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AESTECHNO
Diviseur

Voltage divider calculator

A voltage divider gives Vout = Vin x R2 / (R1 + R2). Enter Vin, R1 and R2: the calculator shows Vout live, snaps R1 and R2 to the nearest E12, E24 or E96 standard value, and reports the divider current and the power dissipated in each resistor.

Inputs
Mode

Compute Vout from R1 and R2, or find R1/R2 for a target output voltage.

The wanted mid-point voltage. Must be strictly between 0 and Vin.

Sets the impedance level; we search R2 in the E-series. 10 kohm suits most cases.

Vout is taken at the node between R1 and R2.

Tolerance of the input supply. It passes straight through to Vout.

Preset from the series (E24 ±5%, E96 ±1%, E12 ±10%), still editable. Widens the Vout range.

Value series

Snap R1 and R2 to the nearest standard value (IEC 60063), or Ideal to keep your values.

Account for the load
Load type

Resistance seen at the output (an ADC input, a comparator). It sits in parallel with R2.

Current direction

Drawn: Vout drops. Injected: Vout rises. Both: unknown direction (e.g. an op-amp bias current spec as ±I), the Vout range widens on both sides.

Current magnitude at the output node; the direction is chosen above.

Vin R1 Vout R2

Voltage divider (schematic)

Result

3.235 V

max 3.348 V
min 3.119 V
Breakdown

R1 / R2 1.8 kΩ / 3.3 kΩ (0%)
Vout tolerance ±3.6%
Zout 1.165 kΩ
Noise 4.38 nV/√Hz
Current I(R1) 980.4 µA
Power R1 / R2 1.73 mW / 3.172 mW

Ideal divider. Zout = R1||R2 is the source impedance; Noise is its Johnson thermal-noise density at 25°C (multiply by the square root of your bandwidth for an RMS voltage). A real load lowers Vout; use the load option.

Frequently asked questions

FAQ

What is the voltage divider formula?
The divider law (Horowitz and Hill, The Art of Electronics) gives Vout = Vin x R2 / (R1 + R2), where R2 is the bottom resistor to ground and R1 the top resistor to Vin. Vout is taken at the node between R1 and R2. The calculator guards R1 + R2 > 0 to avoid a divide by zero.
How does the calculator snap R1 and R2 to E12, E24 or E96 values?
The E-series are defined geometrically by IEC 60063 (12, 24 or 96 values per decade). The calculator picks the nearest standard value in log space: the boundary between two adjacent values is their geometric mean sqrt(p_low x p_high). R1 and R2 are snapped independently, then the real Vout is recomputed with the error in percent versus the ideal.
Why account for the load on the output?
A load connected to Vout (an ADC input, a comparator, etc.) sits in parallel with R2 and pulls the voltage down. This is the most common voltage-divider mistake. The calculator replaces R2 with R2p = (R2 x Rload) / (R2 + Rload), then reports the loaded Vout and the drop versus the unloaded divider.
How do I size R1 and R2 without wasting current?
The current through the divider is i = Vin / (R1 + R2), and each resistor dissipates P = i squared x R. Values too low waste current (critical on battery); too high, the source impedance becomes sensitive to input bias current and noise. The calculator shows the current and the power in R1 and R2 to help you trade off.
Can a voltage divider power a circuit?
No. A divider provides a reference voltage unloaded or under a very high-impedance load (ADC input, comparator threshold, battery sensing). As soon as meaningful current is drawn, Vout collapses. For a stable supply under load you need a regulator or a buffer, not a plain resistive divider.
Go further
AESTECHNO TOOL

A divider that drifts under load or runs hot usually points to an impedance choice worth revisiting. Let us look at it in a free 30-min audit: contact AESTECHNO.

Free 30-min audit

Calculator built by AESTECHNO, an electronic design bureau based in Montpellier, France. 10+ years of experience in board design and first-pass CE/FCC certification.