Voltage Divider Calculator
Design or analyze a two-resistor voltage divider. Enter 3 values to get the 4th value — input voltage (Vin), resistance R1, resistance R2 and output voltage (Vout) — with units in millivolts, volts or kilovolts and ohms, kiloohms or megaohms. For R1 and R2 in series with Vout taken across R2, everything runs locally in your browser.
Pick which value is unknown
The calculator solves for whichever field you leave empty: the output voltage Vout, the input voltage Vin, or either resistor R1 or R2. Exactly one field must stay blank.
Enter 3 values with their units
Type the three known values and choose each unit next to its field — voltages in millivolts (mV), volts (V) or kilovolts (kV); resistances in ohms (Ω), kiloohms (kΩ) or megaohms (MΩ). Mixed units are fine.
Press Calculate, read the result
The missing value appears in the unit selected for its own field, with up to 10 significant digits. Press Reset to clear all fields and start over.
Voltage divider rule
- What is the voltage divider formula?
- For two resistors R1 and R2 connected in series across a supply voltage Vin, with the output Vout taken across R2: Vout = Vin × R2 / (R1 + R2). The output is always a fraction of the input — the fraction is R2's share of the total resistance.
- How do I get the 4th value from the other 3?
- The same rule rearranged four ways: Vout = Vin × R2 / (R1 + R2); Vin = Vout × (R1 + R2) / R2; R1 = R2 × (Vin − Vout) / Vout; R2 = R1 × Vout / (Vin − Vout). Leave the field you want to find empty and press Calculate.
- Can you give a worked example?
- A 12 V supply with R1 = 4.7 kΩ and R2 = 10 kΩ gives Vout = 12 × 10000 / (4700 + 10000) = 120000 / 14700 ≈ 8.163265306 V. Conversely, if you need 3.3 V out from a 5 V supply using R2 = 10 kΩ, then R1 = 10000 × (5 − 3.3) / 3.3 ≈ 5151.515 Ω, i.e. about 5.15 kΩ.
- Why do some combinations return an error?
- Physically impossible or undefined inputs are rejected instead of showing nonsense: fewer or more than exactly three known values, negative resistance, R1 = R2 = 0 (the divider ratio is undefined), R2 = 0 while solving for Vin, Vout = 0 while solving for R1, Vin = Vout while solving for R2, or values that would require a negative resistor.
- What does this calculator ignore about real circuits?
- It is the ideal rule: resistors are exact and no current flows out of the Vout terminal. Any load attached to the output forms a parallel path with R2 and drags Vout down, so keep the load resistance much larger than R2 (rule of thumb: at least 10×) or buffer the output with an op-amp.
Privacy: every calculation runs in your browser tab — no value you type is sent anywhere. Results are ideal steady-state values; real dividers sag under load and depend on resistor tolerance, so verify with a multimeter.