Voltage Divider Calculator - Solve Vout, R1, R2, or Vin

How to use this tool

1. Enter any three values and leave one field blank.
2. The blank field is solved and highlighted in green.

Voltage divider formula

Vout = Vin × R2 / (R1 + R2)

The two resistors form a series circuit. The output is taken across R2. Current through the divider: I = Vin / (R1 + R2).

Common uses

• Level shifting - reduce a 5 V signal to 3.3 V for a microcontroller
• Biasing transistors
• Reference voltages for ADC inputs
• Volume controls (potentiometer)

Circuit diagram

A voltage divider consists of two resistors in series between the supply voltage and ground. The output is measured at the junction between R1 and R2:

   Vin
    │
   [R1]
    │
    ├───► Vout
   [R2]
    │
   GND

R1 is the top resistor (between Vin and Vout). R2 is the bottom resistor (between Vout and ground). To get a higher Vout, decrease R1 or increase R2. The formula Vout = Vin × R2 / (R1 + R2) confirms that Vout is always less than Vin (for positive resistances).

Load effect warning

When you connect a load (RL) across R2, it is in parallel with R2. The effective lower resistance is R2 ∥ RL = (R2 × RL) / (R2 + RL), which is always less than R2. This lowers Vout. Use the optional load panel to estimate the loaded output and its percentage error from the ideal result. The 10× guideline compares RL with the divider’s Thevenin resistance R1 ∥ R2; when RL is at least ten times R1 ∥ R2, the loading error is under about 10% in this model.

Resistor selection guide

Standard resistor values follow the E12 and E24 series, which contain 12 or 24 values per decade. When designing a voltage divider, choose a ratio R1/R2 matching your target and round to the nearest standard values.

Example: 5 V source to 3.3 V output requires Vout/Vin = 0.66, so R2/R1 ≈ 0.515. From the E24 series: R1 = 15 kΩ, R2 = 27 kΩ gives Vout = 5 × 27/(15+27) = 3.21 V (about 2.6% below the target). The calculator can suggest E12 or E24 pairs near a preferred total resistance and reports each pair’s actual output and error. Standard values are approximations; confirm that the resulting voltage meets your circuit’s input thresholds.

Power dissipation calculation

The calculator reports unloaded power for both resistors and the full network. With I = Vin / (R1 + R2), the individual values are P_R1 = I² × R1 and P_R2 = I² × R2; total dissipation is P = Vin × I = Vin² / (R1 + R2).

P_R1 = I² × R1     P_R2 = I² × R2     P_total = Vin × I

For battery-powered circuits, use large resistor values (100 kΩ+) to keep quiescent current low. For a 5 V source and R1+R2 = 200 kΩ, current is 25 µA and total dissipation is 0.125 mW. Compare each resistor’s estimate with its rated power and leave margin for temperature, tolerance, and enclosure conditions. A connected load changes circuit current and requires a separate power calculation.

Zener diode comparison

A Zener diode shunt regulator maintains a more stable output voltage under varying load conditions than a resistive divider:

• Stability: Zener ≪ resistive divider under variable load.
• Cost: a Zener diode + one resistor is often inexpensive.
• Current draw: Zener regulators waste current as heat; not ideal for ultra-low-power designs.
• Recommendation: use a resistive divider for signal conditioning and level shifting; use a Zener or IC regulator for powering loads.