Voltage Divider Calculator: Formula and Examples
The voltage divider formula is Vout = Vin × R2 / (R1 + R2): the same current flows through R1 and R2, so the voltage splits in proportion to the resistances. Enter Vin, R1 and R2: the calculator shows Vout live with the formula worked on your values, snaps R1 and R2 to the E12, E24 or E96 series, and reports the divider current and the power in each resistor.
Common cases
Voltage divider (live schematic)
3.235 V
Vout = 5 V × 3.3 kΩ / (1.8 kΩ + 3.3 kΩ) = 3.235 V
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.
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.
The voltage divider formula
Two resistors in series between Vin and ground carry the same current, I = Vin / (R1 + R2), as long as nothing draws current from the midpoint. The voltage across R2 is Vout = R2 × I, which gives Vout = Vin × R2 / (R1 + R2). The ratio R2 / (R1 + R2) is always below 1: a resistive divider can only lower a voltage, never raise it.
Worked example: Vin = 12 V, R1 = 20 kΩ, R2 = 10 kΩ. The current is 12 V / 30 kΩ = 0.4 mA, so Vout = 0.4 mA × 10 kΩ = 4 V, which the ratio confirms: 12 V × 10 / (20 + 10) = 4 V. Only the ratio sets Vout: 2 kΩ and 1 kΩ also give 4 V, with ten times the current (4 mA). To work backwards, fix R1 and take R2 = R1 × Vout / (Vin − Vout); that is what the Find resistors mode does before snapping R2 to a standard value.
Why a voltage divider is not a power supply
Seen from its output, a divider is a voltage source equal to the unloaded Vout in series with a resistance R1 ∥ R2 (Thévenin's theorem). With R1 = 1.8 kΩ and R2 = 3.3 kΩ, that output impedance is about 1.16 kΩ. Any load on Vout sits in parallel with R2 and pulls the voltage down: in this example a load drawing 1 mA costs about 1.16 V, and Vout moves whenever that current moves.
The divider draws Vin / (R1 + R2) all the time, whether the load takes current or not, and that current turns into heat in the resistors. Lowering R1 and R2 to stiffen the output raises that waste in proportion. To power a circuit, use a regulator: an LDO for a small voltage drop at low current, a switching converter when efficiency matters. The divider remains the right tool for a reference, a comparator threshold, a regulator feedback setpoint or scaling a measurement.
Voltage divider for an ADC input
To measure a voltage above an ADC's full scale, the divider brings the highest expected voltage just under the reference, 12 V down to 3.3 V for example. Keep a margin: E24 rounding and resistor tolerance can push Vout past full scale, so check the Vout you get with the real values.
A successive-approximation ADC charges a sampling capacitor at every conversion. The higher R1 ∥ R2, the slower that charge, and the reading goes wrong if the sampling time is too short: the microcontroller datasheet gives the maximum source impedance. A ceramic capacitor across R2 supplies that charge instantly and filters noise, at the cost of an (R1 ∥ R2) × C time constant.
On a battery, the divider draws Vin / (R1 + R2) continuously: 4.2 V across 460 kΩ is about 9 µA, against 420 µA across 10 kΩ. Values of a few hundred kΩ with that capacitor, or a MOSFET that disconnects the divider between readings, protect battery life. Our battery life calculator puts a number on that current, and the power management glossary entry gives the wider context.
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. When snapping changes a value you typed, we say so under the result, with the Vout of your exact values; pick Ideal to keep your values.
- 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.
Calculator built by AESTECHNO, an electronic design bureau based in Montpellier, France. 10+ years of experience in board design and a 100% success rate on CE/FCC certifications, across dozens of campaigns run over more than 10 years of practice.