_R1 = I²·R1     P_R2 = I²·R2
Rout (Thevenin) = R1 ∥ R2 = R1·R2 / (R1 + R2)

Worked example — reading a 12 V battery with a 3.3 V ADC

A first guess of R1 = 10 kΩ and R2 = 3.9 kΩ gives Vout = 12 × 3.9 / 13.9 = 3.37 V — already above a 3.3 V ADC reference. Worse, a lead-acid or automotive battery sits near 14.4 V while charging, and load dumps go higher still, so the divider must be designed for the maximum input, not the nominal one. Aim for about 3.0 V at 14.4 V to leave headroom: k = 3.0 / 14.4 ≈ 0.208. Enter Vin = 14.4 and Vout = 3.0 with R1 as the unknown and the tool solves it; the standard-pairs table then lists real E24 or E96 combinations sorted by ratio error. A 1 MΩ-scale divider draws about 12 µA at 12 V — fine for a car, but on a coin-cell product even that is worth switching off between readings.

The formulas assume nothing draws current from Vout. Any load RL appears in parallel with R2, lowering the effective bottom resistance and pulling Vout down. A useful rule: keep RL at least 10× larger than R1 ∥ R2 for about 10% accuracy at worst, and 100× for about 1%. The optional load field in the calculator computes the exact loaded voltage so you don't have to guess.

ADC inputs are a special case. A SAR ADC's sample-and-hold capacitor must charge through the divider's Thevenin resistance within the acquisition time, so many microcontroller datasheets specify a maximum source impedance for a given sampling time. With a high-value divider the usual fix is a 10–100 nF capacitor from the ADC pin to ground: it acts as a charge reservoir for the sampling capacitor, at the cost of a slower response (time constant τ = (R1 ∥ R2) × C). Check your MCU's datasheet for its recommended source impedance.

• Low values (100 Ω – 1 kΩ total): stiff output and low noise, but they waste current and get warm. Rarely needed except to drive a low-impedance input.
• Mid values (10 kΩ – 100 kΩ): the sweet spot for most ADC and feedback dividers.
• High values (≥ 1 MΩ): micro-amp current for battery products, but PCB leakage (especially with flux residue or humidity), op-amp bias current and ADC sampling errors start to matter.
• Tolerance: with two ±1% resistors the ratio error can reach about ±2% × (1 − k) worst case. For voltage references and precision measurement, use 0.1% thin-film parts with matched temperature coefficients.
• Voltage rating: chip resistors have a maximum working voltage that depends on size (small 0402/0603 parts are commonly rated at only tens of volts). For high-voltage sensing, split R1 into several resistors in series.

• Powering a load from a divider: the output collapses as soon as the load draws current. Use an LDO or a buck converter instead.
• Designing for nominal instead of maximum input, so the ADC pin is over-driven when the source peaks.
• Ignoring the next stage's input resistance or the ADC's source-impedance limit.
• Placing the divider far from the ADC pin with a noisy trace running past the high-impedance output node.
• Using a divider as a level shifter on fast signals: the input capacitance of the receiving pin forms an RC low-pass filter with the divider and rounds off the edges.

Adjustable regulators set their output through a feedback divider: the feedback pin regulates to a reference voltage Vref across R2, so Vout = Vref × (1 + R1/R2). You can solve this directly here: choose R1 as the unknown, put the desired regulator output in the Vin field, the datasheet Vref (often 0.6 V, 0.8 V or 1.25 V) in the Vout field, and pick R2 from the datasheet's recommended range. On battery products, gate the divider with a small MOSFET so it only draws current while measuring. And on the layout, keep the divider next to the pin it feeds and route the feedback node away from switch nodes and inductors.