Calculators

Resistor Divider Calculator

Calculate the output voltage, divider current, resistor power and equivalent resistance of a two-resistor voltage divider.

What Is a Resistor Divider?

A resistor divider uses two resistors connected in series across a voltage source. The output voltage is taken from the junction between the two resistors. 

             R1
Vin โ”€โ”€โ”€โ”€โ”€โ”€โ”€/\/\/\โ”€โ”€โ”€โ”€โ”€โ”€โ”ฌโ”€โ”€โ”€โ”€ Vout
                       โ”‚
                       โ”‚
                      R2
                       โ”‚
                      GND

The output voltage depends on the ratio between R1 and R2.

Voltage Divider Formula

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For an unloaded divider: 

Vout = Vin ร— R2 / (R1 + R2)

Where R1 is the upper resistor and R2 is the lower resistor.

Resistor Divider Calculator

Enter Vin, R1 and R2.

Example โ€” 12 V Divider

Consider a 12 V supply with two 10 kฮฉ resistors. 

Vin = 12 V
R1  = 10 kฮฉ
R2  = 10 kฮฉ

Vout = 12 ร— 10000 / (10000 + 10000)

Vout = 6 V

Because both resistors have the same resistance, the voltage is divided equally.

Divider Current

The current flowing through an unloaded divider is: 

I = Vin / (R1 + R2)

This is also the current flowing through both resistors because they are connected in series.

Divider Current Calculator

Enter the divider values.

Resistor Power

The power dissipated by each resistor can be calculated using: 

P = Iยฒ ร— R

Therefore: 

P1 = Iยฒ ร— R1

P2 = Iยฒ ร— R2

The resistor power rating should be greater than the calculated dissipation with an appropriate safety margin.

Divider Power Calculator

Enter the divider values.

Calculate R1 for a Desired Output

If the desired output voltage and R2 are known, R1 can be calculated using: 

R1 = R2 ร— (Vin / Vout - 1)
Enter the required values.

Calculate R2 for a Desired Output

If R1 is known, R2 can be calculated using: 

R2 = R1 ร— Vout / (Vin - Vout)
Enter the required values.

Loaded Resistor Divider

The basic divider equation assumes that nothing is connected to the output. In a real circuit, the load connected to Vout affects the result. 

             R1
Vin โ”€โ”€โ”€โ”€โ”€โ”€โ”€/\/\/\โ”€โ”€โ”€โ”€โ”€โ”€โ”ฌโ”€โ”€โ”€โ”€ Vout
                       โ”‚
                      R2
                       โ”‚
                       โ”œโ”€โ”€โ”€โ”€ RL
                       โ”‚
                      GND

The load resistance RL is effectively connected in parallel with R2. 

R2effective = R2 || RL

The loaded output voltage is then calculated using R2effective.

Loaded Divider Calculator

Enter all four values.

Voltage Divider for an ADC

A resistor divider is frequently used to reduce a voltage before connecting it to an analog-to-digital converter. 

Higher Voltage
      โ”‚
      โ”‚
     R1
      โ”‚
      โ”œโ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€ ADC
      โ”‚
     R2
      โ”‚
     GND

The divider must be designed so that Vout never exceeds the maximum permitted ADC input voltage.

The ADC input impedance and leakage current should also be considered because they can affect the divider output.

Choosing Resistor Values

There is usually more than one possible combination of R1 and R2 that produces approximately the required output voltage.

When choosing the values, consider:

  • Desired output voltage
  • Input voltage range
  • Divider current
  • Load resistance
  • Resistor tolerance
  • Resistor power rating
  • ADC input impedance, if applicable
  • Noise and leakage

High Resistance Values

Higher resistor values reduce the current consumed by the divider. However, the divider becomes more sensitive to loading, leakage and noise.

Low Resistance Values

Lower resistor values make the divider less sensitive to many loading effects, but they increase current consumption and power dissipation.

Common Applications

  • Battery voltage measurement
  • ADC input scaling
  • Sensor signal conditioning
  • Reference voltage generation
  • Logic-level voltage reduction
  • Biasing circuits
  • Transistor bias networks
  • Analog electronics

Common Mistakes

  • Reversing R1 and R2.
  • Forgetting the effect of the load.
  • Using excessively high resistor values.
  • Ignoring resistor tolerance.
  • Ignoring resistor power dissipation.
  • Applying an excessive voltage to an ADC or other input.
  • Assuming Vout can supply significant current like a power supply.

Key Points

  • R1 is normally the resistor connected to Vin.
  • R2 is normally the resistor connected to ground.
  • Vout is measured between R1 and R2.
  • Increasing R2 increases Vout.
  • Increasing R1 decreases Vout.
  • The divider current is Vin divided by R1 + R2.
  • The load connected to Vout changes the actual output voltage.
  • Resistor power ratings must be checked.
  • Voltage dividers are useful for signal and measurement circuits but are not normally intended to provide substantial load current.

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