Voltage Divider Calculator
Calculate the output voltage of a resistor voltage divider. Enter Vin, R1 and R2 to find Vout, division ratio and current through the divider.
How to use this tool
- Enter input voltage (vin), r1 (top resistor) and r2 (bottom resistor) in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your output voltage (vout) and the full breakdown beneath it.
A voltage divider uses two resistors in series to produce a lower voltage: Vout = Vin × R2 / (R1 + R2). It is commonly used to scale voltages for ADC inputs, bias circuits, or level shifting.
Formula
Output Voltage: Vout = Vin × R2 / (R1 + R2)
Division Ratio: ratio = R2 / (R1 + R2)
Divider Current: I = Vin / (R1 + R2)
How it works
A resistor voltage divider splits an input voltage proportionally between two series resistors. The output is tapped at the junction between R1 (top) and R2 (bottom), giving a fraction of Vin equal to R2 divided by the total resistance.
The calculation assumes an unloaded divider — no current is drawn from the output node. Adding a load in parallel with R2 will lower the effective R2 and reduce Vout; use a buffer (op-amp follower) if load current is significant.
Worked example
Worked example — 12 V input, 10 kΩ / 4.7 kΩ
- Inputs: Vin = 12 V, R1 = 10,000 Ω, R2 = 4,700 Ω.
- Total resistance = 10,000 + 4,700 = 14,700 Ω.
- Division ratio = 4,700 / 14,700 ≈ 0.3197.
- Vout = 12 × 0.3197 ≈ 3.8367 V.
Vout: 3.8367 V | Division Ratio: 0.3197
Key terms
- Voltage Divider
- A passive circuit of two series resistors used to produce a lower output voltage that is a fixed fraction of the input voltage.
- Division Ratio
- The fraction Vout/Vin, equal to R2/(R1+R2). A ratio of 0.5 means the output is half the input.
- Loading Effect
- The reduction in Vout that occurs when a load (another circuit) draws current from the output node, effectively reducing R2.
- R1 (Top Resistor)
- The resistor connected between Vin and the output node; a larger R1 relative to R2 gives a lower output voltage.
- Divider Current
- The quiescent current flowing through the divider chain from Vin to ground, equal to Vin/(R1+R2). Keep this much larger than the expected load current to maintain accuracy.
Frequently asked questions
- Why does the output voltage change when I connect a load?
- A load in parallel with R2 reduces the effective resistance, dropping Vout. For accurate division, the load resistance should be much larger than R2 (≥10×).
- How do I choose R1 and R2 for a target Vout?
- Rearrange: R1/R2 = (Vin − Vout)/Vout. Pick a convenient R2, then calculate R1.