Voltage Divider Calculator
Enter the input voltage and two resistor values to calculate the output voltage, divider current and resistor power dissipation. Ideal for signal scaling, ADC reference voltages and sensor biasing.
Last updated: May 2026
Enter Vin, R1 and R2 to calculate.
Vout = Vin × R2 ÷ (R1 + R2)
Common presets
How a resistor voltage divider works
A voltage divider uses two resistors in series to create an output voltage that is a fixed fraction of the input. Because the same current flows through both resistors, the voltage is distributed proportionally to resistance. The output taps between R2 and ground. It is one of the building blocks gathered in the Electronics Hub, which collects the related resistor and power tools you tend to reach for in the same design.
The formula: Vout = Vin × R2 ÷ (R1 + R2). To halve a voltage, use equal resistors. To scale 5 V to 3.3 V, use R1=5.1 kΩ and R2=10 kΩ, giving exactly 3.33 V.
Typical use cases
- Scaling a 5 V sensor output down to 3.3 V for an ESP32 or STM32 ADC input
- Creating a reference voltage from a regulated supply
- Biasing a transistor base or FET gate
- Measuring higher voltages with a microcontroller ADC by dividing them down into the 0-3.3 V range
Where the divider fits between Ohm's Law and a real signal
The Ohm's Law calculator gives you the four base numbers of any DC circuit, and the LED resistor calculator applies them to the specific case of taming an LED with a series resistor. You are here, at the next step: taking those same resistor values and arranging two of them in series so the tap between them delivers a scaled fraction of the input voltage. That output is what sets a reference for an ADC, biases a sensor, or brings a 12 V signal down to the 3.3 V range an ESP32 or Arduino can read safely. Once you have Vout confirmed, the chain continues: the resistor color code decoder identifies the physical bands on the parts you pull from the bin, the watts, volts and amps calculator sizes the supply that feeds Vin, and the voltage drop calculator accounts for any loss in the wiring before Vin even reaches your divider. For AC signals the picture changes: reactance and loading interact in ways that go beyond a simple resistor network, which is where the impedance calculator and the capacitor and inductor reactance tools take over. One thing worth keeping in mind at this step: a divider is for reference and sensing, not for supplying current to a load. Draw real current from the output tap and R2's effective value drops, Vout sags, and the whole ratio shifts. For anything that needs to hold voltage under load, a regulator is the right tool, not a divider.
Frequently Asked Questions
Why does loading affect the output voltage?
Connecting a load in parallel with R2 reduces the effective resistance of the lower half, shifting Vout lower. A load impedance of 10× R2 or more is generally acceptable; it causes less than 10% deviation. If your load is low-impedance (e.g. a microcontroller GPIO input is typically >1 MΩ, but a motor driver can be a few hundred ohms), choose lower divider resistor values to make the load effect negligible.
How do I choose R1 and R2 to minimise power waste?
Higher resistance values (10 kΩ-100 kΩ range) draw less current and waste less power. A 10 kΩ + 10 kΩ divider on 5 V wastes only 1.25 mW total. A 100 Ω + 100 Ω divider on 5 V wastes 62.5 mW, significant in battery-powered designs. Use 100 kΩ or higher for always-on sensor biasing to minimise quiescent current.
Can I use a voltage divider to step down 12 V for a 3.3 V microcontroller?
Yes, but only for measurement or signal inputs, not to power the microcontroller or drive high-current loads. A common 12 V → 3.3 V divider: R1 = 8.2 kΩ, R2 = 3.3 kΩ (gives 3.30 V). For powering an MCU, use a voltage regulator (e.g. AMS1117 or LDO) instead; a divider cannot maintain stable voltage under varying load current.
How do I work out R1 and R2 when I need Vout to hit a specific ADC reference, such as scaling 5 V down to 3.3 V?
Rearrange the divider formula: R1/R2 = (Vin - Vout) / Vout. For 5 V down to 3.3 V the ratio is (5 - 3.3) / 3.3 = 0.515, so R1 must be roughly half of R2. Pick R2 first to set your quiescent current: 10 kΩ keeps the divider at 0.25 mA, acceptable for a GPIO-level load. That gives R1 = 0.515 x 10 kΩ = 5.15 kΩ; the nearest E24 standard value is 5.1 kΩ, which lands at 3.33 V, well inside a 3.3 V ADC's input range. Always confirm with the calculator above before soldering, because small tolerance stacks in both resistors shift the output by a few percent.
Methodology and sources
This tool models an unloaded two-resistor series voltage divider: R1 from Vin to the tap, R2 from the tap to ground, with the output taken across R2. It applies the divider relation derived from Ohm's law and Kirchhoff's voltage law, then reports the divider current and the power dissipated in each resistor.
- Method: Vout = Vin × R2 / (R1 + R2); divider current I = Vin / (R1 + R2); voltage across R1 = Vin − Vout; power per resistor P = I² × R (so PR1 = I² × R1, PR2 = I² × R2, Ptotal = Vin × I). The optional nearest-standard-value hint snaps R1 and R2 to the closest E24 preferred value.
- Standards and sources: standard DC circuit physics, Ohm's law (V = I × R) and Kirchhoff's voltage law. Preferred resistor values follow the IEC 60063 (E24) series. No safety-rating standard is implemented; the tool is pure circuit arithmetic.
- Assumptions and limits: assumes an ideal, unloaded divider with no current drawn at the output tap. Connecting a real load in parallel with R2 lowers the effective lower-half resistance and pulls Vout down, so use this for signal scaling and measurement, not to power a load. It also assumes ideal resistors at their nominal values; real-world tolerance, temperature drift and source resistance shift the result.
Reviewed and maintained by Rick Oosterling, who builds and wires 12 V, solar and EV systems hands-on. Last reviewed: June 2026. This calculator is a planning and design aid, not a substitute for a qualified professional or your local wiring and building code; verify any safety-critical design against the relevant standards.
Next step in this workflow
Divider designed: now calculate the current-limiting resistor for your LED.