Voltage Divider Calculator
Calculate the output voltage of a two-resistor voltage divider. Solve for Vout, R1, R2, or Vin. Includes current, power dissipation, and loaded-output effects.
How to Use
- Pick which value to solve for — Vout is the default.
- Enter the three known values. Resistors accept k, M suffixes (e.g., "10k").
- The circuit visualization updates live with current and voltage labels.
- Optionally add a load resistor (Rload) to see the loaded-output voltage.
- Check Show Work for the step-by-step math.
Show Work
Formulas
History of the Voltage Divider
The voltage divider is a direct consequence of Gustav Kirchhoff's 1845 circuit laws — his voltage law requires the sum of potential differences around a loop to equal zero, which for two series resistors forces the voltage across each to be proportional to its resistance. Kirchhoff published these rules as a graduate student in Königsberg; combined with Ohm's 1827 linear V=IR law, they are the bedrock of every DC circuit analysis taught since.
Early vacuum-tube radio designers of the 1920s and 30s relied on resistor dividers to set cathode bias, grid-leak voltages, and screen-grid tapped points — there was no op-amp, no precision reference, just carbon-composition resistors and the divider equation. When semiconductors arrived in the 1950s, dividers became the standard way to bias transistor bases; when microcontrollers arrived in the 1980s, they became the standard way to scale external signals into ADC ranges. The circuit hasn't changed in 180 years; only what's on either side has.
The limitation has also been known for just as long: the formula Vout = Vin × R2/(R1+R2) assumes no current drawn at the tap. The instant a load is connected, its impedance parallels R2 and the ratio drifts. The response — make R2 much smaller than the load impedance, or buffer with an emitter follower / op-amp — has been standard practice since the early analog era.
About This Calculator
Pick what to solve for (Vout, Vin, R1, or R2), enter the three known values with standard engineering suffixes (k, M), and this calculator returns the missing quantity plus current draw and total power dissipation. Add an optional Rload value to see how connecting a real load pulls the output down — useful for reality-checking sensor bias networks or ADC front-ends.
The circuit visualization is live: current arrows, node voltages, and resistor values update as you type. Everything runs entirely in your browser — no values are transmitted or stored. For anything above low-signal levels, consider a regulator instead of a divider; dividers waste power as heat through R1 and R2 continuously.
Frequently Asked Questions
What is a voltage divider?
A voltage divider is two resistors in series that produce a reduced voltage at the tap between them. It's the simplest way to scale a voltage — used for sensor biasing, reference generation, ADC input scaling, and level shifting. Formula: Vout = Vin × R2 / (R1 + R2).
Why does the output drop when I connect a load?
The load resistor (Rload) is effectively in parallel with R2. This lowers the equivalent R2 and changes the divider ratio. A rule of thumb: Rload should be at least 10× R2 to keep the output within ~10% of the unloaded value.
How do I pick R1 and R2 values?
Choose resistor values that give acceptable current draw. Low resistance wastes power; high resistance is sensitive to loading. For a 5V → 2.5V divider with ~100µA draw: R1=R2=25kΩ. Match R2 to be 10× (or more) the input impedance of whatever you're driving.
Why use a voltage divider instead of a regulator?
Dividers are cheap and simple but unregulated — output sags with load and drifts with supply. Use them for low-current signals (ADC inputs, bias points) where precision isn't critical. Use a regulator or buffer amp when you need stable, load-independent output.
How much power do the resistors dissipate?
P_R1 = (Vin − Vout)² / R1. P_R2 = Vout² / R2. Total power = Vin² / (R1 + R2). Pick wattage ratings with 2× headroom minimum.
Common Use Cases
ADC Input Scaling
Step a 12V automotive rail down to within a 3.3V microcontroller ADC range. R1=10kΩ, R2=3.3kΩ gives ~3.0V at the ADC pin.
Bias Point Generation
Set a transistor base voltage or op-amp reference. A matched pair of 10kΩ resistors from a 5V rail makes a clean 2.5V bias.
Battery Level Monitor
Scale a LiPo pack voltage down to a microcontroller-safe range, with high-value resistors (100kΩ+) to minimize current drain.
Pull-Up / Pull-Down Estimation
Predict the voltage at a GPIO when both pull-up and external load are active — treat them as a two-resistor divider.
Sensor Interfacing
Thermistors, LDRs, and pressure sensors form half of a divider. Pair with a fixed resistor to turn resistance changes into voltage changes.
Signal Level Shifting
Drop a 5V logic signal down to 3.3V (use 1kΩ/2kΩ or similar). Works for slow signals; use a proper level shifter for fast I/O.
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