Voltage divider calculator
A voltage divider uses two resistors to turn a higher voltage into a lower one. Choose what to solve for, fill in the other three values and read the result, along with the current and power the divider uses. Prefixes such as k work.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Choose what to solve for: output voltage, R1 or R2.
- Enter the input voltage and the two other known values.
- Read the result and the divider current.
Vout = Vin × R2 ÷ (R1 + R2)
R1 = R2 × (Vin − Vout) ÷ Vout
R2 = R1 × Vout ÷ (Vin − Vout)
Worked example
To scale 9 V down to 3.3 V with R2 = 4.7 kΩ: R1 = 4700 × 5.7 ÷ 3.3 = 8118 Ω. The nearest E24 value is 8.2 kΩ, which gives 9 × 4700 ÷ 12900 = 3.279 V.
Tips
- The calculation ignores resistor tolerance; 1 % parts give a closer result than 5 %.
- Use the “R1 and R2 in parallel or series” tool to build an exact value.
Frequently asked questions
What is the voltage divider formula?
Vout = Vin × R2 ÷ (R1 + R2), where R1 is between the input and the output and R2 is between the output and ground. With 9 V, 10 kΩ and 4.7 kΩ the output is 9 × 4700 ÷ 14700 = 2.878 V.
How do I choose resistor values for a target voltage?
Pick R2 or R1 first, usually in the kilohm range, and let the tool find the other one. To get 3.3 V from 9 V with R2 = 4.7 kΩ, R1 = 4700 × (9 − 3.3) ÷ 3.3 = 8.118 kΩ. The tool also shows the nearest E24 value and the output you get with it.
Why does my divider output drop when I connect a load?
The formula is for an unloaded divider. A load in parallel with R2 lowers the effective R2 and so the output. As a rule of thumb, keep the load resistance at least ten times larger than R2, or buffer the output.
Can I power a circuit from a divider?
Not reliably. The output voltage changes with the current drawn. Use a voltage regulator for supplying circuits; dividers suit measuring and signal levels, such as scaling a battery voltage for an ADC input.
What do the current and power rows show?
The current is Vin ÷ (R1 + R2), flowing through both resistors, and the power is Vin times that current. Higher resistor values waste less power but are more sensitive to loading.