Voltage Divider Calculator
This Voltage Divider Calculator solves an ideal two-resistor divider. Enter Vin, R1, and R2 to calculate Vout, or select a reverse mode to calculate the resistor required for a target output voltage.
The tool is intended for electronics engineers, PCB designers, technicians, and students working with sensor scaling, ADC inputs, bias networks, level detection, and low-current reference circuits.
Results include divider current, total resistance, individual resistor dissipation, and total power so the ratio and its practical electrical cost can be reviewed together.
Voltage Divider Calculator
Calculate divider output voltage or solve either resistor value. All power and current results assume an unloaded, ideal two-resistor divider.
Supply voltage applied across the complete divider.
Resistance between Vin and Vout.
Resistance between Vout and ground.
Calculated result
6 V
Vout = Vin × R2 / (R1 + R2)
- Output voltage (Vout)
- 6V
- Upper resistor (R1)
- 10kΩ
- Lower resistor (R2)
- 10kΩ
- Total resistance
- 20kΩ
- Divider current
- 600µA
- Power in R1
- 3.6mW
- Power in R2
- 3.6mW
- Total power
- 7.2mW
Voltage Divider Circuit Diagram
Voltage Divider Formulas
Vout = Vin × R2 / (R1 + R2)I = Vin / (R1 + R2)R1 = R2 × (Vin - Vout) / VoutR2 = R1 × Vout / (Vin - Vout)P1 = I² × R1; P2 = I² × R2- Vin = Input voltage across the full divider
- Vout = Unloaded voltage at the R1 and R2 midpoint
- R1 = Upper resistor connected between Vin and Vout
- R2 = Lower resistor connected between Vout and ground
- I = Current through both resistors in the unloaded divider
How to Use This Calculator
- Select Calculate Vout when both resistor values are known. Select Calculate R1 or Calculate R2 when designing for a target Vout.
- Enter voltage values in volts or millivolts and resistance values in ohms, kilohms, or megohms. Units can be mixed because all internal calculations use base SI units.
- Press Calculate and review the highlighted requested value plus the complete current, resistance, and power result set.
- Compare the calculated resistor with a preferred resistor series, then recalculate using the selected standard values to find the real divider output.
Worked Example
Consider an unloaded divider with Vin = 12 V, R1 = 10 kΩ, and R2 = 10 kΩ.
Vout = 12 × 10 kΩ / (10 kΩ + 10 kΩ) = 6 V.
I = 12 V / 20 kΩ = 0.6 mA.
P1 = (0.6 mA)² × 10 kΩ = 3.6 mW.
P2 = (0.6 mA)² × 10 kΩ = 3.6 mW.
Total divider power is 7.2 mW. Equal resistors produce half of Vin, but their absolute values determine current consumption and source loading.
Engineering Notes
Load resistance effect
A load appears in parallel with R2 and lowers Vout. For accurate scaling, include the load resistance or buffer the divider with a high-input-impedance stage.
Divider current trade-off
Lower resistor values reduce sensitivity to leakage and loading but waste more current. Higher values save power but increase source impedance and noise sensitivity.
Power dissipation
Check power in R1 and R2 separately. Select resistor wattage with thermal and reliability margin rather than matching the calculated dissipation exactly.
Not a voltage regulator
A passive divider should not power a variable load. Load current changes its output, so use a regulator, reference, buffer, or power converter when a stable supply is required.
Common Mistakes
- Swapping R1 and R2 and therefore calculating the wrong ratio.
- Ignoring the input resistance of an ADC, sensor, or measurement tool.
- Choosing very low resistor values without checking wasted current.
- Choosing very high values without checking leakage, settling time, and noise.
- Using a divider as a supply for a changing or substantial load.
FAQ
What is a voltage divider?
A voltage divider is a series pair of resistors that produces a fraction of an input voltage at their midpoint. Its ideal output depends on the ratio of R1 and R2.
How do you calculate Vout in a voltage divider?
For an unloaded divider, use Vout = Vin × R2 / (R1 + R2), where R1 is connected to Vin and R2 is connected from Vout to ground.
How do you choose R1 and R2 for a voltage divider?
Choose the resistor ratio for the required output, then select the overall resistance to balance current consumption, loading error, noise, and input impedance requirements.
Why does load resistance affect voltage divider output?
A load connected to Vout appears in parallel with R2, reducing the effective lower resistance and changing the divider ratio. The load should be much larger than R2 or included in the calculation.
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Reference Links
Return to the resistor calculator category for component tools, or browse the Engineering Reference Center for circuit-design resources and component guidance.
Disclaimer
This calculator models an ideal unloaded resistor divider. Verify loading, tolerance, temperature, leakage, transient behavior, and component ratings with simulation, datasheets, and hardware testing.