Resistor network
What is a voltage divider?
A voltage divider is a simple series circuit that produces an output voltage lower than its applied input voltage. It normally consists of two resistors connected in series.
The output is measured across the lower resistor, commonly identified as R2. The voltage is divided in proportion to the two resistance values.
Main equation
Voltage divider formula
Output voltageVout = Vin × R2 ÷ (R1 + R2)
- Vout is the voltage measured across R2.
- Vin is the total applied input voltage.
- R1 is the upper resistor between the input and output nodes.
- R2 is the lower resistor between the output node and ground.
Solve every variable
Rearranged voltage divider equations
Input voltageVin = Vout × (R1 + R2) ÷ R2
Upper resistanceR1 = R2 × (Vin − Vout) ÷ Vout
Lower resistanceR2 = Vout × R1 ÷ (Vin − Vout)
Worked calculation
Calculate output voltage
Suppose the input voltage is 12 V, R1 is 2,000 Ω, and R2 is 1,000 Ω.
- Write the equation: Vout = Vin × R2 ÷ (R1 + R2).
- Substitute the values: Vout = 12 × 1,000 ÷ (2,000 + 1,000).
- Add the resistances: R1 + R2 = 3,000 Ω.
- Calculate the result: Vout = 4 V.
Divider proportion
Understanding the divider ratio
The divider ratio is the output voltage divided by the input voltage:
Divider ratioRatio = Vout ÷ Vin = R2 ÷ (R1 + R2)
In the worked example, the ratio is one-third, so the output voltage is one-third of the applied input voltage.
Series current
Current through the divider
In an ideal unloaded divider, the same current flows through both series resistors.
Circuit currentI = Vin ÷ (R1 + R2)
For 12 V across a total resistance of 3,000 Ω, the ideal divider current is 0.004 A, or 4 mA.
Consistent values
Voltage and resistance units
- Enter voltage values in volts.
- Enter both resistances using the same resistance unit.
- Both resistances may be entered in ohms, kilo-ohms, or mega-ohms when the same unit is used for each.
The calculator displays ideal circuit current assuming resistance values were entered in ohms.
Real circuit behavior
Voltage-divider loading effect
A connected load draws current from the output and appears electrically in parallel with R2. This reduces the effective lower resistance and changes the output voltage.
For accurate loaded-divider analysis, first calculate the parallel combination of R2 and the load resistance, then use that effective resistance as the lower resistance.
Model assumptions
Assumptions and limitations
This calculator assumes an ideal DC source, ideal resistors, and no connected load. Real circuits may be affected by resistor tolerance, temperature, source resistance, wiring resistance, and measurement-device impedance.
A passive two-resistor divider cannot produce an output voltage equal to or greater than its input voltage.
Common questions
Voltage divider calculator FAQ
What formula does the voltage divider calculator use?
The main equation is Vout = Vin × R2 ÷ (R1 + R2), where R1 is the upper resistor and R2 is the lower resistor connected to ground.
Can the calculator find either resistor value?
Yes. It can rearrange the divider equation to calculate R1 or R2 when the input voltage, output voltage, and other resistance are known.
Why must output voltage be below input voltage?
A passive two-resistor voltage divider can only reduce the applied input voltage. An output equal to or above the input would require a different circuit arrangement or an active component.
Can I enter resistance in kilo-ohms?
Yes, provided both resistor values use the same unit. For example, both may be entered in ohms or both in kilo-ohms because the divider ratio depends on their proportion.
Does this calculator account for a connected load?
No. It models an ideal unloaded voltage divider. A load connected across R2 changes the effective lower resistance and must be included as a parallel resistance.