How to use the voltage divider calculator
- Choose what to find: the output voltage, the bottom resistor R2 or the top resistor R1.
- Enter the input voltage and the values you know. Resistor values are in ohms, so enter 10000 for 10 kilohms.
- To see the effect of a load, enter its resistance in the optional field. Leave it blank for no load.
- Read the output, the current, the power in each resistor and the share of the input you get at the output.
Formula
Vout = Vin x R2 / (R1 + R2)
R2 = R1 x Vout / (Vin - Vout)
R1 = R2 x (Vin - Vout) / Vout
With a load RL: use R2 in parallel with RL = R2 x RL / (R2 + RL) in place of R2
Current = Vin / (R1 + bottom resistance), power = current squared x resistance
R1 is the resistor connected to the input voltage and R2 is the one connected to ground. The output is taken at the point between them.
Worked example
With 12 V across R1 = 10 kohms and R2 = 5 kohms, the output is 12 x 5,000 / 15,000 = 4 V. The current is 12 / 15,000 = 0.8 mA, so R1 dissipates 6.4 mW and R2 dissipates 3.2 mW.
Connect a 10 kohm load across R2 and the bottom becomes 5,000 x 10,000 / 15,000 = 3,333 ohms, so the output falls to 12 x 3,333 / 13,333 = 3 V.
To get 4 V from 12 V with R1 = 10 kohms, R2 must be 10,000 x 4 / 8 = 5 kohms. With R2 = 5 kohms, R1 must be 5,000 x 8 / 4 = 10 kohms.
What a voltage divider is for
A voltage divider uses two resistors in series to produce a lower voltage from a higher one. It is common for scaling a sensor signal to the range an input can accept, setting a reference level, or reading a battery voltage. The output depends only on the ratio of the resistors, not on their absolute size.
The effect of a load
A divider only holds its voltage if whatever you connect to the output draws little current compared with the current through the resistors. A load acts like a resistor in parallel with R2 and pulls the output down. As a rule of thumb the load should be many times larger than R2, or the divider resistors should be made smaller, which wastes more current. Enter the load to see exactly what happens.
Choosing resistor values
Larger resistors waste less power but are more sensitive to loads and noise. Smaller ones hold up better but draw more current. Real resistors have tolerances, so the output will differ slightly, and the resistor color code calculator helps you read what you have. To check currents and power use the Ohm's law calculator.
Assumptions and limits
- The source is ideal and holds its voltage regardless of current.
- Resistors have exactly the values you enter. Tolerance and temperature effects are not included.
- The load, if given, is a plain resistor connected between the output and ground.
- A divider is not a voltage regulator and should not power loads that draw real current.
Frequently asked questions
What is the voltage divider formula?
Vout = Vin x R2 / (R1 + R2), where R1 is the top resistor and R2 is the bottom one. The output is the fraction of the input that falls across R2.
How do I choose R1 and R2 for a target voltage?
Pick a value for one resistor, then solve for the other with this tool. A common choice is a current of about a milliamp or less, which sets the size of the resistors.
Why is my measured voltage lower than calculated?
Whatever you connect to the output, including a meter with a low input resistance, acts as a load and pulls the voltage down. Enter the load resistance to see the expected value.
Can I use a voltage divider to power a device?
Usually not. The output changes whenever the load current changes. A regulator or a converter is the right tool for supplying power.