Voltage divider formula
A two-resistor voltage divider produces an output voltage that is a fraction of its input voltage.
R1 is connected between the input supply and the output node. R2 is connected between the output node and the reference or ground.
Voltage divider example
With a 5 V input, a 10 kΩ top resistor and a 20 kΩ bottom resistor, the divider output is approximately 3.333 V.
The output is two-thirds of the input because R2 represents two-thirds of the total series resistance.
Solving for input voltage
If Vout, R1 and R2 are known, the divider equation can be rearranged to calculate the required input voltage.
Solving for R1
If Vin, Vout and R2 are known, the required top resistor is:
Solving for R2
If Vin, Vout and R1 are known, the required bottom resistor is:
Divider ratio
The voltage ratio is the output voltage divided by the input voltage.
Multiplying that ratio by 100 expresses the divider output as a percentage of the input.
Voltage divider current
In an ideal unloaded divider, the same current flows through R1 and R2 because the resistors are connected in series.
Higher resistor values reduce divider current and power consumption, while lower values make the divider less sensitive to loading.
Resistor power
Each resistor dissipates power as current flows through the divider.
The calculator estimates the ideal power dissipated by R1 and R2 so you can compare the result with the resistor power rating required by a real circuit.
Using Ω, kΩ and MΩ
R1 and R2 can use different units. The calculator converts ohms, kilohms and megohms to a common internal resistance before performing the calculation.
One kilohm equals 1,000 ohms. One megohm equals 1,000,000 ohms.
Loaded vs. unloaded voltage divider
The basic divider equation assumes the output has no significant load. A real device connected to Vout can draw current and effectively place resistance in parallel with R2.
When the load resistance is not much larger than the divider resistance, the measured output voltage can be lower than the ideal unloaded result.
Voltage divider vs. voltage regulator
A resistor divider is useful for signals, references and measurement inputs, but it is generally not a substitute for a regulated power supply when powering a changing load.
Related electrical calculations
For voltage, current and resistance relationships, use the Ohm's Law Calculator.
To combine resistor values, use the Series & Parallel Resistor Calculator.
If you need to identify a resistor from its color bands, use the Resistor Color Code Calculator.
This calculator models an ideal unloaded two-resistor divider. Real circuits can be affected by resistor tolerance, temperature, source resistance, load resistance and measurement equipment.
Frequently asked questions
What does a voltage divider do?
It produces an output voltage equal to a fraction of the input voltage using two resistors in series.
Which resistor is R2?
In the standard divider formula, R2 is the bottom resistor between Vout and ground.
Can Vout be higher than Vin?
Not in a passive two-resistor divider with positive resistor values. Vout must be lower than Vin.
Why is my measured Vout lower than the calculated value?
The connected load may be drawing current and acting in parallel with R2, which changes the effective divider resistance.
Can R1 and R2 use different units?
Yes. The calculator supports Ω, kΩ and MΩ independently for each resistor.
Using this calculator effectively
The Voltage Divider Calculator solves an ideal two-resistor divider for output voltage, input voltage, R1 or R2. It also reports divider ratio, current, total resistance and estimated resistor power.
Common uses
- Calculating the unloaded output voltage from a known input voltage and two resistor values.
- Finding the resistor value needed to produce a target output voltage from a known supply.
- Checking divider current and resistor power when selecting values for signal or reference circuits.
Tips for a better estimate
- Remember that R1 is the top resistor and R2 is the resistor between Vout and ground in the standard divider formula.
- Use the unloaded result only when the connected load has sufficiently high resistance compared with the divider.
- Check real resistor tolerance, power rating, source resistance and load resistance before using a calculated value in hardware.
Related calculations
Amps to Watts Calculator · Ohm's Law Calculator · Resistor Color Code Calculator